The following is the discussion on the static pressure influence of differential pressure transmitter by lotingson technology R & D center. You are welcome to read it.
When the differential pressure transmitter is in linear calibration, it is usually carried out under the condition that the negative pressure chamber is open to the atmosphere. In other words, the static pressure is 1 atmospheric pressure. However, once installed in the field for actual use, a certain working pressure will be added to the positive and negative pressure chambers. At this time, it will be found that the zero position is offset and the full position output is also offset (the full position offset is generally read out by comparing with the standard instrument). When the working static pressure is added, the zero position and full position output of the transmitter will deviate from the zero position and full position during atmospheric calibration, which is called static pressure influence error.
2. The influence of static pressure on transmitter performance and field examples
The static pressure error of differential pressure transmitter directly affects its comprehensive accuracy. The comprehensive accuracy (%) of differential pressure transmitter generally consists of three factors, which are accuracy (%), influence of ambient temperature change (% / 30o) and static pressure change (% / 7MPa). Their calculation formula is as follows:
Thus, static pressure error is a very important factor for the comprehensive accuracy of differential pressure transmitter.
This point has also been confirmed in various practical application conditions.
For example, when the differential pressure transmitter is applied to the field application of orifice flow detection, orifice plate or nozzle and other throttling parts are installed in the pipeline. Because the orifice diameter of the throttling piece is smaller than the inner diameter of the pipe, when the fluid flows through the throttling part, the beam cross-section suddenly shrinks and the flow rate accelerates. After throttling, the static pressure of the fluid at the back end decreases, so there is a static pressure difference before and after the throttling piece. There is a definite numerical relationship between the static pressure difference and the fluid flow, which conforms to q = K. The differential pressure transmitter is used to measure the differential pressure before and after the throttling parts to realize the flow measurement.
See Figure 1：
When it is used to measure the flow rate of high pressure steam in power plant, if the static pressure effect is not corrected or compensated, it will bring large error to the flow measurement, especially when the relative flow rate is small, the influence is more significant. For example, a metal capacitive differential pressure transmitter and throttling device constitute a differential pressure flowmeter. Under the condition of 32Mpa working static pressure, the static pressure error of full scale is ≤± 2% FS. Although the zero position error can be eliminated by zero adjustment, the full output error is always unavoidable. Therefore, this error directly affects the flow test and has a great influence. In this application condition, the static pressure performance of differential pressure transmitter is particularly important. If the static pressure error is compensated or its static pressure error is very small, its measurement accuracy will be greatly improved.
Causes of static pressure influence of metal capacitance sensor
Metal capacitive sensor is a kind of structural sensor, its static pressure effect is particularly prominent. This is related to its own structural characteristics.
Working principle introduction:
The medium pressure is transmitted to the measuring diaphragm located in the center of "δ" chamber through isolation diaphragm and silicone oil, and the measuring diaphragm deforms with the differential pressure on both sides. The displacement of the diaphragm is directly proportional to the differential pressure, and the maximum displacement is 0.1 mm. The differential capacitance between the measuring diaphragm and the capacitor plate is converted into a two-wire 4-20madc output signal through an electronic conversion circuit.
From the analysis of the above two reasons, the metal capacitance sensor will inevitably produce measurement drift error under the influence of working static pressure. There is a certain linear relationship between the full position drift and the working static pressure, and the direction uncertainty for the zero position drift.