PRESSURE HYSTERESIS MEASUREMENTS ON DIGIQUARTZ BAROMETERS

Dr. Theo P. Schaad  

Dr. Richard B. Wearn, Jr.

Laboratory measurements of pressure hysteresis on a group of 23 standard production Paroscientific barometers are reported.  None of the barometers showed any measurable hysteresis when cycled over pressures from 827 to 1069 hPa (mbar).  The mean observed hysteresis was 0.001 hPa and the largest measured value was 0.0077 hPa.  All observed values are less than the estimated measurement uncertainty of 0.008 hPa.

As described in Reference 1, Paroscientific pressure transducers use a vibrating quartz beam with an output frequency which varies with applied pressure.  Reference 2 reports in detail on properties of higher pressure models of these transducers.  Reference 3 reports on the long-term stability of the Paroscientific barometers.  The present report describes the testing techniques and results of high precision measurements of hysteresis over the barometric pressure range.

Pressure hysteresis is usually measured by applying a sequence of test pressures from a lower limit to an upper test limit and then repeating the series of pressures in the decreasing pressure direction.  Typical high quality dead weight testers and other precision pressure sources for the barometric range have point-to-point non-repeatability of 0.01 to 0.05 hPa or more.  Therefore, measurement of pressure hysteresis smaller than about 0.1 hPa requires very careful experimental technique, averaging over a number of measurements, and correcting for real fluctuations in delivered test pressure.

A production build of 42 Paroscientific barometers was available for testing.  These were divided into a test group of 23 barometers and a control group of 19 barometers.  The test group was cycled repeatedly up and down through test pressures of 827, 965, 1069, 965, and 827 hPa.

The control group was held continuously at 965 hPa, and was connected to the test group by opening a valve only when the test pressure was 965 hPa.  Thus, the test group would experience hysteresis while the control group would not.  The control group could therefore be used to monitor and correct for real changes in delivered test pressure at the 965 hPa test points.

To eliminate temperature effects on the barometers, all tests were conducted in a temperature-controlled chamber.  Temperature during the tests varied by less than 0.04 degrees C, and residual temperature effects on the barometers are less than 0.001 hPa.

Test pressures were applied with a CEC Model 6-201 precision dead weight pressure standard.  This is an absolute pressure standard with weights spinning inside a vacuum bell jar.  True delivered pressure was calculated by correcting for dead weight piston temperature, bell jar back pressure, and local gravity.  At each test pressure, delivered pressure was continuously monitored with another Paroscientific barometer sampling once per second to detect weight bounce and other short term pressure fluctuations, similar to techniques used by Wearn and Paros, Reference 4.  These fluctuations were typically ± 0.005 hPa during data taking at a single test pressure.  The test group of barometers was cycled twice through the test pressures before taking data.  Data from the subsequent three cycles were used to calculate hysteresis.

For each transducer, hysteresis was calculated as the average indicated pressure at 965 hPa taken on the decreasing pressure part of the cycle minus the average indicated pressure at 965 hPa taken on the increasing pressure part of the cycle.  Corrections were made for differences in actual delivered pressure because of changes in piston temperature and bell jar back pressure.

Data from the control group of barometers indicated actual delivered pressure at 965 hPa on the various cycles varied by ±0.008 hPa.  The measurements from the control group were used for a further correction to actual delivered pressure in calculating hysteresis for the test group of barometers.  The effect of this correction was a shift in calculated hysteresis for the test barometers by 0.0034 hPa.

Individual estimates of hysteresis for each of the 23 barometers in the test group are histogrammed below.  The mean hysteresis observed was -0.0013 hPa.  The largest individual estimate obtained for any of the barometers was 0.0077 hPa.  Random errors and possible unknown systematic errors are estimated to total ±0.008 hPa, or approximately 8 ppm of full-scale barometric pressure. We therefore conclude that all of the 23 Paroscientific barometers tested have hysteresis smaller than can be measured with our present experimental errors of approximately 0.008 hPa.

This result is consistent with measurements reported by Wearn and Larson, Reference 2, for Paroscientific pressure transducers with full scale ranges of 28 bar and 62 bar.  They reported hysteresis over a 300 hPa excursion for those transducers less than or equal to 1 part per million of rated full scale pressure.