Zeroing Absolute Pressure Transducers, Transmitters and Sensors

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Zeroing absolute pressure transducers can be problematic.

Problem #1:  It is impossible to reach absolute zero pressure – a total and complete vacuum.
Problem #2:  Complete pressure equilibrium within a closed system containing relatively isolated volumes can be elusive when the system pressure approaches absolute zero.

An explanation and solutions to the problems encountered when zeroing absolute pressure transducers are discussed below.

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.

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. Robert Clayton, director of research at Mensor, wrote a white paper on this subject. The excerpt below explains this condition:

“The major reason for the low pressure non-equalization is a matter of flow. Above a certain pressure, largely determined by the geometry of a system, the flow is viscous in nature. This is referred to as the Viscous Flow Region. The mean free path of the molecules is such that the interactions of the particles play a dominant role in determining the flow.  In this region, pressures across systems equalize rapidly and this is where low [absolute] pressure measurements should be made. As the pressure continues to drop, a transition area is reached. This region is known as the Knudsen Flow Region. This area is somewhat suspect for making accurate low-pressure measurements. Below the transitional area is the Molecular Flow Region. In this region, the mean free path between molecules becomes equal to or greater than the distance to the walls of the tube.  The molecules bounce around and randomly go through the restrictions in the system. In this region, it is often impractical to ever have a system stabilize in pressure.”

The result is that 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. The solution to this dilemma is to choose an absolute pressure low enough – but not too low. The tubing diameters, materials used, and the restrictions between the standard and the transducer will have an effect on how quickly the system will equilibrate.

Solutions for zeroing absolute pressure transducers

For more detailed guidance on zeroing absolute pressure transducers, read the white paper by Robert Clayton. The paper reviews fundamental physical properties of pneumatic calibration systems, limitations associated with these, and also provides solutions to overcome these limitations.  Experimental data is reviewed, and conclusions and recommendations are drawn from the results.

Mensor recommends in all our instruction manuals that absolute zero calibration should be performed between 600 mTorr and 20% of the full scale range of the transducer.