Why Absolute Pressure Transducer is More Prone to Zero Drift

It is well known that pressure transducers can be classified into absolute pressure transducers, gauge pressure transducers and differential pressure transducers depending on the type of measured pressure. In this paper, the absolute pressure transducer is compared with the gauge pressure and differential pressure transducer. The reasons for the zero drift of the absolute pressure transducer are analyzed from the aspects of principle, practical application and operation.

• Overview of the zero drift in the absolute pressure transducer

Structurally, the diaphragm of the gauge pressure transducer and the low pressure chamber in the differential pressure transducer are in direct contact with the atmosphere. The pressure value at atmospheric Environment is its zero pressure. In the atmospheric environment, the theoretical output current value (referred to as the theoretical output value) of the 4-20 mA gauge pressure transducer or differential pressure transducer is 4 mA. If this type of transducer has a zero drift during the calibration process, the adjustment tool can be used to directly adjust the zero point in the atmosphere. For an absolute pressure transducer with 4-20 mA output signal, it will only have a theoretical output value of 4 mA under vacuum. We all know that a complete vacuum is impossible under realistic conditions. Therefore, in the actual calibration process, when adjusting the zero point of the absolute pressure transducer, the influence of the system vacuum on its zero point should be considered.

• Reasons for the zero drift of absolute pressure transducer

According to practical experience, absolute pressure transducers are more prone to zero drift than gauge and differential pressure transducers. Even for some transducers, its output value are out of tolerance in the periodic verification. For example, when users perform annual cycle inspections on a series of pressure transducers, they find that the cycle stability of the series of gauge and differential pressure transducers is better. The zero drift is small, and the measurement error and backhaul error are within the allowable range. In contrast, the series of absolute pressure transducers often have serious zero drift and large deviations. Analysis for the reasons, mainly the following points.

1. Measuring principle and structural reasons of absolute pressure transducer

When the pressure transducer measuring the pressure, the medium pressure is transmitted to the measuring diaphragm through the isolation diaphragm at high and low sides and filling oil. The measuring diaphragm at the center of the measuring chamber. Using the direct digital circuit, the position of the measuring diaphragm is detected through the fixed plate on both sides. As shown in following figure.

For gauge and differential pressure transducers, if the atmospheric pressure is applied to the low pressure side and the sensing diaphragm at the same time, the diaphragm has a symmetrical structure at the high pressure side without pressure. It makes it easy to achieve pressure balance. However, for the absolute pressure transducer, the low pressure side always maintains a reference pressure (that is, it is enclosed in a high vacuum reference chamber). Thus, the sensor diaphragm has an asymmetric structure at both ends. That is to say, regardless of the reference cavity leakage or material deformation and other factors will lead to zero point change, that is, zero drift of absolute pressure transducer.

2. The inference of the on-site environment on absolute pressure transducers

The stability of the absolute pressure transducer is greatly affected by temperature. Take the application of the absolute pressure transducer in the nuclear plant as an example. For pressure transducers installed in cooling water pipes, the temperature of the contact medium is often as high as 100 ℃. When these absolute pressure transducers are operating under high temperature and vacuum conditions, the isolation diaphragm in the cavity is drummed due to the vacuum. Then the pressure in the sealing system reduces and the viscosity of the filling liquid also reduces. It leads to volatile enhancement for silicone oil, lower boiling point and enhanced thermal expansion. Part of the silicone oil appears to be vaporized and volume-expanded, which deforms the isolating diaphragm and causes zero drift.

3. Calibration for absolute pressure transducer is incorrect

When the user periodically calibrating the absolute pressure transducer, the indication value is often out of tolerance. The reason is that the user often makes adjustment errors when checking the absolute pressure transducer. For example, take an absolute pressure transducer with range of 0-10 MPa and output signal of 4-20 mA as an example. In the vacuum state (ie, the very large vacuum degree that the metering device can achieve), its actual output current value (referred to as the actual output value) is smaller than the theoretical output value. The analysis found that the theoretical output of the 0-10 MPa range absolute pressure transducer at atmospheric pressure (standard atmospheric pressure is 101.321 kPa) is 4.162 mA, which is close to 4 mA. At this time, many users will mistakenly treat these absolute pressure transducers as gauge pressure transducers. It is directly cleared in the atmospheric pressure environment. It causes the actual output of the absolute pressure transducer to be lower than the theoretical output value under vacuum status.
Some users often make incorrect adjustments to the full scale of the absolute pressure transducer relative to the misalignment for the zero adjustment. Take an absolute pressure transducer with range of 0-100 kPa and output signal of 4-20 mA as an example. Its theoretical output at 100 kPa absolute should be 20 mA. If the local atmospheric pressure is 102 kPa, the theoretical output current should be 20.32 mA. When the user finds that the output current of these absolute pressure transducers is higher than 20 mA, the pressure transducer will be erroneously adjusted to full-scale current at atmospheric pressure. It causes the range of the absolute pressure transducer to expand and it also affects its zero current.