- Email: [email protected]
An evaluation of an in-situ salinity-temperature-depth measuring system
Marine Geology - Elsevier Publishing C o m p a n y , A m s t e r d a m - Printed in The Netherlands
AN EVALUATION OF AN IN-SITU SALINITY-TEMPERATURE-DEPTH M E A S U R I N G SYSTEM M. R. HOWE AND R. I. TAIT
Oceanography Department, Liverpool University, Liverpool (Great Britain) (Received July 8, 1965)
SUMMARY
An in-situ Salinity-Temperature-Depth Measuring System, a product of the BissettBerman Corporation, San Diego, U.S.A., was evaluated during sea trials in March, 1965 by comparing its performance with data obtained from a series of standard hydrographic water-bottle casts. The system not only compared most satisfactorily as a means of measuring the absolute values, but also it provided continuous profiles of salinity and temperature in which the fine structure could be readily resolved.
INTRODUCTION
The classical hydrographic method for the in-situ measurement of salinity and temperature, although capable of high accuracy, has two major limitations, viz., the time required for measurement (several hours for a deep cast) and the necessity to interpolate between observed readings. There has long been a need for a device of comparable accuracy giving a continuous profile of salinity and temperature as a function of depth, but the technical problems involved in the construction of such an instrument are formidable. These problems are now being surmounted with the aid of sophisticated electronic techniques (BRow?4, 1962) and the performances of the new instruments are of great interest to physical oceanographers at the present time. The Oceanography Department at Liverpool University has recently acquired a Hytech Model 9006 in-situ Salinity-Temperature-Depth Measuring System (S.T.D. system) which is a product of the Bissett-Berman Corporation, San Diego, U.S.A. The Liverpool team intends to use this instrument for heat exchange studies in the surface layers of the ocean where it is planned to measure short-term changes in the salinity and temperature profiles to a depth of 500 m. The performance of the S.T.D. system was therefore tested not only to check the manufacturer's specifications for the overall accuracy, but also from the point of view of resolving small variations in the parameters under observation. The sea trials were conducted aboard the A.C.S. St. Margarets during March, 1965 in the South Biscay area. Marine Geol., 3 (1965) 483-487
484
M . R . HOWE AND R. I. TAIT
THE S.T.D. SYSTEM
The Hytech Model 9006 S.T.D. system is designed to measure salinity, temperature and pressure in situ and to record graphically on board ship salinity and temperature as a function of depth. The sea-water salinity is determined by sensing conductivity, temperature, and pressure. The conductivity is measured by means of an inductivelycoupled sensor and since conductivity is a function of temperature, pressure and salinity, electronic circuits are included to compensate for the effects of temperature and pressure and so the resultant output is dependent only on salinity. The temperature sensor incorporates a platinum resistance thermometer and the depth sensor utilizes a pressure transducer containing a strain-gauge bridge. In operation the strain-gauge bridge is in balance at zero p.s.i, and becomes increasingly unbalanced as the pressure increases. The data from the three underwater sensors are transmitted as three frequency analogues via a single conductor cable to the deck unit where the signals are amplified, filtered and fed to three demodulators which operate an X1X2Y recorder. A description of the system is given in the Hytech Products Technical Report No. 102. This instrument measures salinity from 30°/0o to 400/00 in seven ranges (selectable at the deck unit); temperature from --2°C to 35°C in ten ranges; and depth from 0 to 1,500 m in three ranges (units are available for depths to 6,000 m). The design accuracy was specified as !0.03°/00 salinity, ~0.05°C temperature and i 0 . 5 ~ of the full scale depth. The weight of the underwater unit was approximately 185 lb. and the suspension cable was a double armoured co-axial of diameter 0.3 inches, weight 143 lb. per 1,000 ft. and breaking strain in excess of 7,000"lb. The depth of operation was limited to 500 m by the length of cable that was available.
DEPTH CALIBRATION
The cable was marked at 50 ft. intervals and the underwater unit was lowered successively by this amount while the recorder and depth-frequency readings were noted. The wire angle was also observed. At maximum depth the ship was manoeuvered to maintain zero wire angle while the instrument was raised through the same 50 ft. sequence. Within the limitations of the method, the probe was found to give a correct depth reading. However, the overall experimental error (approximately 1 ~), due mainly to the uncertainty of the wire angle, was greater than the overall design accuracy of the depth sensor and from the results there was no reason to suggest that the manufacturer's calibration, which was done with a dead-weight pressure gauge, was incorrect. Therefore the depth recorded by the instrument was regarded as being more accurate than could otherwise be measured.
Marine Geol., 3 (1965) 483-487
485
IN-SITU SALINITY-TEMPERATURE-DEPTHMEASURINGSYSTEM SALINITY AND TEMPERATURECALIBRATION
Since the salinity a n d temperature gradients in the area were small, the calibration was confined to a limited range of temperature a n d salinity, b u t the absence of large gradients was also an advantage in that the overall experimental error was thereby reduced. The performances of the salinity a n d temperature sensors were evaluated by c o m p a r i n g the recorder profiles with the results of a n u m b e r of standard hydrographic water-bottle casts. The salinity samples were determined on b o a r d ship with a portable salinometer, (Hytech M o d e l 6210) based on the design described by BROWN a n d HAMON (1961). The procedure a d o p t e d was to synchronize the water-bottle casts
TABLE I A COMPARISON OF THE DATA FROM THE S.T,D. SYSTEM AND THE SAMPLING BOTTLES
Depth (m)
Temp. (°C )
Salinity (°/oo)
S.T.D.
Bottles
Difference
S.T.D.
Bottles
Difference
Cast H 60 119 298 407 515
12.98 12.95 11.68 11.34 11.24
13.01 12.96 11.70 11.39 11.26
--0.03 --0.01 - - 0.02 - - 0.05 --0.02
35.835 35.850 35.640 35.640 35.675
35.827 35.837 35.635 35.620 35.669
+ ÷ + ÷ ÷
Cast IH 59 118 278 398
13.16 12.96 11.64 11.15
13.17 12.96 11.66 11.19
--0.01 0.00 --0.02 --0.04
35.840 35.850 35.640 35.595
35.822 35.832 35.620 35.569
+ 0.018 -t- 0.018 ÷ 0.020 + 0.026
Cast I V 61 122 183 306 412 519
13.21 12.96 12.85 11.67 11.30 11.17
13.21 12.96 12.83 11.69 11.16 10.93
0.00 0.00 + 0.02 --0.02 + 0.141 + 0.241
35.825 35.850 35.830 35.630 35.640 35.730
35.815 35.839 35.816 35.619 35.574 35.601
+ + + + + +
Cast V 62 123 185 307 411 514
13.12 12.94 12.77 11.58 11.20 11.26
13.20 12.94 12.81 11.64 11.31 11.25
--0.08 0.00 --0.04 --0.06 --0.11 a q- 0.011
35.815 35.840 35.810 35.630 35.655 35.730
35.808 35.835 35.817 35.621 35.635 35.707
+ 0.007 + 0.005 --0.0071 ÷ 0.009 + 0.0201 ÷ 0.023
0.008 0.013 0.005 0.020 0.0061
0.010 0.011 0.014 0.011 0.0661 0.129t
1 Denotes depths of relatively large gradients in temperature or salinity and therefore rejected in the final evaluation.
Marine Geol., 3 (1965) 483--487
486
M.R. HOWEAND R. I. TAIT
and the S.T.D. profiles as much as possible by releasing the messenger to trigger the water-bottles when the S.T.D. probe was at mid-depth during the lowering to 500 m. A suitable winch with slip rings ~;s normally required to operate the system efficiently, but at the time such a winch was unavailable and by using existing facilities it was not possible to lower or raise the probe in less than about 12 min. The data from four such casts were used to evaluate the performance of the S.T.D. system and the results are presented in Table I. The values of salinity and temperature from the sampling bottles at the depths shown are compared with the corresponding values from the S.T.D. recorder. The differences are tabulated, but in the final calibration, the values associated with relatively large salinity or temperature gradients, as revealed by the S.T.D. profiles, were ignored, regardless of whether the discrepancies were great or small. A typical example of the profiles obtained from the S.T.D. system, together with the sampling bottle values of temperature and salinity, is shown in Fig.1. The final result of the comparison was that the mean of the temperature differences between the S.T.D. system and the reversing thermometers was --0.02°C with a standard deviation of 0.02°C, and the salinity differences were such that the S.T.D. system indicated an average value of + 0.014°/0o with a standard deviation
~#tmi
m m g m tm m m R~ #tit i mt tu m i mt
mm m
Fig.1. A typical record from the S.T.D. system showing also the salinity and temperature values from a cast of water-sampling bottles. Marine Geol., 3 (1965) 483-487
IN-SITU SALINITY--TEMPERATURE-DEPTH MEASURING SYSTEM
487
of 0.006%o. The comparison is only strictly valid if the S.T.D. probe and the waterbottles are sampling the same water mass simultaneously. In fact, due to the shipboard arrangements, the measurements were made from points about 150 ft. apart and there was probably a maximum time lag of about 6 min between the bottles and the S.T.D. probe measurements at the surface and at the greatest depth. An estimate of the errors in the calibration due to these factors was possible, however, because a total of ten S.T.D. casts were made over a period of 8 h during the calibration experiments, and an examination of successive profiles showed that changes in the salinity and temperature structure occurred slowly and were of relatively small amplitude. It was estimated that the mean errors in the calibration, due to the time and space factor, were less than -- 0.015°C and ~ 0.004% 0 in salinity, and since the comparison depths were selected for regions of smallest gradient, these errors were probably less than ~ 0.005°C and :J: 0.002°/0o .
CONCLUSION
Since the S.T.D. specifications quote an overall absolute accuracy of :t: 0.05°C and :J: 0.030/00 salinity, then the observed discrepancies of ---0.02°C and +0.014°/00 salinity can be regarded as entirely satisfactory. The S.T.D. system also demonstrated the additional advantage of providing continuous profiles in which fine structure of the order of 0.01°C in temperature and 0.005%0 in salinity can be resolved.
ACKNOWLEDGEMENTS This project is being supported by a D.S.I.R. grant. The authors are also grateful to the A.U.W.E., Portland, for the use of the A.C.S. "St. Margarets" and to the officers and crew for their assistance during the sea trials.
REFERENCES
BROWN,N. L., 1962. A proposed in-situ salinity sensing system. I.S.A. Mar. Sci. Instrum., 2 : 19-24. BROWN,N. L. and HAMON,V., 1961. An inductive salinometer. Deep-Sea Res., 8 : 65-75.
Marine Geol., 3 (1965) 483-487
AN EVALUATION OF AN IN-SITU SALINITY-TEMPERATURE-DEPTH M E A S U R I N G SYSTEM M. R. HOWE AND R. I. TAIT
Oceanography Department, Liverpool University, Liverpool (Great Britain) (Received July 8, 1965)
SUMMARY
An in-situ Salinity-Temperature-Depth Measuring System, a product of the BissettBerman Corporation, San Diego, U.S.A., was evaluated during sea trials in March, 1965 by comparing its performance with data obtained from a series of standard hydrographic water-bottle casts. The system not only compared most satisfactorily as a means of measuring the absolute values, but also it provided continuous profiles of salinity and temperature in which the fine structure could be readily resolved.
INTRODUCTION
The classical hydrographic method for the in-situ measurement of salinity and temperature, although capable of high accuracy, has two major limitations, viz., the time required for measurement (several hours for a deep cast) and the necessity to interpolate between observed readings. There has long been a need for a device of comparable accuracy giving a continuous profile of salinity and temperature as a function of depth, but the technical problems involved in the construction of such an instrument are formidable. These problems are now being surmounted with the aid of sophisticated electronic techniques (BRow?4, 1962) and the performances of the new instruments are of great interest to physical oceanographers at the present time. The Oceanography Department at Liverpool University has recently acquired a Hytech Model 9006 in-situ Salinity-Temperature-Depth Measuring System (S.T.D. system) which is a product of the Bissett-Berman Corporation, San Diego, U.S.A. The Liverpool team intends to use this instrument for heat exchange studies in the surface layers of the ocean where it is planned to measure short-term changes in the salinity and temperature profiles to a depth of 500 m. The performance of the S.T.D. system was therefore tested not only to check the manufacturer's specifications for the overall accuracy, but also from the point of view of resolving small variations in the parameters under observation. The sea trials were conducted aboard the A.C.S. St. Margarets during March, 1965 in the South Biscay area. Marine Geol., 3 (1965) 483-487
484
M . R . HOWE AND R. I. TAIT
THE S.T.D. SYSTEM
The Hytech Model 9006 S.T.D. system is designed to measure salinity, temperature and pressure in situ and to record graphically on board ship salinity and temperature as a function of depth. The sea-water salinity is determined by sensing conductivity, temperature, and pressure. The conductivity is measured by means of an inductivelycoupled sensor and since conductivity is a function of temperature, pressure and salinity, electronic circuits are included to compensate for the effects of temperature and pressure and so the resultant output is dependent only on salinity. The temperature sensor incorporates a platinum resistance thermometer and the depth sensor utilizes a pressure transducer containing a strain-gauge bridge. In operation the strain-gauge bridge is in balance at zero p.s.i, and becomes increasingly unbalanced as the pressure increases. The data from the three underwater sensors are transmitted as three frequency analogues via a single conductor cable to the deck unit where the signals are amplified, filtered and fed to three demodulators which operate an X1X2Y recorder. A description of the system is given in the Hytech Products Technical Report No. 102. This instrument measures salinity from 30°/0o to 400/00 in seven ranges (selectable at the deck unit); temperature from --2°C to 35°C in ten ranges; and depth from 0 to 1,500 m in three ranges (units are available for depths to 6,000 m). The design accuracy was specified as !0.03°/00 salinity, ~0.05°C temperature and i 0 . 5 ~ of the full scale depth. The weight of the underwater unit was approximately 185 lb. and the suspension cable was a double armoured co-axial of diameter 0.3 inches, weight 143 lb. per 1,000 ft. and breaking strain in excess of 7,000"lb. The depth of operation was limited to 500 m by the length of cable that was available.
DEPTH CALIBRATION
The cable was marked at 50 ft. intervals and the underwater unit was lowered successively by this amount while the recorder and depth-frequency readings were noted. The wire angle was also observed. At maximum depth the ship was manoeuvered to maintain zero wire angle while the instrument was raised through the same 50 ft. sequence. Within the limitations of the method, the probe was found to give a correct depth reading. However, the overall experimental error (approximately 1 ~), due mainly to the uncertainty of the wire angle, was greater than the overall design accuracy of the depth sensor and from the results there was no reason to suggest that the manufacturer's calibration, which was done with a dead-weight pressure gauge, was incorrect. Therefore the depth recorded by the instrument was regarded as being more accurate than could otherwise be measured.
Marine Geol., 3 (1965) 483-487
485
IN-SITU SALINITY-TEMPERATURE-DEPTHMEASURINGSYSTEM SALINITY AND TEMPERATURECALIBRATION
Since the salinity a n d temperature gradients in the area were small, the calibration was confined to a limited range of temperature a n d salinity, b u t the absence of large gradients was also an advantage in that the overall experimental error was thereby reduced. The performances of the salinity a n d temperature sensors were evaluated by c o m p a r i n g the recorder profiles with the results of a n u m b e r of standard hydrographic water-bottle casts. The salinity samples were determined on b o a r d ship with a portable salinometer, (Hytech M o d e l 6210) based on the design described by BROWN a n d HAMON (1961). The procedure a d o p t e d was to synchronize the water-bottle casts
TABLE I A COMPARISON OF THE DATA FROM THE S.T,D. SYSTEM AND THE SAMPLING BOTTLES
Depth (m)
Temp. (°C )
Salinity (°/oo)
S.T.D.
Bottles
Difference
S.T.D.
Bottles
Difference
Cast H 60 119 298 407 515
12.98 12.95 11.68 11.34 11.24
13.01 12.96 11.70 11.39 11.26
--0.03 --0.01 - - 0.02 - - 0.05 --0.02
35.835 35.850 35.640 35.640 35.675
35.827 35.837 35.635 35.620 35.669
+ ÷ + ÷ ÷
Cast IH 59 118 278 398
13.16 12.96 11.64 11.15
13.17 12.96 11.66 11.19
--0.01 0.00 --0.02 --0.04
35.840 35.850 35.640 35.595
35.822 35.832 35.620 35.569
+ 0.018 -t- 0.018 ÷ 0.020 + 0.026
Cast I V 61 122 183 306 412 519
13.21 12.96 12.85 11.67 11.30 11.17
13.21 12.96 12.83 11.69 11.16 10.93
0.00 0.00 + 0.02 --0.02 + 0.141 + 0.241
35.825 35.850 35.830 35.630 35.640 35.730
35.815 35.839 35.816 35.619 35.574 35.601
+ + + + + +
Cast V 62 123 185 307 411 514
13.12 12.94 12.77 11.58 11.20 11.26
13.20 12.94 12.81 11.64 11.31 11.25
--0.08 0.00 --0.04 --0.06 --0.11 a q- 0.011
35.815 35.840 35.810 35.630 35.655 35.730
35.808 35.835 35.817 35.621 35.635 35.707
+ 0.007 + 0.005 --0.0071 ÷ 0.009 + 0.0201 ÷ 0.023
0.008 0.013 0.005 0.020 0.0061
0.010 0.011 0.014 0.011 0.0661 0.129t
1 Denotes depths of relatively large gradients in temperature or salinity and therefore rejected in the final evaluation.
Marine Geol., 3 (1965) 483--487
486
M.R. HOWEAND R. I. TAIT
and the S.T.D. profiles as much as possible by releasing the messenger to trigger the water-bottles when the S.T.D. probe was at mid-depth during the lowering to 500 m. A suitable winch with slip rings ~;s normally required to operate the system efficiently, but at the time such a winch was unavailable and by using existing facilities it was not possible to lower or raise the probe in less than about 12 min. The data from four such casts were used to evaluate the performance of the S.T.D. system and the results are presented in Table I. The values of salinity and temperature from the sampling bottles at the depths shown are compared with the corresponding values from the S.T.D. recorder. The differences are tabulated, but in the final calibration, the values associated with relatively large salinity or temperature gradients, as revealed by the S.T.D. profiles, were ignored, regardless of whether the discrepancies were great or small. A typical example of the profiles obtained from the S.T.D. system, together with the sampling bottle values of temperature and salinity, is shown in Fig.1. The final result of the comparison was that the mean of the temperature differences between the S.T.D. system and the reversing thermometers was --0.02°C with a standard deviation of 0.02°C, and the salinity differences were such that the S.T.D. system indicated an average value of + 0.014°/0o with a standard deviation
~#tmi
m m g m tm m m R~ #tit i mt tu m i mt
mm m
Fig.1. A typical record from the S.T.D. system showing also the salinity and temperature values from a cast of water-sampling bottles. Marine Geol., 3 (1965) 483-487
IN-SITU SALINITY--TEMPERATURE-DEPTH MEASURING SYSTEM
487
of 0.006%o. The comparison is only strictly valid if the S.T.D. probe and the waterbottles are sampling the same water mass simultaneously. In fact, due to the shipboard arrangements, the measurements were made from points about 150 ft. apart and there was probably a maximum time lag of about 6 min between the bottles and the S.T.D. probe measurements at the surface and at the greatest depth. An estimate of the errors in the calibration due to these factors was possible, however, because a total of ten S.T.D. casts were made over a period of 8 h during the calibration experiments, and an examination of successive profiles showed that changes in the salinity and temperature structure occurred slowly and were of relatively small amplitude. It was estimated that the mean errors in the calibration, due to the time and space factor, were less than -- 0.015°C and ~ 0.004% 0 in salinity, and since the comparison depths were selected for regions of smallest gradient, these errors were probably less than ~ 0.005°C and :J: 0.002°/0o .
CONCLUSION
Since the S.T.D. specifications quote an overall absolute accuracy of :t: 0.05°C and :J: 0.030/00 salinity, then the observed discrepancies of ---0.02°C and +0.014°/00 salinity can be regarded as entirely satisfactory. The S.T.D. system also demonstrated the additional advantage of providing continuous profiles in which fine structure of the order of 0.01°C in temperature and 0.005%0 in salinity can be resolved.
ACKNOWLEDGEMENTS This project is being supported by a D.S.I.R. grant. The authors are also grateful to the A.U.W.E., Portland, for the use of the A.C.S. "St. Margarets" and to the officers and crew for their assistance during the sea trials.
REFERENCES
BROWN,N. L., 1962. A proposed in-situ salinity sensing system. I.S.A. Mar. Sci. Instrum., 2 : 19-24. BROWN,N. L. and HAMON,V., 1961. An inductive salinometer. Deep-Sea Res., 8 : 65-75.
Marine Geol., 3 (1965) 483-487