Page 1 Setra 470 Pressure Transducer Tests
INSTRUMENT TEST REPORT NUMBER 622
Setra 470 Pressure Transducer Compliance Tests for A.W.S. Applications
Kent Gregory Position : PO2
Physics Laboratory, OEB 10th September, 1992
Jane Warne Senior Physicist Instruments and Laboratory for Director of Meteorology
24 pages including 2 appendices
TABLE OF CONTENTS
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1. INTRODUCTION 3 2. EXPERIMENTAL METHOD 4 2.1 Pressure Calibration Method 4 2.2 Sustained Temperature Test Method 5 2.3 Variable Temperature Test Method 5 3. RESULTS 6 3.1 Accuracy 6 3.2 Error Due to Non-Linearity 7 3.3 Error Due to Hysteresis 10 3.4 Response to Sustained Temperatures 11 3.5 Effect of Temperature on Accuracy 12 4. DISCUSSION 14 4.1 Accuracy at Room Temperature 14 4.2 Non-Linearity at Room Temperature 14 4.3 Hysteresis at Room Temperature 14 4.4 Accuracy at Various Temperatures 15 4.5 Stability with Time 15 5. RECOMMENDATIONS 16 6. REFERENCES 16 Appendix A 17 Appendix B 20 1. INTRODUCTION.
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The Physical Laboratories performed several tests on a Setra Pressure Transducer model 470 serial number 254649 (herein referred to as the Setra 470) to determine its suitability for use in an A.W.S. and to check its compliance to Bureau of Meteorology and factory specifications. The Setra 470 requires 5V DC at 90 mA to operate. Pressure readings are output digitally via a serial port. The claimed specifications  of the Setra 470 are presented in Table 1. Figures relating to thermal effects have been converted from the Fahrenheit scale to the Celsius scale and are valid for the 800 hPa to 1100 hPa pressure range. Setra Systems Inc. define Absolute Accuracy as the root sum square (R.S.S.) of non-linearity, hysteresis and non-repeatability but does not include errors due to temperature variability or instrument ageing. From correspondence with Setra Systems Inc. in Australia and the United States, the implied confidence interval used to derive the figures in Table 1 is 99% . The thermal effects quoted in Table 1 are valid for the range -1C to 43C .
The uncertainty bounds quoted in Table 1 have been broadened to allow for uncertainty due to the reference barometer. These uncertainties appear in the middle column of Table 1. A R.S.S. of the reference barometer total uncertainty (0.08 hPa) and the original Setra 470 absolute accuracy specification ( 0.06 hPa) yield the experimental specification ( 0.10 hPa). Also, the non-repeatability of the reference barometer during the term of the experiment was 0.06 hPa. This was also incorporated into Table 1 as per the absolute accuracy specification.
Characteristic Uncertainty due to Reference
Setra 470 Uncertainty
Absolute Accuracy 0.08 hPa 0.06 hPa 0.10 hPa at 21C Non-linearity - 0.04 hPa 0.04 hPa
Hysteresis - +0.03 hPa +0.03 hPa Non-repeatability 0.06 hPa 0.03 hPa 0.07 hPa Thermal zero shift - 0.0033 hPa/C 0.0033 hPa/C
Thermal sensitivity shift - 0.0016 hPa/C 0.0016 hPa/C Stability - < 0.15 hPa for 1
year < 0.15 hPa for 1 year
Table 1. Specifications for Setra 470 .
The reference barometer has no hysteresis uncertainty, due to the measurement method. Any temperature or non-linearity characteristics of the reference barometer are removed after each measurement. 2. EXPERIMENTAL METHOD.
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The sequence of tests used to check the specifications of the Setra 470 is given in Table 2.
Test Test Type Temperature (C) Pressure Range(hPa) Start Date End Date (a) Pressure
Calibration 24.5 0.2 800 to 1050 7/3/91 7/3/91
(b) Pressure Calibration
25 0.2 800 to 1050 13/3/91 13/3/91
(c) Sustained Temperature
40 1002 to 1017 14/3/91 26/3/91
(d) Pressure Calibration
40 1 800 to 1050 28/3/91 28/3/91
(e) Sustained Temperature
23 993 to 1025 1/4/91 3/5/91
(f) Pressure Calibration
22.6 800 to 1050 13/5/91 13/5/91
(g) Variable Temperature
0 to 50 1011 7/6/91 12/6/91
Table 2. The sequence of tests performed on the Setra 470. The Reference Barometer was a Mechanism Ltd. digital aneroid barometer (serial number DA 670). It had a resolution of 0.01 hPa and an absolute accuracy of 0.08 hPa. This instrument is traceable to the W.M.O. Region V pressure standard (maintained by the Bureau of Meteorology) through direct comparisons at weekly intervals. The temperature of the air surrounding the reference barometer was measured every time a pressure measurement was made. Correction tables, based on pressure measured and air temperature, were used to correct the reference barometer readings for non-linearity and temperature effects. The pressure regulator was a Bell and Howell Pressure Volume Regulator (serial number 2491). The oven used to heat the Setra 470 was a Laboro (serial number CA1997) and a Muller McQuay refrigerator was used to cool the Setra 470 to temperatures below 25C. 2.1 Pressure Calibration Method. The purpose of the pressure calibrations was to determine the absolute accuracy of the Setra 470 against the W.M.O. Region V pressure standard. Tests (a) and (b) were used to determine the hysteresis, non-repeatability and non-linearity uncertainties of the Setra 470. Tests (d) and (f) were carried out during and after the Setra 470 had been subjected to sustained high temperatures, to determine if any change in the Setra 470 performance occurred.
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The pressure regulator was connected via a 'T' piece to the reference barometer and the Setra 470 by lengths (less than 500 mm) of rubber tubing. The reference barometer always remained at room temperature. Using the Bell and Howell pressure regulator, the pressure in the line was decreased to 800 hPa. Both the Setra 470 and the reference barometer were allowed to stabilise for 60 seconds. After this time, the pressure indicated on the Setra 470 and the reference barometer were recorded. The pressure regulator was used to increase the pressure by 20 hPa. Again the sensors were allowed 60 seconds to stabilise and readings were taken. This continued up to a pressure of 1050 hPa. The process continued in the same manner, but the pressure was decreased in steps of 20 hPa. This test method was used in Tests (a), (b), (d) and (f). 2.2 Sustained Temperature Test Method. The purpose of Test (c) was to accelerate the ageing of the Setra 470 and to check its pressure response at elevated temperatures. Test (e) was designed to monitor how the Setra 470 recovered after being maintained at the elevated temperatures in Test (c). The Setra 470, the reference barometer and the ambient pressure of the laboratory were connected by lengths of tubing (less than 500 mm). The temperature of the Setra 470 was held within 1C of the desired test temperature for the term of the experiment. The reference barometer always remained at room temperature. At 9 am and 4 pm a pair of readings from the reference barometer and the Setra 470 were taken. This test method applies to Tests (c) and (e). 2.3 Variable Temperature Test Method. A.W.S. pressure sensors can be exposed to a variety of temperatures every day. Test (g) was designed to examine the Setra 470's performance at various temperatures to check the claimed specifications regarding temperature effects. The Setra 470 was placed in an oven held at room temperature. The Setra 470 was allowed 25 minutes to stabilise at this temperature. A reading from the Setra 470 and the reference barometer was taken. The oven temperature was increased by 5C and the Setra 470 was allowed another 25 minutes before readings were again taken. These steps were repeated until the oven reached a temperature of 50C. The Setra 470 was removed from the oven and allowed to cool to room temperature for 60 minutes. The Setra 470 was then placed in a refrigerator held at room
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temperature. The temperature of the refrigerator was lowered by 5C. The Setra 470 was allowed 30 minutes to come to thermal equilibrium inside the refrigerator. Readings were taken from the Setra 470 and the reference barometer in the same way as readings were taken in the oven. The temperature was lowered in steps of 5C and the process repeated to a temperature of 0C. This test method applies to Test (g).
The data from the tests is tabulated in Appendix B. Statistical proofs referenced in this section can be found in Appendix A. The difference between the Setra 470 and the reference barometer is expressed as a correction to 99% confidence. That is, the reference barometer reading minus the Setra 470 reading.
Since the confidence interval used by Setra Systems, Inc. is assumed to be 99% , all calculations and statistical proofs have been based on this confidence interval. 99% confidence intervals are also referred to as the random component of the total uncertainty. A correction cannot be applied to reduce this type of uncertainty. It describes the scatter of the data. Bias errors, in the context of this report, are the mean of the corrections. The bias is the mean diff