NDBC TAO: Calibration Summary

Calibration Summary

Calibration Procedures and Instrumental Accuracy Estimates of TAO Temperature, Relative Humidity and Radiation Measurements

H. Paul Freitag, Yue Feng, Linda J. Mangum, Michael J. McPhaden, LT Julia Neander, and Linda D. Stratton

SUMMARY

The following generalizations and conclusions may be drawn from the above discussion.

Calibration residuals for sensors were generally equal to or larger than calibration residuals for I/O boards.
Pre/post-deployment calibration differences (drifts) were generally equal to or larger than calibration residuals.
Sensor calibration differences (drifts) were generally larger than I/O board drift.
AT and SST sensors performed as well as or better than specified by the manufacturer.
SBT sensor-board systems did not meet the specifications of the sensor manufacture. An apparent drift in the SBT board which caused this large calibration drift probably has less affect on the data itself. A modification of the calibration procedure may improve the error estimate.
While no drift specifications are published for the SWR sensor, our experience was similar to what the manufacturer has informally suggested under conditions on TAO moorings.
The RH sensor drift was 4 times larger than the manufacturer's specifications. We believe the larger than expected error was due to a combination of calibration method and environmental fouling of the sensors.

The combined effect of board and sensor drifts shown in Table 6 is computed as (board drift2 + sensor drift2)1/2, where it is assumed that board and sensor drifts are independent of one another. PROTEUS AT combined error was 80% of the ATLAS value. We speculate that the ATLAS value would approach the PROTEUS value if the ATLAS I/O board calibration coefficients were computed in a fashion similar to the PROTEUS. PROTEUS SST and RH combined errors were half those for ATLAS. Possible reasons for the larger ATLAS values include longer ATLAS deployments (both SST and RH), errors in the calibration data base (SST only), and calibration procedures (RH only).

Table 6. Combined instrumental error for each measured parameter.

CONCLUSIONS

This study quantified calibration accuracy and sensor performance on ATLAS and PROTEUS moorings of the TAO Array and in most cases found that measurement errors met the specifications of the sensor's manufacturers. In addition, this study highlighted the need for modification in calibration procedures. ATLAS air temperature I/O board calibrations, for example, no longer include a 0°C calibration point. Also, the RH sensor calibration method described above is fairly time consuming and labor intensive. PMEL technicians have found that at higher humidity values the sensor response time is much longer than that quoted by the manufacturer. A calibration over the range 20% RH to 95% RH can take an elapsed time of 1 day and only one sensor can be used per calibration chamber. In order to decrease the time required for sensor calibration and to hopefully decrease errors in the calibration procedure, we have modified humidity calibrations in two ways. First, calibrations will only be performed over the range 50% RH to 95% RH since tropical humidity rarely (if ever) is below 50% RH. Secondly, a series 2500 Humidity Generator has been purchased from Thunder Scientific Corp. of Albuquerque, New Mexico. This chamber can accommodate up to 20 sensors at once and can be monitored and controlled by an unattended computer program. A series of experiments will be conducted to more accurately determine sensor response time and the effect of the filter presence and condition.

As a result of our study, ATLAS RH data based on the previous calibration techniques have been recomputed using the manufacturer's specified coefficients as we sense that these will give better values than the 4% error indicated in Table 4. Nevertheless, we adhere to 4% as a conservative error for ATLAS RH measurements for the present, but expect that future calibrations will lower this error estimate.

ACKNOWLEDGMENTS

This work was supported by NOAA's Equatorial Pacific Ocean Climate Studies (EPOCS) Program and the U.S. TOGA Project Office. Engineering and technical support was provided by Pat McLain, Hugh Milburn, Ben Moore, Andrew Shepherd, and David Zimmerman.

REFERENCES

Hayes, S.P., L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi (1991): TOGA-TAO: A moored array for real-time measurements in the tropical Pacific Ocean. Bull. Am. Meteorol. Soc., 72, 339 - 347.

McPhaden, M.J. (1993): TOGA-TAO and the 1991 - 93 El Niño-Southern Oscillation Event. Oceanography, 6, 36 - 44.

McPhaden, M.J., H.B. Milburn, A.I. Nakamura, and A.J. Shepherd (1990): PROTEUS Profile Telemetry of Upper Ocean Currents. In: Proceedings of the Marine Tech. Soc. Conference, September 25-28, 1990, The Marine Technology Society, Washington, D.C., 353 - 357.

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