Views: 0 Author: Site Editor Publish Time: 2022-08-01 Origin: Site
Differential pressure transmitters are widely used in factories. In order to ensure their normal operation and accuracy, regular inspection and calibration are necessary. A method for on-site calibration without removing the pressure guiding tube is now introduced.
We know that the differential pressure transmitter is connected to the pressure guiding pipe in the application. The usual practice is to disassemble the joint of the pressure guiding pipe and the differential pressure transmitter, and then connect to the pressure source for calibration. This is very troublesome, and the work and labor intensity are high. The most worry is that the pressure guiding tube will be broken or the leakage problem will occur when disassembling and assembling the joint.
We know that no matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valves or cocks; this provides convenience for us to calibrate the differential pressure transmitter on site, that is to say, there is no need to remove the guide The differential pressure transmitter can be calibrated by pressing the tube.
When calibrating the differential pressure transmitter, first close the positive and negative valves of the three-valve group, open the balance valve, then loosen the exhaust, drain valve or cock to empty, and then replace the positive pressure chamber with a self-made joint The exhaust, drain valve or cock; and the negative pressure chamber is kept loose, so that it is open to the atmosphere. The pressure source is connected to the self-made joint through the rubber tube, the balance valve is closed, and the air circuit is checked for sealing. Then, the ammeter (voltmeter) and the hand communicator are connected to the output circuit of the transmitter, and the calibration is started after power-on and preheating.
First adjust the damping to the zero state, first adjust the zero point, and then add the full pressure to adjust the full scale, so that the output is 20mA. The on-site adjustment is fast. Here we introduce the fast adjustment method of zero point and span. When the zero point is adjusted, it has almost no effect on the full scale, but when the full scale is adjusted, it has an impact on the zero point. When there is no migration, the effect is about 1/5 of the range adjustment amount, that is, when the range is adjusted up by 1mA, the zero point will move up by about 0.2mA. ,vice versa. For example: the input full scale pressure is 100Kpa, the reading is 19.900mA, and the range adjustment potentiometer makes the output 19.900+(20.000-19.900)×1.25=20.025mA. The range increases by 0.125mA, then the zero point increases by 1/5×0.125 =0.025. The zero-point potentiometer makes the output 20.000mA. After the zero-point and full-scale adjustment are normal, check the middle scales to see if they are out of tolerance? Make fine adjustments if necessary. Then carry out the adjustment work of migration, linearity and damping.
It is impossible to calibrate the smart transmitter with the above-mentioned conventional method, because it is determined by the structural principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20mA current signal, in addition to machinery and circuits, there is also the operation of the microprocessor chip on the input data. Therefore, the adjustment is different from the conventional method.
In fact, the manufacturer also has instructions on the calibration of smart transmitters. For example, ABB's transmitters have the following categories for calibration: "set range", "re-measurement range" and "fine adjustment". Among them, the "set range" operation is mainly to complete the configuration work through the digital setting of LRV.URV, while the "re-range" operation requires the transmitter to be connected to a standard pressure source, guided by a series of instructions, from variable The transmitter directly senses the actual pressure and sets the value. The initial and final setting of the range is directly dependent on the actual pressure input value.
However, see that although the analog output of the transmitter is correctly related to the input value used, the digital readout of the process value will show a slightly different value, which can be calibrated by trimming. Since each part needs to be adjusted individually and must be adjusted jointly, the actual calibration can be carried out according to the following steps:
1. First do a 4-20mA fine-tuning to correct the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required.
2. Do a full fine-tuning again to make the 4-20mA, digital readout match the actual applied pressure signal, so a pressure signal source is required.
3. Finally, make the re-quantity range, and make the analog output 4-20mA match the external pressure signal source through adjustment, and its function is exactly the same as that of the zero (Z) and range (R) switches on the transmitter shell.
Some people think that the range of the smart transmitter can be changed as long as the HART Communicator is used, and the adjustment of the zero point and the range can be performed without the need to input a pressure source, but this method cannot be called calibration, but can only be called "Set Range". True calibration requires a standard pressure source input to the transmitter. Because range adjustment (LRV, URV) is not a calibration without the use of a standard, output adjustment (transmitter conversion circuit) ignoring the input part (pressure input to the transmitter) is not a correct calibration. Furthermore, the relationship between pressure and differential pressure detection components, A/D conversion circuit and current output is not equal. The purpose of calibration is to find out the changing relationship of the three. Emphasize one point: Only when the input and output (the pressure of the input transmitter, the A/D conversion circuit, and the loop current output circuit) are debugged together, can it be called calibration in the true sense.
After the adjustment work, the exhaust and drain valves or cocks should be turned back to their original positions, and the raw material belt should be wrapped. It should be tightened to ensure no leakage, but the positive and negative pressure chambers should be discharged before tightening. Air and drainage work. At this time, the process pressure can also be used to perform a simple static pressure error check of the transmitter.