Analysis of the Performance of a Pressure Transducer for Sea
Level Measurements
A Thesis Submitted for the Award of the Degree of
Doctor of Philosophy
In The Faculty of Natural Sciences Goa University
Goa
by
Antony Joseph K
Department of Physics Goa University
Goa
5 34.4
"c
ANT/AN
October, 1996
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DECLARATION
The author hereby declares that this thesis represents work which has been carried out by him and that it has not been submitted to any other University or Institution for the award of any Degree, Diploma, Associateship, Fellowship or any other such title.
Place: Taleigao Plateau.
Date : October, 1.996 Antony Joseph K.
Scientist, MICR
National Institute of Oceanography Dona Paula
Goa 403 004 •i ce
CERTIFICATE
I hereby certify that the above declaration of the candidate Antony Joseph K. is true arid that this thesis represents his independent work.
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en. i0. g6 I, I- "1 Dr. J.A.E. Desa r ka,
(Research Guide) • 4
Reader G 0 it
.Depart•iiient of Physics Goa University
Goa 403 205
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ACKNOWLEDGEMENTS
During the course of this work, I have received support and help from a num- ber of colleagues and well-wishers. The study reported here were initiated after the observation of an unexpected tinder-estimation of tidal range during tidal measurements using a pressure transducer deployed in the turbid waters of the
"'ugh estuary in January 1991. Logistic support and encouragement from Cap- tains A.C. Dutta and R.C. Paul of the Marine Department of Calcutta. Port Trust, and laboratory and field services extended by Drs. E. Desa and E.S. Desa, and S/Shri. R.G. Prabhudesai,. V.N. Chodankar, Dennis Rodrigues and E. D'elva, of the National Institute of Oceanography (NIO) were very valuable for the measurements in the Hugh estuary. Excellent co-operation was received from S/Shri. D.B. Kulkarni (Executive Engineer), D.M. Damle (Asst. Exec. Engi- neer) and Rawat (Marine Surveyor) of Bombay Port Trust during measurements in Bombay. Services from S/Shri. Raut Dessai (Dy. Chief Engineer), Balan (Asst. Engineer) and D.M. Hinge (Marine Surveyor) of Mormugao Port Trust, and Shri. Vijaykumar and Snit. Vani Peshwe of NIO were•valuable in erecting a tide station in the Zuari estuary, Goa and making the sea level measurements re- ported here. The generous support I received from the Prottdman Oceanographic Laboratory (POL) during measurements in UK was extremely valuable. I am grateful to Joe Rae (Head of Technology Group), Dave Smith and Les Bradley (Tide Gauge Inspectorate), Peter Foden, Kevin Taylor, Jim McKeown, John Cas- son, John Mackinnon, Tom Wylie, Jackie I lardcastle and many other colleagues
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of POL for their active co-operation in the completion of the experiments dur- ing my short stay at POL. I am thankful to Ms Caroline Marriott (Programme Officer, British. Council, Manchester) for providing funds for some of the exper- iments undertaken in the U.N. l also acknowledge with thanks all the help I received from. Dr. Adrian M.illwa•d, Bill Lanceley and Prof.
T.
Hedges of the University of Liverpool, U.K. Acknowledgement is also due to Dr. David Pugh and Dr. Peter Collar (Institute of Oceanographic Sciences (10S), Worniley, U.K) for many useful discussions during wave-flume experiments at IOS. Inputs from Dr. Prasann.akumar of NIO on oscillatory boundary layer and fluid mud phe- nomena were valuable. I am thankful to Dr. Erwin Desa who guided me in this research work and extended painstaking efforts in helping to present the results of the studies in a logical sequence. My thanks are also due to Dr. G.R. Bhat (Reader, Physics Department, Goa University), Ms. Alka Shikerkar, S/Shri.Shantann Gain's and kaustlibli Priolkar (Research Fellows, Goa University) and Smt. Nikita Kamat for their help in the use of the TEX package and to Smt.
Linda Veliath for typing this manuscript. I am grateful to Dr. E. Desa (Direc- tor, N10) at whose initiative my visit to POL was made possible, and to Shri.
M.R. Nayak (Head, MICD, NM) for his generous co-operation. During this visit, with active co-operation from POL, most of the experiments reported here were carried out. Finally, I thank .Dr.P.B.Sarode (Head of Physics Department, Goa University) for all the administrative support during the submission of this thesis.
Last but not least I would like to recall my gratitude to my wife Lissy and my
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daughters Runa and Rini for their generous support during the period over which this work was executed.
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CONTENTS
List of Figures iv
List of Tables viii
List of Symbols viii
Abstract xi v
Chapters
Chapter 1 Introduction
1.1 Historical background •/1
1.2 A Review of Measurement Techniques 3
1.2.1 Tide Staff 3
1.2.2 Electric Step Gauge 4
1.2.3 Float-Driven Gauge 4
1.2.4 Pressure Measuring Systems 6
1.2.4.1 Pneumatic System 6
1.2.4.2 Pressure Transducers 7
1.2.4.3 Importance of a Pressure Transducer 9
1.2.5 Acoustic Gauge 10
1.2.6 Satellite Altimetry 12
1.2.7 Radiowave Interferometry 13
1.2.8 Differential Global Positioning System (DGPS) . 15 1.3 Comparison of Different. Methods of Sea Level Measurement 17
1.4 Scope of the Present Work 18
Chapter 2
Apparatus and Experimental Facilities
2.1 Quartz Differential Pressure Transducer 21
2.1.1 Vibrating beam 22
2.1.2 Signal Processing 26
2.1.3 Specifications 26
2.1.4 Temperature Compensation 26
2.2 Sea Unit and Data Logger for Hugli Estuary 28
2.3 Experimental Setup for Measurement at Bombay Harbour 32
2.3.1 Intelligent Pressure Transmitter 32
2.3.2 Data Logger • 35
2.4 Experimental Setup for .Performance Evaluation under Different
Flow Conditions 36
2.4.1 Transducer Mounting Mechanism 36
2.4.2 High Speed Flow Channel 39
2.4.3 Data Logger 41
2.5 Experimental. Setup for Performance Evaluation under Different
Wave- and Coinbination of Flows and Wave-Conditions 42 2.6 Experimental Setup for Measurements in the Humber Estuary,
North Sea, ILK 45
2.7 Experimental Setup for Measurement in Zuari Estuary 49 Chapter 3
An Evaluation and Improvement of a Pressure Transducer Under Difi, ferent Flow Conditions
3.1 Introduction 55
3.2 Response of Pressure Transducer to a Flow Field 56
3.3 Description of Experiments and Results 57
3.3.1 Response of a Pressure Transducer to Laminar Flow 57
3.3.2 Role of a Large Circular Thin Plate 62
3.3.3 Response of a Pressure Transducer to Turbulent Flow 65
3.4 Discussion of Results 71
3.4.1 Role of a Flat Plate (Laminar Flow) 75
3.4.2 Influence of a Vertical Cylindrical Piling 79
3.4.3 Role of a Thin Perforated Sheet 82
3.4.4 Role of a Single Horizontal Plate (Turbulent Flow) 83 3.4.5 Role of a Pair of Parallel Horizontal. Thin Plates
(Turbulent Flows) 86
Chapter 4
Evaluation and Performance Enhancement of a Pressure Transducer Under Waves and Combination of Flows and Waves
4.1 Introduction 87
4.2 Response of a Pressure Transducer to Shallow Water Waves 87
4.3 Experiments and Results 97
4.3.1. Response of a Pressure Transducer to Progressive
Gravity Waves Propagating on Quiescent Water 97
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4.3.2 Response of a Pressure Transducer to Progressive
Gravity Waves Riding on Positive and Negative Currents 102
4.4 Discussion of Results 106
4.4.1 Role of a Flat Plate 109
4.4.2 Role of Parallel Plates and Stand-Offs
(Waves Propagating on Quiescent Water) 112
4.4.3 Response of the Pressure Transducer to Waves
Riding on Currents 117
Chapter 5
Evaluation of Performance of a Pressure Transducer Under Turbid Natural Waters
5.1 Introduction 127
5.1.1 Turbidity Maxima 128
5.1.2 Turbidity Profile 130
5.1.3 Lutocline 130
5.1.4 Concentration and Density 131 °,
5.1.5 Settling Velocity 132
5.1.6 Air Bubbles 134
5.2 Deployment of a Pressure Transducer in the Turbid Waters of
Hugli Estuary, India 134
5.3 Influence of Fluid Density on Measurement Accuracy of Tide 138 5.4 Reduction in the Effective Density of Turbid Natural Waters 139 5.5 Measurements hi the Turbid Waters of a Partially Constrained
Water Body at Bombay Harbour, India 143
5.6 Measurements in the Turbid Waters of the Humber Estuary, U.K
Using Two Pressure Probes 147
5.6.1 Experimental Setup 148
5.7 Measurements in the Clear Waters of the Zuari Estuary,
India, Using Two Pressure Transducers 152
5.8 Probable Reasons for an Effective Reduction in the In-:situ
Density of Turbid Natural Waters 154
5.8.1 Influence of Suspended Sediments 155.
5.8.2 Influence of Lift Force on Densities 157
Chapter 6 Conclusions
6.1 The Present Work 160
6.1.1 Response to Flows 160
6.1.2 Response to Waves and Combination of Waves and Flows 163
6.1.3 Response in Turbid Natural Waters 165
6.2 Suggestions for Future Work 167
6.2.1 Modified Bubbler Design 168
6.2.2 Tilt-Response Enhancement of a Pressure Transducer
and Miniaturisation of Its Front-End 171
6.2.3 Design of an Enhanced Sea Level Measuring System
with Compensation for Fluid Density Changes 173
Bibliography 178
Figures
2.1 Single-beam quartz resonator for pressure transducer 24 2.2 Quartz crystal re'sonator's mounting isolation 24 2.3 Conversion of input; pressure to au axial force
applied to the crystal resonator 24
2.4 Torsional tuning fork temperature-sensitive quartz resonator
used for temperature-compensation of pressure transducer 29 2.5 A sectional view of the in-water unit of tide gauge
deployed at Hugh estuary 29
2.6 Schematic block diagram of the Data Terminal of tide gauge
deployed at Hugh estuary 31
2.7 Partially constrained body of turbid water in Bombay harbour where a pressure transducer and a float-driven gauge
measured sea, level 33
2.8 Block schematic of Intelligent Pressure Transmitter 34 2.9 Modified positive pressure inlet on pressure housing 38 2.10 Support mechanism for pressure transducer housing
for flow flume experiments 38
2.11 Schematic of the flow flume 40
2.12 Working section of the flow channel 40
2.13 Block schematic of Data logger used in Flow channel
and wave flume experiments 43
2.14 Schematic of the wave flume/tow-tank facility 43 2.15 Support mechanism for pressure transducer housing
for wave flume experiments 44
2.16 Location of measurement (turbid waters), Immingham, U.K 46 2.17 Bubbler mounting arrangement at Humber Estuary, Immingham
with reference to the phase
of
the tide 472.18 Schematic diagram of bubbler 47
2.19 Scheme for density-compensated water level measurements,
employed in Zuari estuary, Goa 51
2.20 Block schematic of de► sity-con►pensated sea level
measuring equipment 52
2.21 Schematic diagram of multiplexed serial communication
interface 53
3.1 Differing hydro-mechanical front-ends of pressure transducer
(No piling ;11 the vicinity) 59
3.2 Response of a pressure transducer for differing hydro-mechanical
front-ends of Figs. 3.1(a,b,e) to a steady laminar flow field 59 3.3 Spread in horizontal azimuthal response of a pressure transducer for
small parallel plates of Figs. 3.1(c,d) for a steady laminar flow field 61 3.4 Spread in horizontal azimuthal response of a pressure transducer for
large parallel plates of Figs. 3.1. (f,g) for a steady laminar flow field 61
3.5 Dependence of on ( 20-r ) 64
3.6 Differing experimental settings for the pressure transducer housing
mounted near a cylindrical piling 66
3.7 Performance of a pressure transducer for differing relative
orientations of its housing and a cylindrical vertical piling go o
for various mainstream flows 67
3.8 Effect of a perforated curved sheet in the vicinity of the pressure
transducer housing on the performance of the pressure transducer 70 3.9 Effect of a single large end-plate and a parallel pair on the
performance of the pressure transducer 70
3.1.0 Transverse distance from the pressure inlet with increasing (u117,; ) for indicated mainstream flows in the case of
plate diameter D, as in Fig. 3.1(b) 73
3.1.1. Flow velocity u with.in the laminar boundary layer for differing transverse distances from the pressure inlet for indicated mainstream
flows in the case of plate diameter D 73
3.12 Transverse distance from the pressure inlet with increasing
(u/U00 )
for indicated mainstream flows in the case ofplate diameter 3D, as in Fig. 3.1.(e) 74
3.13 Flow velocity u within the laminar boundary layer for differing transverse distances from the pressure inlet for indicated mainstream
flows in the case of plate diameter 3D 74
3.14 Growth of laminar boundary layer thickness over a flat plate,
from its leading edge, for indicated mainstream flows U 00 77 3.1.5 Laminar boundary layer thickness at the centre of the end-plates of
diameter 1) and 31), for indicated mainstream. flows U 00 77 3.16 Variation of experimentally observed and calculated values of
AP with mainstream flow velocities 11,,
for protruding pressure inlet as in Fig. 3.1.(a) 78 3.1.7 Experimental and theoretical values of AP for various flow
speeds and directions when the pressure transducer was mounted
near a cylindrical piling as in Fig. 3.6(a) 81
3.18 Dependence of difference between experimental and theoretical values of AP on flow directions for various flow speeds 81 3.19 Growth of laminar sub-layer thickness over a flat plate
for indicated mainstream turbulent flows
Uoo
853.20 Laminar sub-layer thickness at the centre of the end-plates
of diameters D and 3D for various mainstream turbulent flows 85 4.1 Definition sketch of parameters used in eqn.(4.1) 90 4.2 Predicted dependence of average pressure reduction AP
on wave period, in shallow waters, for indicated wave heights
for a typical low-tide situation 93
4.3 Predicted dependence of average pressure reduction AP on wave period, in shallow waters, for indicated wave heights
for a typical high-tide situation 93
4.4 Predicted dependence of average pressure reduction AP on wave height, in shallow waters, for indicated wave periods
for a typical low-tide situation • 94
4.5 Predicted dependence of average pressure reduction aY/3 on wave height, in shallow waters, for indicated wave periods
for a typical high-tide situation 94
4.6 Predicted dependence of average pressure reduction AP
on sea floor depth, in shallow waters, for indicated wave periods
for wave height of 150 cm 95
4.7 Predicted dependence of average pressure reduction AP
on sea floor depth, in shallow waters, for indicated wave heights
for a wave period 7 secs 95
• 4.8 Predicted dependence of average pressure reduction AP
on the height of pressure inlet from sea floor
for
indicated waveheights of period 7 secs for a typical low-tide situation 96 4.9 Predicted dependence of average pressure reduction
A P
on the height of pressure inlet from sea floor for indicated wave
heights
of
period 7 secs for a typical high tide situation 96 4.10 Response of a pressure transducer for hydro-mechanical front-ends ofFigs. 3.1(a,b,e) to progressive gravity waves propagating on quiescent
water 100
4.11 Spread in horizontal azimuthal response of a pressure
transducer for small parallel plates of Fig: 3.1(c,d) for progressive
gravity waves propagating on quiescent water 101 4.12 Spread in horizontal azimuthal response of a pressure transducer
for large parallel plates of Figs. 3.1(f,g) for progressive gravity waves
propagating on quiescent water 101
.13 Response of a pressure transducer with a protruding inlet as in Fig. 3.1(a) for waves riding on positive and negative
currents 104
.14 Response of a pressure transducer for pressure inlet located at the centre of the end-plate of the transducer housing and flush with the plate surface as in Fig. 3.1(b) for waves propagating
on positive and negative currents 104
.15 Response of a pressure transducer for pressure inlet located at the centre of a. large end-plate as in Fig. 3.1(e) for waves
propagating on positive and negative currents 105 .16 Response of a pressure transducer when its pressure inlet
is attached to large parallel plates [Fig. 3.1(f)], for waves
propagating on positive and negative currents 105 .17 Laminar boundary layer thickness over a flat plate for different
wave periods 111
.18 Root-mean-square flow velocity u„,„ as a function of yb within the laminar boundary layer for differing experimental wave heights
i.19 Comparison of experimentally observed and predicted values of AP using four calculational schemes for combinations
of waves and currents in the case of a. protruding pressure inlet as in Fig. 3.1(a). The parameters used were y=38 cm, d=165 cm,
g=981.21 cm/sec t and T=2 sec 120
i.1 A section of Hugh estuary, India 136
i.2 Tidal range under-estimation by a pressure gauge deployed in the turbid waters of the Hugh estuary when average bulk density was used. (1) offset adjusted at high tide,
(2) offset adjusted at low tide 137
5.3 Correction of tidal range under-estimation by a pressure gauge deployed in the turbid waters of the Ilugli estuary by the use of
effective density. (1) ebb tide phase, (2) flood tide phase 140 5.4 Percent errors in water level of Fig. 5.2 when bulk density
and effective density of turbid water were used 141 5.5 Tidal range under-estimation by a pressure gauge deployed in
a partially constrained turbid water body in Bombay when
the density of clear water was used 145
5.6 Correction of tidal range under-estimation by a pressure gauge deployed in a partially constrained turbid water body in Bombay by the use of effective density
5.7 Percent error in water level of Fig. 5.5 when clear water density and effective density of turbid water were used 5.8 Comparison of bulk density, clear-water density and effective
145 146
density of turbid waters of the Humber estuary, North Sea (U.K)
as a function of tidal height 151
5.9 Comparison of measured and effective densities of the clear waters of the Zuari estuary, Arabian Sea (Goa) as a function of
tidal height 153
6.1 Proposed schemes to reduce flow- and wave-effects on a pneumatic bubbler. Bubbler protected by: (1) a portion of a perforated copper
sphere, (2) a perforated copper cylinder 170
6.2 Proposed schemes to reduce flow- and wave-effects on a pressure transducer. Pressure inlet protected by: (1) a single-layered
perforated copper hemisphere, (2) two concentric perforated copper
hemispheres 172
6.3 Proposed scheme for density-compensated sea level measurement
using pressure transducers 174
Tables
1.1 Comparison of accuracies of sea level measuring devices '19 2.1 Operation of multiplexed serial communication interface 54 4.1 Comparison of experimental and theoretical values of AP
in the case of a protruding pressure inlet (Fig. 3.1a) for a 2 sec
wave, and y = 38 cm, d = 165 cm, T = 28.125 sec 108 4.2 Predicted lengths and heights of major and minor axes of elliptical
motions of water particles at the measurement location for
the indicated wave heights 114
4.3 Mean pressure difference for differing mass transport reverse
current for indicated wave heights 122
5.1 Depth-profile of conductivity, temperature, and density of water in the Humber estuary, Immingham, U.K
(Presence of sediment not taken into account) 149
Symbols
Symbols Definitions
A (i) Semi-major axis of elliptical path described by water particles under shallow water progressive wave motion.
(ii) Projected area of a suspended solid particle facing a flow.
•
(i) Wave amplitude.
Symbols Definitions
(ii) Radius of a suspended sediment particle.
ao Radius of cylinder.
B
Semi-minor axis of elliptical path described by water particles under shallow water progressive wave motion.Radius of cylinder.
Concentration of sediment in a turbid water suspension.
C1 Coefficient of lift.
C2 , C3 Constants.
D (i) Diameter of Hat-plate hydro-mechanical front-end.
(ii) Diameter of a suspended sediment particle.
d Depth of sea floor below mean water level.
Lift force.
f Wave frequency.
f, Strauhal frequency.
g Acceleration due to gravity.
H Height of wave propagating on quiescent water.
Hf Modified height of wave propagating on a flow.
Rh(t) True mean height of fluid column above a pressure transducer at high tide.
True mean height of fluid column above a pressure transducer at low tide.
k (i) Wave number.
Symbols Definitions
(ii) Velocity coefficient.
ki , k2 Empirical constants.
L Length of wave propagating on quiescent shallow water.
f Modified length of wave propagating on a flow.
L o Length of wave propagating on quiescent deep water.
P Pressure of fluid in presence of flow or wave.
Ph Mean pressure at high tide.
Pi Mean pressure at low tide.
Po Hydrostatic pressure.
PD Pressure difference across a vertical fluid column of height D.
Initial value of fluid pressure in the vicinity of a solid body outside its boundary layer.
P2 Value of fluid pressure in the vicinity of a solid body outside its boundary layer over a small distance Ax.
AP Deviation in pressure; i.e., (Po - P) or (P2 - AP Time-average value of P.
Pexpi Experimental value of P.
A
Pad Calculated value of P.I. Reynolds number.
R„(crii) Critical Reynolds number.
R t True tidal range.
ie,.
Calculated tidal range.Symbols Definitions
Rerro 'r Error in tidal range, i.e.,
(R t
- Re ).r Radial distance of a. given point in a flow field from the axis of a submerged vertical cylinder.
S
Strouhal number.s specific gravity of sediment particle.
Wave period.
t Time.
Free stream velocity.
u
• (i) Velocity within the boundary layer of a solid body. • (ii) Horizontal component; of the wave orbital velocity.
Urms Root-mean-square value of u.
no Drift-induced reverse current in the presence of a wave in a closed tank.
V Velocity of fluid in the vicinity of a solid body and outside its boundary layer.
Vl Initial value of V.
V2 Value of V over a small distance As.
Vr Radial component of the flow velocity field around a vertical cylinder.
Flow component vector normal to Vr . v settling velocity of a suspended particle.
• vsc, settling velocity at which hindered settling begins.
W,. Effective weight of a suspended particle tinder turbulent flows,
• xi
Symbols Definitions
14/3 Submerged weight of a solid particle under quiescent condition.
w vertical component of the wave orbital velocity.
Radian frequency of wave propagating on quiescent water.
co./ Radian frequency of wave in the presence of a flow.
Horizontal distance or co-ordinate.
y Vertical distance or co-ordinate.
Yb Value of y within the boundary layer of a fiat plate.
z Height or depth with reference to the mean water level. •
() latitude.
Coefficient of molecular viscosity.
Time span for which averaging is performed.
0 Angle between r and the vector U o.3 . Boundary layer thickness.
cS Laminar sub-layer thickness.
To Horizontal shearing stress.
P, Pw Density of water.
Depth-mean value of p.
Puff Effective depth-mea ► density of turbid natural water.
Ps Density of a, sediment particle.
• Pb Bulk density of turbid water suspension.
Pw Density of water with suspended sediment removed.
Symbols Definitions
Pw(1) True depth-mean density of fluid.
Pw(c) Density of fluid used for conversion of pressure to fluid height.
Kinemati• viscosity of fluid.
(i) ✓L4'' LS
(ii) ye,r,
ABSTRACT
In the present study, the response of a pressure transducer to laminar and turbulent flows; waves and combination of flows and waves; and suspended sedi- ments have been investigated. Flow- and wave-flume experiments conducted on a pressure transducer whose pressure inlet was attached to differing configurations of hydro-► echanical front-ends have shown that the transducer's performance for steady laminar flows and regular progressive gravity waves propagating on quiescent waters can be significantly improved (i.e., a small value of AP which was the difference between the average hydrostatic and measured pressures). In
• this arrangement, the pressure inlet was located at the centre of a flat, thin and smooth horizontal circular plate whose diameter was 3 times that of the pressure housing and gave an accuracy of 0.12% of KS as compared to 0.34% in the case of a protruding pressure inlet. This enhancement was likely to have been achieved on account of the isolation of the pressure inlet from the separated flow and from- vortices generated by the transducer housing. Deterioration in performance of a pressure transducer, arising from the flow disturbances generated by nearby solid structures, could be significantly reduced by protecting the pressure housing by 'a sturdy curved perforated sheet whose radius was approximately twice that of the pressure housing. The enhanced performance in this case was 0.08% of F.S as compared to 0.38% in the absence ()I' the sheet, and arose from the flow retarding
• jet action of the numerous orifices of the perforated sheet. For turbulent flows, use of a, thin parallel-plate front-end mechanism whose diameter was 3 times that
of the pressure transducer housing, a,nd separated by a distance equal to the diameter of the housing led to a much improved horizontal azimuthal response for the transducer upto a flow speed of 1.00 cm/sec. At this speed the spread in AP was <1 mbar compared to 6.29 inbais without a plate. The 'flow stabilising capability of such a parallel-plate front-end mechanism was likely to have yielded the observed improvement in this case.
In the case of waves propagating on quiescent waters, the use of this parallel- :,
plate front-end led to negative AP values. The total value of AP for waves riding on currents (e.g., 0.15 mbar at wave hei
•
approximately a sum of a positive AP due to currents alone and a negative AP due solely to waves. Thus, for combinations of waves and currents a relatively small positive AP (0.5 mbar, typically) was found. This result is of special significance to tidal measurements of coastal waters in which waves propagate on tidal currents.
Pressure measurements made in turbid natural water bodies have led to the inference that the effective in-situ density values pelf of such water bodies are less than their static bulk densities and also that of the density of the same water without suspended sediment. The values of pelf in
a
given site differed from one tidal cycle to another. These values varied slightly also from mid-tide to slack waters of the same tidal cycle. It was found that the use of bulk density for conversion of measured pressure to tidal height gave an under-estimation of tidal range (upto 7%). The error could be reduced to negligible values with the use of gilts of 16 cm and P.1 50 cm/sec) wasPeff•
For clear waters, there was a close agreement between the effective density as estimated from pressure measurements and the density of water samples measured using a precision density meter. Thus, the observed reduced in-situ effective density of turbid natural waters implies that suspended sediments play a hitherto unknown role in reducing the effective density of turbid natural waters. A close agreement of the pressure-derived water levels in turbid natural waters with those measured using a tide staff when the reduced effective density values were applied on the pressure data clearly indicates that, the concept of reduced effective density
• has a practical significance.
Chapter 1
INTRODUCTION
1.1 Historical background
The theory of the tide generating force was first propounded by Sir Isaac New- ton in his celebrated work 'Principia Mathematica Philosophae Naturalis'. The theory of tidal motion has since been further investigated by Daniel Bernoulli, Laplace, Lord Kelvin and G.H. Darwin. The earliest tide-prediction tables wove compiled in China in order to satisfy the interests of sight-seers of spectacular estuarine bores and for the benefit of travellers wishing to cross a tidal river (Zu- osheng (4, al, 1989). Later, the times mid heights of high and low tides were needed by pilots for convenience of shipping. Tide data were also used for hydro- graphic survey applications.
From a scientific point of view, long-term sea level data series significantly contributes to the explanation of long-term secular behaviour of low-frequency sea level oscillations. Such data records also help in revealing seasonal and inter- annual variabilities of coastal currents. Sea level records are also useful in de- termining the trend of submergence or emergence along the coast. Progress in the forecast arid timely warning of impending natural disasters such as storm surge and El Nino events depends largely on climate and sea level data (Wyrtki,
a positive surge, can give rise to serious coastal flooding especially when it coin- cides with high water on a spring tide. Storm surge warning is only one aspect of sea level monitoring. An understanding of the climate and global sea level change over lung periods depends on the innovative interpretation of historical sea level records from numerous tide stations spread over the globe. Such records are useful to take a closer look at the likely implications of the sea level rise to the coastal environment (Pirazzoli, 1986., Dias and Taborda, 1992). Emery and Aubrey (1989) have made some studies of the mean sea level (MSL) variations in some regions of India. Shetye et al., (1990) investigated the vulnerability of the Indian coastal regions to damage from sea level rise.
•
Recent studies are indicative of the mean sea level rise as a result of increasing concentration of greenhouse gases in the atmosphere and the consequent rise in the global mean surface temperature (Gornitz et al., 1982., Hoffman, 1984., Titus and Barth, 1984., Barnola et al., 1987, and Genthen et al., 1987). Bird and Koike (1986) have examined man's impact on sea level changes. Dolotov (1992) examined some possible types of coastal evolution associated with the expected rise of the world's sea level caused by the greenhouse effect.
When theoretical predictions are uncertain, the role of careful monitoring of the sea level are important in that subtle changes may be identified at. an early stage through proper analysis. The concern over mean sea level rise has led to a number of assessment of the quality of sea level data. The need for mea- surement of sea level with significantly improved accuracy, and exchange of sea
level data on a global scale has developed since 1985 when the Intergovernmen- tal Oceanographic Commission (IOC) initiated the Global Sea Level Observing System (GLOSS).
1.2 A review of measurement techniques
Sea level is defined as the distance of the sea surface above a reference datum known as chart datum (C.D). By international agreement, the level used as C.D should be just low enough so that low waters do not go far below it. The device that carries out sea level measurement has been called a `ticlegaugg, rather tian a• 'sea level gauge', primarily because at most locations the astronomical tide is the largest part of the sea level variations. Coastal measurements of sea level have a long history and have been made from time immemorial. In this section, sea level measurement techniques are briefly reviewed.
1.2.1 Tide staff
The oldest device used to measure sea level is a 'tide staff' which is a vertically mounted graduated pole driven into the sea bed. The resolution of a tide pole is usually 10 cm. In a flow field, tide staff readings can be corrupted by flow- induced piling at the staff. Reading accuracy in the presence of flows and waves may be increased by fitting a transparent tube alongside the staff, which connects to the sea through a narrower tube preventing immediate response to external
and recording the data makes it; unsuitable for long-term measurements.
1.2.2 Electric step gauge
This is an electric version of the tide staff, where a vertical row of manganese bronze electrodes are used to detect the position of the water surface. The elec- trodes are scanned at least twice a second to provide digital data that can be processed to yield tidal height information. The main output signal is the binary value of the highest submerged electrode that is detected while scanning from top to bottom. Though the electrodes are usually 5 cm apart, the averaging process permits estimation of tidal height with a resolution better than 0.5 cm in niost applications. Molina and Leenhonts, 1993). However, a source of error is water piling at the staff due to flow and waves. A layer of thick oil on the water surface can be a serious impediment to the reliable functioning of this gauge as it inhibits electrical contact of the electrodes with sea water.
1.2.3 Float-driven gauge
The oldest standard instrument for automatic recording of sea level is a mechan- ical device known as float gauge, and was conventionally used worldwide. The main component in this gauge is a cylindrical or conical float resting on the sur- face of water within a large cylindrical hollow pipe called a stilling-well. At the bottom of the well is a cone or a narrow tube whose end, known as orifice, is usually located at a point at least 1 m below C.D and at least 1 m above the sea floor. The resistance to flow of the orifice in combination with the area of the
water surface in the well form a low-pass hydraulic filter which suppresses wave- induced high-frequency water level fluctuations in the well. Thus, the float rests on this approximately quiet level, rising and falling with the slow oscillations of the tide with a much reduced disturbance from short-period wind-wave action.
The float is connected to a pen via a set of gears and a taut flexible wire attached to a counter-weight and passing over a pulley. While a mechanical clock-work gives the recorder paper on a drum a continuous motion along the time-axis, the float drives a pen over the paper along the tide height axis, thereby continuously
tracing the sea level on the paper.
•
Although a stilling-well system is relatively simple to operate, it has a num- ber of disadvantages. Studies by Lennon . (1971) showed that local water density changes have a profound influence on the accuracy of a float-gauge because the stilling-well traps low density fluid. Further investigations by Noye (1974) indi- cated that the conventional stilling-well system suffers from inherent problems, including non-linear response. An apparent variation in the mean sea-level (MSL) can conceivably result from an asymmetric non-linear response of the measuring instrument to short-term processes that do not average out. Studies by Shih and Baer (1991) indicated that draw-down errors in a stilling-well can be signif- icantly reduced by modification Of the shape of the orifice and use of a suitable hydro-mechanical front-end at the orifice.
1.2.4 Pressure measuring systems
A widely used approach to the estimation of sea level is to measure pressure at some fixed point below the C.D and to convert this pressure to water level with the use of water density. In coastal measurements, the underwater pressure may be transmitted to a shore-based recorder through a pneumatic tube. In simple systems, underwater pressure is sensed by connecting the sea- ward end of the tube to a partially inflated bag or an air-filled drum open at the base, in which
pressures adjust as water enters and leaves during the tidal cycle.
1.2.4.1 Pneumatic system
A traditionally used pressure'systein for sea level measurement is a bubbler at- tached to a pneumatic system. A bubbler works on the principle that if a com- pressed gas is bubbled freely into a liquid from a submerged fixed end of a tube the pressure in the entire length of the tube, irrespective of the elevation of its other end, is equal to the pressure head of the liquid column over the bubbler's orifice, provided the tube is not too long 00 in). In operation, compressed air from a cylinder is reduced in pressure through one or two valves so that there is a small steady flow down the connecting tube to escape through an orifice in an underwater canister called bubbler orifice chamber. At this underwater outlet, for low rates of gas escape, the gas pressure is equal to the water pressure. This is also the pressure transmitted along the tube to the ►eas ►ring and recording system, apart from a small correction for pressure gradients in the connecting
•
tube. The gas pressure at the landward end of the tube is measured by a differ- ential pressure transducer which responds to the difference between the system pressure and the atmospheric pressure, so that only Lite water head pressure is recorded.
The advantages of a pneumatic bubbler system include the stability of a clearly defined datum (i.e., bubbling point) and the expendable nature of the vulnerable underwater parts. However, in locations having a broad beach and a shallow topography the application of pneumatic pressure transmission is not desirable.
This is because a long pneumatic tube, usually more than 10 in, introduces a delay in pressure transmission and also causes errors induced by pressure gradient in the tube. A mathematical treatment of the bubbler gauge principle has been given by Pugh (1972) and Ling and Pao (1994).
1.2.4.2 Pressure transducers
For sea level measurements two types of pressure transducers are in common use, namely; absolute and differential. An absolute transducer has only one pressure port, and senses the total pressure (i.e., atmospheric pressure plus the pressure due to water column above the transducer). The output of a differential pressure transducer does not respond to atmospheric pressure variations. A number of dif- ferent types of pressure transducers have been used for tidal measurements over the continental shelf and in deep sea. These include strain gauges (analogue), vi-
et al., 1973., Peshwe et al., 1980., Joseph and Desa, 1984., Cox et al., 1984., Spencer et al., 1994., Spencer et al., 1996, and Joseph et al., 1996). As the inher- ent accuracy of a transducer is best maintained in a digital system, an inherently analoge device such as a strain gauge is usually incorporated in an oscillator to provide a digital output (Sidor, 1983). Temperature sensitivities of various pres- sure transducers have been reported by Rae (1976). Temperature-compensated quartz absolute and difkrential pressure transducers with an accuracy of 0.01 %
(in the quiescent condition) are presently available for sea level measurements.
The quartz technology for water pressure measurements was first introdwced by Hewlett-Packard in 1970. Subsequently quartz transducers were introduced by Paroscientific Inc., under the trade name `Digiquartz'. A description of the quartz pressure transducer is given in Chapter 2. Temperature-correction of quartz pres- sure transducer is adequate only when the ambient temperature changes slowly enough that the pressure transducer can be considered to be in thermal equilib- rium (Kusters, 1976). Diqiquartz transducers have time-constants of the order of half-an-hour when adjusting to changes in temperature. In situations where water temperatures vary in time-scales of this order, pressure errors arise that cannot be ignored (Chiswell, 1991). Measurement, in the shelf region, by Boss and Gonzalez (1995) indicated that pressure signal from Digiquartz transduc- ers is related to the time-derivative of temperature signal. Similar results were
• observed by researchers from Proudinau Oceanographic Laboratory, U.K. (J.M.
Vassie, 1995, private communication).
1.2.4.3 Importance of a pressure transducer
A unique advantage of a pressure transducer is that it can be deployed even in re- mote estuarine locations, in the shelf region and also the deep ocean. For example, bottom pressure recorders have been deployed by the Proudman Oceanographic Laboratory for many years on a routine basis and have been used to measure oceanic tides and low-frequency variations in the South Atlantic (Cartwright et al., 1988., Spencer et at, 1994). Pressure transducers have also been found to be ideal for island measurements. In the open ocean, moored pressure transducers
• are required for validation of satellite altimeter data. Because altimetry plays a major role in the study of world ocean circulation, such validation assumes greater significance (Wunsch., 1986).
In the coastal waters, bottom-mounted pressure transducers would be a pow- erful tool for the study of low-frequency coastally-trapped waves such as Internal Kelvin Waves (IKWs) and barotropic shelf waves (BSWs) or their hybrids where most of the energy of low-frequency motions in the shallow seas and at the bor- ders of oceans is concentrated (Schmidt and Johnson, 1993). Measurement of pressure fields is particularly suited to evaluating the scattering of these low- frequency waves when they travel from one topography to another, and also for the investigation of the irregular occurrence of the unique fluid mud suspension events in some areas (Wells et al., 1978., Shenoi and Murthy, 1986., NIallik et
been found especially useful is for the study of infra-gravity energy modulation by tides (Okihiro and Cuza, 1995). As this phenomenon occurs outside the surf zone and a few kilometres from the shore at depths in the range 8-30 in, con- ventional sea level measurement systems are not suitable for this study. All the studies reported in this field have been carried out using bottom-mounted pres- sure transducers. (Seymour et al., 1985, Elgar et al., 1992 and Herbers et al.,
199/1).
1.2.5 Acoustic gauge
• Acoustic gauges have been used in two modes namely under-water-mode and air-mode. In the former the acoustic transducer is placed on the sea floor and operated as an inverted echo-sounder. In operation, a piezo-electric transducer transmits a short-time narrow beam acoustic pulse. The pulse is reflected by the sea surface and returns to the transducer, which converts it into an analogous electric pulse. The electronic circuitry associated with the transducer determines the two-way travel time `ti of the pulse. The water head above the transducer is determined from the measured value of t and a knowledge of sound velocity in water.
Measurements by Cartwright (1982) showed that variation of t depends not only on the vertical motion of the sea surface with changing tide but also on
• the internal layers of differing sound velocity. The acoustic path was found to have been lengthened by refractions and internal tides. For these reasons, an
under-water acoustic gauge is not suitable for accurate sea level measurements.
I lowever, this system is usable for hydrographic survey applications offshore where an accuracy of a few tens of cm is acceptable. The sea level data collected by under-water acoustic gauges are usually transmitted to the survey ship via under- water acoustic telemetry, upon interrogation (Morgera et al., 1986).
For coastal measurements of sea level using the acoustic principle the trans- ducer is operated in the air-mode. The air-acoustic gauge has the advantage of operation from land and is usually installed in harbours. Its operating principle is the same as that for the under-water acoustic gauge, hilt the sound velocity in air has to be used for acoustic path length calculations. To ensure contin- uous and reliable operation, by avoidance of wave-induced loss of the reflected acoustic signal from the sea surface, the acoustic pulses are generally restricted to travel within a vertical downward-looking narrow tube mounted within a pro- tective well. The tide height with reference to C.D is obtained by subtracting the estimated height of the transducer face, with reference to the sea surface, from its altitude above C.D. Averaging a number of measurements will smooth out wave-induced fluctuations in the water level.
An important consideration in the ai•-acoustic tide gauge is the dependence of velocity of sound in air on atmospheric pressure, temperature and moisture. This dependence is more significant on temperature than on atmospheric pressure and moisture. Application of the air-acoustic gauge for sea level measurements was restricted until corrective measures were devised to compensate for errors due to
• II
•
temperature effects.
A method for temperature-compensation of the air-acoustic gauge is the use of an acoustic reflector at a fixed level in the air column. By relating the time of reflection from the sea surface to that from the fixed reflector, a direct compen- sation for variations in sound velocity between the acoustic transducer and the fixed reflector can be achieved. However, this technique still .does not account for variations in sound velocity between the fixed reflector and the sea surface. Per- formance evaluation of a temperature-compensated air-acoustic gauge has been given by Woodworth et al., (1995). The question of sound speed as a function
• of temperature profile in the well remains to be fully studied. Errors arising from trapped fluid in the sounding tube would also be a major concern when considering the overall accuracy attainable from an air-acoustic. gauge.
1.2.6 Satellite altimetry
Satellite altinietry is a fairly new and very important development in the mea- surement of MSL. The utility of this technique for sea level measurement has been demonstrated by a series of altimeters of increasing accuracy and precision flown successively on several satellites (Cartwright, 1991).
Nleasnring the sea, surface topography with a satellite altimeter system results from the combination of two techniques, namely radar altimetry and precise orbit
• determination. The former is the precise measurement of the distance between
the satellite and the ocean surface. The latter is the measurement of satellite's
orbital distance from the centre of mass of the earth. The difference between the two measurements gives the height of the sea surface (average over the footprint of the altimeter) in a co-ordinate system relative to the centre of the earth. While the ground coverage repeats at regular intervals, any dilkreuces in the levels measured by repeated orbits must be due to sea surface variability.
The orbit of the satellite (800 to 1300 km above the earth's surface) does not remain precisely stable due to many atmospheric, astronomical, and geophysical forces which act to disturb the dynamics of the satellite. Other small orbit dis- tortions are introduced by changes of the satellite's mass after manoeuvres l. by gravitational attractions of the moon and sun, and even by the changes of gravity due to water mass movements of the ocean tides. Because of these uncertainties, the satellite positions and velocities must be tracked at intervals relative to fixed
0
ground stations. Several satellite altimeter systems have been launched, and af- ter suitable tracking, these have the capability of measuring the range of radar altimeter signals returned from the sea surface with a variance of about ±3 cm for sea state of about /I, = 2 are. Using altimeter signals, high precision modelling of mean sea level and ocean tides have become possible (Schwiderski, 1991).
1.2.7 Radiowave interferometry
Remote radiowave interferometric techniques, used to measure sea level in coastal regions, employ a bistatic scheme where a radiowave transmitter sends a radio
separation ranging from 400 to 2700 m (Glassman, 1981, 1982). The signal at
the point of observation is composed of two coherent parts, one direct from the transmitting antenna to the receiving antenna, and the other by way of low-angle reflection from the water surface. Water level information is contained in the path difference of signals arriving over the two paths. This path difference causes the two signals to interfere at the receiving antenna with a resultant that de- pends upon the signal frequency and the geometry of the system, incorporating the instantaneous sea level. Thus, with the rise and fall of the sea level this geometry also undergoes a corresponding change. With a bistatic scheme, it
pe-
comes possible to derive information about the sea level height by monitoring the interkrence Id along a vertica,1 direction at the receiver. A change in the
sea level height causes not only a vertical shift of the entire interference pattern but also affects the distance between individual interference fringes. Using two receiving antennas spaced by a known vertical distance, it becomes ideally pos- sible to obtain unambiguous information on sea level height, independent of sea surface roughness caused by wind-waves. In practice, the accuracy of sea level measurements using this method is in the range of 3 to 9 cm, depending on the sea state and tidal range.
An advantage of this system is that it has a remote character of operation and works round the clock regardless of weather conditions. A disadvantage is that it can be operated only on such locations where the transmitter and the receiver can be suitably located, with the horizontal distance between them not exceeding
a few kilometers. For large distances (many tens of km), the earth's curvature and atmospheric refractions can have a significant effect on the received signal.
1.2.8 Differential global positioning system (DGPS)
Sea level measurement using the DGPS relies on a method called 'kinematic dif- ferential positioning' based on the highly stable carrier phase signals transmitted by NAVSTAR global positioning satellites. By observing the signals from two receivers simultaneously (one placed on a precisely known stable position on land known as 'reference station'), the relative position of the moving receiv:;:- (i.!&., the
`gauge station') can be derived in a geocentric reference frame, or with respect to the reference station. The recently emerging technique using CPS receivers in floating buoys allows reliable sea level measurements to be made as far as 20 kin from the coast, and requires only anchoring of buoys containing GPS receiving antenna, tilt sensor, radio unit and the required power supply housed within a buoy and a solar panel to charge the battery. The measured GPS data from the gauge station along with the tilt sensor data are time-tagged and transmitted to the reference station for processing.
In a DGPS system, analysis of the phase observations in the differential mode, using sophisticated kinematic software, results in a 3-dimensional positioning of the buoy antenna relative to the position on land, resolving cycle slips and carrier phase ambiguities during motion.
ambiguities be determined during motion, since the buoy is never in a static position. An initialisation is also necessary after periods of loss of phase lock to the satellites and/or when switching the GPS receiver on or off for observing (ay over certain time-windows. ll'or longer separations (i.e., more than 20 kin) between the reference station and the buoy antenna, the accuracy will gradually degrade as a function of atmospheric conditions. Thus, separations less than 20 km are preferred in DGPS-based sea level measurements. A discussion on carrier-phase ambiguity resolution has been given by Lachapelle et al., (1992).
Descriptions on GPS instrumentation aspects have been given by Dierendonci et al., (1978)., Rocken et al., (1990) and Hein et al., (1990).
Reduction of the moving antenna's phase centre to the vertical is achieved us- ing observed angles of two orthogonal tilt sensors, having an accuracy better than 0.1°. The dip-in-depth of the buoy into the water can be determined empirically before hand. This allows the instantaneous sea surface to be determined with an accuracy of 1 to 2 cm in the reference system. To take care of wave-induced motions of the sea surface, observations can be carried out once per second over a few minutes and averaged.
While most of the errors associated with UPS measurements get cancelled in a DGPS system, as these are common to both the receivers, some errors are specific to the case of sea level measurements. Such an error is the dynamics of the receiver antenna resulting from the wave-induced motion of the buoy. Receiver dynamics can impair the measurement accuracy by introducing noise in the phase-
lock tracking loop. It may be noted that the noise performance of these loops can be improved by decreasing the loop bandwidth to as small a value as possible.
However, beyond a certain limit this method causes a serious degradation in the dynamic tracking performance of the loop.
Another source of inaccuracy in the MA'S measurement of sea level is that tidal and other ocean current forces pulls the buoy deeper into the water when the buoy arrives at the extreme position allowed by the length of the anchor chain.
Additionally, other sources of errors are ionospheric, toposheric and orbit-related ones. However, these errors cancel for a short baseline. •
1.3 Comparison of different methods of sea level measurement
It has been noted in the previous section that several methods exist to obtain long-term time-series records of sea level, each method having its own merits and limitations. When automated tide measurements are required from locations other than harbour environments such as remote shallow coastal regions and • islands, the use of float-driven and air-acoustic gauges are impractical because such gauges require tide-wells or protective wells and associated purpose-built structures. Electric step gauge may appear to be a simple device in principle, but its long-term performance has not yet been proven. Further, its sensitivity to flow- and wave-induced piling effects and presence of oily filaments in water
records. Although a pair of radiowave interferometers is a modern remote sensing tool, the device eau be installed only ill limited locations such as river mouths and straits. DG I'S system is an einerging partially-remote sensing method for sea level measurements from locations upto 20-km-away from the shore, and . will be most useful for an earlier detection of storm surge events. However, the vulnerability of its satellite-receiver-bearing buoy to fishing activity is likely to limit its usage to protected environments such as harbours. Remote measurements using satellite altimetry has the capability for global coverage. However, time-series data from a given location cannot be obtained using this method as a result of the polar orbiting nature of the altimeter-bearing satellites. Further, measurements using other devices are required for ground truth measurements. A comparison of the accuracies of the above mentioned methods of sea level measurement under particular conditions is given in Table 1.1.
1.4 Scope of the present work
Inspite of worldwide use of pressure transducers for sea level measurements in different environments, studies on the performance enhancement of these trans- ducers of various types were largely limited to laboratory-scale quality improve- ment under quiescent conditions. Reported studies on their field-performance were limited to; pressure probe designs by Carson et al., (1975); calibration and use of pressure transducers by Banaszek, (1985); effects of temperature changes on the performance of a Digiquartz pressure transducer by Boss and Gonzalez
Sea level
measuring tivviev
Accuracy Conditions 1. Tide staff P-2, ±2.5 cm
> 10 cni
Calm sea.
Waves and flows.
2. Electric step gauge
::,--, ±2.5 cm Calm sea. Accuracy deteriorates in presence of flows and waves 3. Float-driven
gauge
r-zt, ±2 cm
>2 cm
>2 cm
Calm sea.
Flow speed of 100 cm/sec resulting in a draw-down of 17 cm.
Wave height of lm resulting in a draw-down of 7 cm.
Additional errors due to density stratification in the well.
4. Pressure transducers (a) Temperature-
compensated strain-gauge.
±0.1% of F.S i.e., ±2 cm (shallow water)
Calm sea, and with fluid density compensation.
Additional errors present due to flows and waves.
(b) 'Temperature- compensated quartz.
±0.01% of F.S i.e., ±0.2 cm (shallow water)
As above. Additional errors present if ambient water temperature changes fast.
5. Acoustic gauge.
(a) Under-water mode.
(b) Air mode
—
±1 cm
Accuracy-not known as this is used only for survey applications offshore.
Calm sea and temperature compensation.
Additional errors present due to flow, waves and density stratification in the sounding tube.
6. Satellite altimetry.
±3 cm Sea state < 118=2 rn 7. Radiowave
interferometry.
3 to 9 cm Small sea state.
8. D(; PS 1 to 2 cm Small sea state, no flow.
Table 1.1: Comparison of accuracies of sea level measuring devices
(1995); and precise datum control of sea level records from pressure tide gauges by Woodworth et al., (1996).
A study by the present author [Joseph et al., (1993)] indicated' that the field performance of a pressure transducer is likely to be significantly influenced by various site-related factors such as flow, waves and suspended sediments. The inference was that flow- and wave-elfects can be significantly reduced by the use of modified pressure inlets. The present, studies focus on these observations. The work reported here involved making measurements using a pressure transducer in order to quantify the latter's response to the following: laminar and turbu- lent water flows; progressive gravity waves; combinations of flows and waves; and suspended sediments. In these studies, different pressure inlets have been inves- tigated in order to identify the configuration most appropriate for field use. In the following chapters these experiments are described, and the results discussed on the basis of theories of fluid dynamics.
Chapter 2
APPARATUS AND EXPERIMENTAL FACILITIES
the present studies of performance enhancement of a pressure transducer
- under different .fieht situations a wide range of apparatus and experimental facili-
ties were used Iu all the experimelits, pressure was measured using a crystalline-
quartz resonator differential pressure transducer. Experiments to investigate flow effects were conducted in a circulating flow flume at the University of Liverpool,
U.K. Effects of progresSive gravity waves and combination of flows and waves were investigated in a wave flume/flow-tank faCility at the Institute of Oceanographic Sciences, Wormley, U.K. Field measurements were conducted in the sediment- laden turbulent waters of the Hugli estuary, West Bengal; a partially constrained turbid water body at Mumbai harbour; sediment-laden waters of the Humber estuary in the North Sea, U.K; and clear waters of the Zuari estuary, Goa. In this chapter brief accounts of the transducers and the data loggers used in tlic experiments, various experimental facilities, the tools used and the methodologies adopted during various field measurements are given.
2.1 Quartz pressure transducer
In the present studies, pressure was measured using a quartz differential pres- sure transducer developed by Paroscientific Inc., U.S.A under the trade name
• 'lligiquartz'. The need for precision measurements, together with the widespread use of, and continuing trend toward digital information have prompted the use
of a quartz pressure transducer in the present investigation. The accuracy of this transducer under quiescent conditions has been claimed to be comparable to primary standards (Maros, 1972) and is quoted as 0.01%.
In contrast to convertably digital.opend.00p devices such as strain gauge trans- ducer connected to an analog-to-digital converter, the inherently digital closed loop transducers designed using quartz crystal resonator possesses high Q, ex- cellent repeatability and negligible hysteresis. The value of Q is proportional to the ratio of energy stored to energy lost per cycle in the vibrating system. High Q of the resonator means that a very small source of external energy needs to be supplied to achieve and maintain the oscillations. In the present transducer, power dissipation is only a few milliwatts, and does not vary with force loading of the crystal.
2.1.1 Vibrating beam
The key sensing element in the present pressure transducer is a quartz crystal oscillating beam (Fig. 2.1) whose resonant frequency of vibration varies with applied pressure-induced loads. In operation, the crystal beam is stressed by fastening it to a structure which can transmit forces to it. For fidelity and re- peatability, the crystal resonator must be isolated from all external forces except the intended pressure-induced loads. A unique mounting isolation system (Fig.
2.2), which is an integral part of the quartz resonator, effectively decouples the fixed-fixed beam from the force producing structure. The mechanical isolation
system on both ends of the beam has a much lower resonant frequency than that of the vibrating be and, therefore, acts as a vibration isolator or low-pass me- chanical filter. The isolator masses and springs are forced into vibration by the end forces and moments of the beam but, because of the decoupled isolation, transmissibility of the beam vibrations to the mounting structure is negligible.
The entire resonator is fabricated from one piece of quartz to minimize energy losses due to joints. This mechanical isolation of the single-beam resonator from the mounting pads by means of low-pass mechanical filters on both ends of the beam allows the single-beam to oscillate in a high-frequency flexural mode with- out coupling energy to the pads. As a consequence, virtually no mechanical energy is lost, and the extremely high operating values that result contribute to the accuracy and stability of the transducer. Careful construction and mounting — an essential criterion. for vibration isolation for the flexural mode — prevents spurious resonances within the resonator body itself, thereby eliminating non-linearities in the output. Further, the quartz crystal's excellent elastic properties, long-term stability characteristics and ease of vibrational excitation make it most suitable for use as a frequency standard.
In the pressure transducer usually bellows, a diaphragm or a Bourdon-tube pressure mechanism are used to convert input pressure to an axial force applied to the crystal resonator that changes the resonant frequency of the crystal. The lower pressure range designs employ bellows, whereas the higher range designs generally employ a 13ourdon tube as the pressure-to-load converter. lti the case
INTERNAL VACUUM PIVOT
QUARTZ CRYSTAL
Figure 2.1: Single-beam quartz resonator for pressure .transducer
ISOLATOR SPRING FLEXURE RELIEF ISOLA OR MASS
I' IdIIIIIIIIIIIIIII•11111V -411111111111
■11
frI II
7,,,\\\n:..-\•...
MOUNTING SURFACE VIBRATING BEAM
( ADDITIONAL ELECTRODES DEPOSITED UNDERNEATH)
Figure 2.2: Quartz crystal resonator's mounting isolation
1
PR ESSURE INPUTFigure 2.3: Conversion of input pressure to an axial force applied to the crystal 'resonator
IN PUT FORCE
of bellows, as in the present case, the force is the product of applied pressure and the bellows' effective area. The force develops a torque about the pivot (Fig. 2.3) that is counteracted by the rigid crystal. This imposed axial compressive stress in the resonator beam decreases the resonant frequency of oscillation, providing a means of pressure measurement. Changing the size and position of the bellows relative to the lever-arm pivot allows the ratio of bellows force to crystal stress to be adjusted over a wide range. In differential-pressure transducers two bellows are placed on either ends of the lever arm, with only the difference in the two pressures hying transmitted to the crystal resonator.
•
The crystal resonator of the pressure transducer uses the phenomenon of piezo- electric drive to achieve and maintain beam oscillation. The electrodes, vacuum deposited on the beam, are used with an oscillator circuit to induce and maintain excitation and mechanical vibration. The resonant frequency of the vibrating beam increases under axial tension. Under compression the frequency decreases.
The frequency of the electrical oscillator freely follows the beam's resonant fre- quency as dictated by the applied axial load.
Although the complete quartz crystal resonator can operate at normal ambi- ent pressures and teixmperatures, the pressure transducer's performance has been significantly improved by locating its quartz crystal resonator in vacuum. With air damping effects removed under vacuum, the Q factor increases front several thousands to over 40,000. Similarly, the sensitivity to temperature is reduced
absorb on the crystal to change its frequency.
2.1.2 Signal processing
The pressure output P of the Digiquartz pressure transducers is obtained from the following:
P = A (1 — — 1b
) — B (1 — 11 ) 2
(2.1)where:
T To A, B :
period output (microsecond)
period output at zero pressure input curve fit coefficients
•
2.1.3. Specifications
Power requirements
Operational temperature range Nominal frequency excursion (Zero to full scale)
Repeatability (% of FS) Hysteresis (% of FS) Vibration sensitivity
: 6V, 0.001. Amp : -54° C to 107° C : 40 kHz to 36 kHz : 0.005%
: 0.005%
: Negligible
2.1.4 Temperature compensation
Water temperature in a field environment undergoes significant daily and seasonal variations. For this reason, temperature-compensated pressure transducers were used for field measurements.
The Digiquartz transducer's pressure output has sonic sensitivity to the ambi- ent temperature. For this reason, in temperature-compensated Digiquartz pres- sure transducers the residual thermal effects between —54° C and 107° C have been compensated for by means of a quartz crystal temperature sensor. Self-
