Zeroing absolute pressure transducers is significantly different than zeroing a gauge transducer. Absolute zeroing can be problematic because standard practices call for creating a deep vacuum on the transducer and having an accurate reference standard capable of measuring this pressure. So, what exactly is challenging about that?
Problem #1: It is impossible to reach absolute zero pressure – a total and complete vacuum.
Problem #2: Setup costs for a vacuum-based zero system are high and cause unreliability in the system.
Downward arrow symbol represents zeroing function
Because it is impossible to reach absolute zero, it is necessary to determine at what pressure the zero calibration should be performed. To complicate the issue, establishing equilibrium in a calibration system at pressures approaching absolute zero can be difficult. For a proper comparison at the chosen zero point, the pressure at the standard and the transducer should be in equilibrium. The more closely the pressure in the calibration system approaches zero, the more time it takes for the system to stabilize.
As pressure in a system approaches absolute zero, the flow of gas molecules goes through a transition. Their ability to flow within the system from one part to another is reduced as the total number of molecules decreases. As a result, the pressure in two different areas of a system may be significantly different. So, when zeroing absolute pressure transducers, the pressure at the standard may indicate a significantly different pressure than the pressure at the transducer being calibrated.
When using a vacuum reference standard or vacuum gauge as the reference, the setup can be tedious. Typically, the vacuum standard is connected to the outside of the transducer under test and both are then connected to a vacuum pump. The evacuation process for a stable vacuum requires identifying if the system is leak free by evacuating it and then monitoring the reading for leaks. Once it is established that the system is leak free, the system is vented and evacuated again with the vacuum pump, all of which can take several hours. Additionally, the setup requires a bleed valve to regulate the vacuum pressure to the desired value when zeroing. Once the system indicates the desired vacuum pressure, the reading is noted between the vacuum standard and the transducer. The system is then vented back to atmosphere, the transducer is replaced and then the complete process is repeated for a new transducer. The complete process could take several hours and a high amount of manual intervention.
Overcoming these problems
The above problems with vacuum-based absolute zeroing can be relieved through potentially two other techniques. One involves a piston gauge or a deadweight tester being used as the reference standard, while the other uses a precision barometer as the reference to set the zero.
When using a piston gauge as a reference, the minimum pressure point is limited by the weight and dimension of the piston cylinder system. Since this pressure is significantly higher than with the vacuum-based zeroing, an advantage of this technique is that pressure differentials across the system are negligible. The advantages of using this technique is the wide availability of deadweight testers in a calibration laboratory environment and the stability and accuracy guaranteed by the piston cylinder systems. It does reduce the relative time required to zero, however, it is still a manual process. It also presents a disadvantage for really low range pressure transducers that have a higher than average non-linearity. As the zero adjustment is now made at a pressure greater than zero, it presents a risk of adding an offset through the range that could negatively impact the span pressure. This scenario is detailed in the paper below.
Another technique that uses significantly less setup time and cost is taking a precision barometer and using it as a reference standard to zero absolute transducers. This technique, if applied correctly, could greatly simplify the zeroing process, but it is important to understand its limitations. Typically, precision barometers range from 0.008% to 0.02% of reading. This uncertainty is significantly larger when compared to the above two methods and this causes false acceptance of pressure deviations. This can be easily overcome in transducers with high pressure ranges where the uncertainty of the barometer is negligible in comparison to the DUT.
A 1200 kPa absolute transducer zeroed using a 0.01% of reading barometer
Zeroing can be a complex process but it doesn't need to be, at least for all the ranges of absolute transducers. The paper highlights the differences of each of these techniques and their pitfalls using a variety of absolute transducers of different ranges and accuracies to help you better understand the benefits of one technique over the other in your application.
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