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Antony Joseph
and
Ehrlich Desa

Abstract

Speed and direction performances of flowmeters, designed by the authors for in-house use, employing an Aanderaa-type curved-bladed Savonius rotor and a free vane and an Aanderaa-type flat-bladed Savonius rotor and a fixed vane, are discussed. It has been observed that accuracy, linearity, and tilt response of a meter using the Aanderaa curved-bladed rotor is superior to those of a meter using the Aanderaa flat-bladed rotor. Analysis showed that the azimuth response of a flowmeter is affected by the presence of support rods surrounding its rotor. The change in azimuth response arises from flow pattern modifications in the vicinity of the rotor, imposed by the changes in the horizontal angle of the support rods of the rotor relative to the flow streamlines. While the use of two support rods may be suitable for a fixed-vane system, it is undesirable for a free-vane system where the meter's orientation with respect to the flow direction is not defined. Flow direction calibration results indicated that a fixed-vane system exhibits superior direction performance compared to a free-vane system. The comparatively poor direction performance of the free-vane system stems from the poor coupling to the “vane-follower” magnet from the external vane.

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Vijay Kumar
,
Antony Joseph
,
R. G. Prabhudesai
,
S. Prabhudesai
,
Surekha Nagvekar
, and
Vimala Damodaran

Abstract

Simultaneous calibrations of three temperature-compensated piezoresistive ruggedized precision “absolute” pressure transducers (Honeywell model PPTR0040AP5VB-BD), which have been designed specially for long-term coastal oceanographic and limnological measurements, have been carried out at four differing temperatures (10°, 20°, 30°, and 40°C) to evaluate their suitability for such applications. The full-scale pressure range of these shallow water absolute pressure sensors is ≈ 2800 hPa (equivalent to water depth of ≈ 18 m). Measurement results have been used to examine the transducers’ performance indicators, such as zero-point offset, accuracy, linearity, hysteresis, temperature sensitivity, and slope. Differing piezoresistive ruggedized precision absolute pressure transducers (PPTRs) exhibited differing zero-point offset values, ranging from 2 to −79 hPa. Temperature sensitivity of zero-point offset was ≈0.3 hPa over the temperature range 10°–40°C. The mean hysteresis over the full-scale absolute pressure range (≈2800 hPa) varied from approximately 2 to 8 hPa over the temperature range 10°–40°C. The slope of the least squares–fitted linear graph (taking the mean of ascending and descending pressures) was close to the ideal value of unity (deviation from 1 over the temperature range 10°–40°C was in the range of −0.001 to +0.005). Linearity was excellent, its mean over the entire pressure range being between ≈ −0.006% and 0.008% of full-scale (FS) over the above temperature range. The worst performance was exhibited at input pressures below ≈1500 hPa. Zero-point offset has played a significant role in deteriorating the accuracy of the PPTR, the mean accuracy (within ≈0.1% and −5%) having been exhibited by those transducers having offsets of 2 and −79 hPa, respectively. The mean accuracy exhibited temperature sensitivity of ≈1% in the range 10°–20°C and negligible sensitivity beyond 20°C. Use of a calibration equation significantly improved the mean static accuracy obtainable from the PPTR, to between −0.04% and 0.01% of FS. Evaluation results have indicated that a suitably calibrated temperature-compensated Honeywell PPTR provides an alternate cost-effective means for pressure measurements for coastal oceanographic and limnological studies.

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Antony Joseph
,
J. A. Erwin Desa
,
Peter Foden
,
Kevin Taylor
,
Jim McKeown
, and
Ehrlich Desa

Abstract

The performance of a pressure transducer, with its inlet attached to differing hydromechanical front ends, has been evaluated in flow flume and wave flume experiments in which laminar and turbulent flows, and regular progressive gravity waves and combinations of flows and waves, were generated. For steady laminar flows, and for waves propagating on quiescent waters, the transducer’s performance improved when the inlet was at the center and flush with a large, thin, and smooth circular horizontal end plate. This enhancement is likely to have been achieved because of the isolation of the pressure inlet from the separated flows and vortices generated by the transducer housing. Flow disturbances, generated by nearby solid structures, deteriorated the performance of the pressure transducer. However, its performance could be significantly improved by protecting the pressure inlet by a sturdy, curved perforated shield. The dynamic pressure error in this case was 2 mb at 100 cm s−1, compared to 8 mb in the absence of the shield. For turbulent flows less than 100 cm s−1, a pair of thin, circular, parallel plates, with a diameter three to four times that of the transducer housing and separation equal to the housing diameter, led to a much improved horizontal azimuthal response. At this speed the spread in the dynamic pressure, ΔP, was less than 1 mb compared to 6 mb without a plate. Beyond this speed the transducer’s horizontal azimuthal response deteriorated faster. For combinations of waves and flows a relatively small ΔP was found. This result is of special significance to tidal measurements of coastal waters, in which waves propagate on tidal currents.

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Antony Joseph
,
Ehrlich Desa
,
Elgar Desa
,
David Smith
,
Vani B. Peshwe
,
Vijaykumar
, and
J. A. Erwin Desa

Abstract

Pressure measurements made in two turbid natural waters have led to the inference that the effective depth-mean in situ density values, ρ eff, of these waters are less than (≈2.70%–6.5%) their bulk densities (i.e., densities of water–sediment mixture), and also less than (≈0.4%–4.5%) that of the density of the same water after removal of suspended sediment. The values of ρ eff in a given site differed from one tidal cycle to another (≈1.9%). These values varied slightly (<0.8%) from midtide to slack water period of the same tidal cycle, with ρ eff being lower at midtide. It was found that the use of bulk density to estimate tidal elevation yielded an underestimation of tidal range (up to 7%). The underestimation has been corrected (to within ±1.5%) with the use of ρ eff parameter. For clear waters there was no measurable underestimation in tidal range. The observations indicate an apparent in situ density reduction for turbid natural waters. With the use of two pressure transducers at a known vertical separation, the value of ρ eff over this vertical column of water may be determined during each sampling of pressure values. The present studies indicate that when pressure transducers are used for water level measurements in turbid natural waters, the use of ρ eff, in contrast to the bulk density, significantly improves the measurement accuracy. For clear waters, precision density measurements made on discrete water samples agreed with ρ eff values derived from pressure measurements to better than ±0.4%. Thus, use of ρ eff is expected to improve the accuracy of water level measurements also from clear water estuaries where depth-mean water density undergoes marginal changes with differing phases of tide and significant changes with seasons.

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