Debugging of Differential Pressure Transmitter

Points for Attention in Field Debugging of Differential Pressure Transmitter

The on-site debugging of differential pressure transmitter is an important work for metrologists in industrial and mining enterprises. After unloading the connecting pipeline of the differential pressure transmitter, the high voltage side enters the standard pressure value, and the low voltage side communicates with the atmosphere. Corresponding to different pressure differences, the differential pressure transmitter will output a corresponding current value. The difference between the current value and the standard current value is the error value of the differential pressure transmitter. Metrologists use different adjustment methods to control the difference within the allowable range. Although the debugging method is simple, if we do not pay enough attention to several details in the debugging process, it will bring great errors to the debugging results of differential pressure transmitter. According to some typical problems encountered in daily work, the author puts forward the following points for discussion. In order to express it more clearly, the author mainly analyses the situation that the pressure conducting medium of differential pressure transmitter is water, and the pressure conducting medium of standard instrument used for debugging is gas, and so on.

1. Before debugging the differential pressure transmitter, the remaining liquid in the chamber should be blown out. After unloading the connecting pipe of the differential pressure transmitter, a part of the liquid will be left in the cavity of the differential pressure transmitter, as shown in Figure 1. When the input pressure of the high-pressure side of the differential pressure transmitter is applied, the liquid in the chamber may be sucked into the pressure guide tube of the pressure-making device in the repeated pressure process and can not be discharged in time. As a result, a pressure value generated by a liquid with a length of H is added to the displayed standard pressure value. If the length of the liquid is 50 mm, 490 Pa pressure can be generated in the vertical direction; if 40 000 Pa pressure is displayed on the standard device, the actual pressure acting on the high-pressure side of the differential pressure transmitter is only 39510 Pa, and the deviation between the pressure shown by the standard device and the actual pressure acting on the high-pressure side of the differential pressure transmitter is 1.24%. The test results are not accurate. True.

2. The internal resistance of testing equipment, wire resistance and contact resistance of connecting parts should not be too large. In order to facilitate measurement and debugging, there are a pair of test terminals beside the signal connection terminals on the differential pressure transmitter circuit board, and the internal circuit is shown in Figure 2. The inner part of the test terminal is a diode through which the loop signal current flows during normal operation. When the test equipment is connected to the test terminal, the test terminal is short-connected, so as long as the voltage at both ends of the test terminal does not exceed the threshold voltage of the diode, there is no current flowing through the diode; but when the test terminal is connected to the test equipment (including the internal resistance of the device, the resistance of the wire and the contact resistance produced by the connection of the wire) When the resistance value is too large, a part of the current will pass through the diode; if the resistance value is greater than 30_, the current value displayed by the external testing equipment will have an error of about 1%. Therefore, in order to ensure that no current passes through the diode, the internal resistance of the test terminal should be as small as possible, generally less than 10 is appropriate.

3. After adjusting the zero position and full range potentiometer, the critical point of regulation should be separated. This point should be paid special attention when the differential pressure transmitter is in the temperature limit or in the case of large vibration. Bad environment will change the resistance of adjustable potentiometer. In order to improve the stability of zero-position and full-range potentiometers in these environments, the potentiometer can be slightly reversed to the opposite direction of the original adjustment after completion of the adjustment, so that the potentiometer knife edge and the internal groove are out of contact.

4. After debugging of differential pressure transmitter, the opening of high and low pressure side valves should follow a certain order. As shown in Figure 3, the correct opening method should be to open the balance valve first, then the high-pressure valve, then close the balance valve, and finally open the low-pressure valve. Such opening sequence can prevent differential pressure transmitter from being impacted by one-way pressure and prolong the service life of differential pressure transmitter.

5. After the differential pressure transmitter is debugged and connected to the upper pipeline, the residual gas in the inner and outer pipelines of the differential pressure transmitter should be discharged in time. This work should be done better in pipelines without self-exhaust function. Assuming that there is a difference of 20 mm in the vertical direction between the high and low pressure side of the differential pressure transmitter due to the different height of the air column, the differential pressure transmitter will introduce an additional pressure of about 196 Pa when it works, which will affect the normal use of the differential pressure transmitter.