What Is Pressure Gauge Hysteresis And How To Detect It
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Pressure gauges are critical tools for monitoring and controlling fluid pressure. However, no matter how good the quality of the pressure gauge is, there may still be "hysteresis", which will affect the accuracy of the measurement. Therefore, this article will delve into the nature, causes and effects of pressure gauge hysteresis and how to effectively detect it to help users obtain more accurate pressure gauge readings.
Pressure gauge hysteresis, simply put, refers to the phenomenon that when the measured pressure goes through a complete cycle of first rising and then falling (or first falling and then rising), for the same actual pressure point, the reading displayed by the pressure gauge during the pressure rising process is inconsistent with the reading displayed during the pressure falling process. Specifically, when the pressure increases and the pointer reaches the pressure value corresponding to a certain scale, the actual pressure may be slightly higher than the theoretical value; when the pressure decreases and the pointer drops back to the same scale, the actual pressure at this time may be slightly lower than the theoretical value. This difference between the "forward" and "return" readings is the hysteresis error of the pressure gauge.
Causes of pressure gauge hysteresis
The hysteresis phenomenon of the pressure gauge is mainly caused by the inherent elastic hysteresis characteristics of the elastic sensitive element. The core of the pressure gauge is a pressure-sensing element, such as a common C-type or spiral Bourdon tube, diaphragm or bellows, which rely on its own elastic deformation to respond to pressure changes. An ideal elastomer, after being deformed by force, should be able to completely and instantaneously return to its original shape when the external force is removed. However, it will not actually return to the exact same state as it was originally. Because during the process of change, it will be resisted by its own internal structure, causing the amount of its retraction to be inconsistent with the amount of compression, which is the phenomenon of "elastic hysteresis".
The existence of pressure gauge hysteresis will directly affect the accuracy and consistency of measurement data. Because at the same actual pressure value, the gauge needle may point to one reading during the pressure increase process, but stop at another slightly deviated position during the pressure reduction process, causing the user to be unable to accurately judge the current real pressure. This situation of "one pressure value corresponding to two readings" destroys the single value correspondence relationship that the instrument should have, thereby causing measurement uncertainty.
This effect is particularly obvious in usage scenarios that require high measurement accuracy. For example, in chemical production, the pressure control in the reactor needs to be accurately maintained within a certain range. If the pressure gauge lags, the operator may misjudge the current pressure state and adjust the air intake or exhaust rate incorrectly, which may affect the reaction efficiency at best, or cause incomplete reaction or even safety hazards.
In addition, in some scenarios that require real-time monitoring of pressure fluctuation trends, such as pulsating pressure analysis in test equipment, differential pressure monitoring in filtration systems, etc., hysteresis will cause the pointer of the pressure gauge to "not keep up" with the rapid changes in the real pressure, resulting in delayed judgment. For example, the actual pressure has begun to decrease, but the needle remains high; or the pressure is rising rapidly, but the needle lags behind. This will cause operators to mistakenly believe that the system is still in a safe or stable state, thereby missing the opportunity for timely adjustment.
Detecting pressure gauge hysteresis is usually done during the pressure gauge calibration process. The standard testing method requires the use of a standard pressure source (such as a piston pressure gauge or a high-precision digital pressure calibrator) that is much more accurate than the pressure gauge being measured and a system that can pressurize smoothly.
The testing steps are roughly as follows: First, slowly and evenly increase the pressure from zero point (or lower limit of scale) to full scale (or upper limit of scale), and record the readings of the pressure gauge after stabilizing at several pre-selected calibration points (such as 0%, 25%, 50%, 75%, 100% of the scale), which constitutes the "upstroke" or "boost" data. Then, the pressure is slowly and evenly dropped from full scale to zero, and the pressure gauge reading is recorded again after stabilizing at the same calibration point as during the rise. This constitutes "downstroke" or "decompression" data. By comparing the reading difference between the upstroke and downstroke at the same calibration point, the hysteresis error at that point can be obtained. The maximum absolute value of the hysteresis error at each calibration point within the entire range is usually defined as the hysteresis of the pressure gauge, and is often expressed as a percentage of the full scale. This testing process must be carried out in strict accordance with relevant metrological verification procedures to ensure the accuracy and comparability of the results.
