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7 Reasons Causing Differential Pressure Sensor Measurement Error

The differential pressure sensor is widely used in industrial applications. It is prone to inaccurate measurement and error during operation. What is the cause? This paper will analyze the error of the differential pressure sensor.

1. Differential pressure sensor zero adjustment is inaccurate causing error

The instrument error generated in installation can be eliminated by the zero adjustment method. The pressure sensor can be zero-checked at the production site using the handheld communicator or directly zeroing on the meter body. Once the zero adjustment is unreasonable, the pressure indication value will not match the actual value. If the pressure sensor indication is not corrected, it will lead to a large error.

2. The unreasonable design and installation of the differential pressure sensor pressure’s piping causing error

The pressure sensor must be equipped with a pressure line when measuring pressure. Improper operation during installation will cause measurement errors to differential pressure sensors, as follows. The pressure tapping point in the pressure tube of differential pressure sensor is completely perpendicular to the container wall during installation. The pressure tube is set too long or without inclination. The instrument and the pressure piping are not tightly sealed, causing pressure delay, resulting in static pressure imbalance and additional error. At the same time, the medium in the pressure pipe of the pressure sensor contains gas or liquid, which causes the variation of the medium density, then causing deviation of the measured value.
In general, when measuring the liquid phase pressure, the pressure port is generally installed at the lower part of the pipeline or equipment. It prevents bubbles that are precipitated in the medium from entering the pressure tube, causing slow measurement. Yet it should not be installed at the bottom to prevent the sediment from clogging the pressure port. In areas with low ambient temperatures, antifreeze equipment should be installed in the middle of the pressure piping to prevent the liquid medium such as crude oil from condensing the pressure tube, which will cause pressure inaccuracy.

3. Long-term pressure fluctuations in the fluid cause damage to the measurement hardware

Under the condition of pressure rise and fall frequently, the isolation diaphragm and the measurement diaphragm in the differential pressure sensor are subjected to the alternating stress generated by the pressure difference, so that the displacement Ad of the measurement diaphragm exceeds the allowable range. Especially in the case where the actual pressure value is larger than the maximum value of the pressure sensor range, the measurement diaphragm is subjected to stress exceeding the yielding pole of the measuring diaphragm. This inevitably causes deformation or even breakage, causing the differential pressure sensor to drift, and then causing the measured pressure value to be inaccurate or machine strike.

4. Effect of ambient temperature on differential pressure sensor

The set range set of the differential pressure sensor is based on the constant density of the measured medium. For some liquids whose density changes with temperature, when the temperature difference between the ambient temperature and the measured point is large, or when the medium in the container contains liquid or gas phase components, the liquid density in the pressure tube will change greatly. At this time, the indication value reflected by the secondary meter and the actual value will have a large error.

5. Unreasonable power line layout and interference will cause differential pressure sensor error

The measuring component of the differential pressure sensor utilizes the principle of the parallel plate capacitor. The electrode plate is made into a curved shape in consideration of some factors such as deformation. However, the influence of the fringe electric field is not considered, resulting in uneven distribution of the generated electric field, leading the displacement generated by the diaphragm and the change in capacitance deviating from the approximate linear relationship. Unreasonable laying of the power line (such as no grounding wire) will inevitably produce a distributed capacitance outside the cross section of the wire, which will interfere with the parallel plate electric field of the differential pressure sensor. It will also cause the approximate proportional relationship to deviate more and more from reality, causing a certain deviation between the measured value and the actual value.

6. Unreasonable selection for differential pressure sensor causing relative error

The absolute error of a meter is the absolute value between the measured value and the true value. The fixed error of a meter is related to its range and accuracy. Pressure sensors with different range and accuracy cause different errors under the same pressure conditions. Measuring a small pressure value with a large-scale differential pressure sensor will produce a large error. For example, a differential pressure sensor with an accuracy of 0.075% has a fixed error of 0.75 Pa. When measuring a pressure of 1 MPa, the sensor accuracy is 0.075%. When measuring a pressure of 500 kPa, the accuracy is 0.75%. When measuring a pressure of 10 kPa, the accuracy is only 7.5%. It shows that the smaller the measured value, the larger the relative error when using the same pressure sensor. Therefore, the unreasonable selection for the differential pressure sensor leads to a large measurement error.

7. Design defect for the isolation diaphragm of the differential pressure sensor causing error

From the working principle of the differential pressure sensor, when differential pressure is generated, it will inevitably lead to more or less displacement of the measuring diaphragm. The differential pressure acts on the isolation diaphragm. Transfer of filling liquid make a displacement to the measurement diaphragm, and there is an approximately linear relationship between them. When measuring the high pressure difference, the measuring diaphragm is designed to be thicker to meet the conditions. When measuring the minor-differential pressure, if the diaphragm is too thick, it will lead to a decrease in sensitivity. Therefore, in consideration of manufacturing factors in design, the measuring diaphragm of the differential pressure sensor cannot satisfy the condition that the displacement is much smaller than the wall thickness, resulting in design error.

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