Thread Content
Differential pressure transmitters are widely used in factories. To ensure their normal operation and accuracy, regular inspection and calibration are necessary. Now we introduce a method for on-site calibration without removing the pressure pipe. one. Preparation work We know that the differential pressure transmitter is connected to the pressure guiding pipe in application. The usual method is to disassemble the joint between the pressure guiding pipe and the differential pressure transmitter, and then connect the pressure source for calibration. This is very troublesome, and the work and labor intensity are high. The most worrying thing is that the pressure guide tube will be broken or leakage will occur when disassembling and assembling the joint. We know that no matter what model of differential pressure transmitter, the positive and negative pressure chambers have exhaust and drain valves or cocks. ； This provides convenience for us to calibrate the differential pressure transmitter on site, which means that the differential pressure transmitter can be calibrated without removing the pressure guiding tube. For this purpose, DLR processed and produced a joint (also called a nipple) with the same thread as the exhaust and drain valve or cock, as shown in the figure. 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 and drain valves or cocks to vent, and then use homemade joints to replace the exhaust and drain valves or cocks connected to the positive pressure chamber. ； The negative pressure chamber remains unscrewed to allow it to ventilate to the atmosphere. The pressure source is connected to the self-made joint through a rubber tube, the balance valve is closed, and the sealing condition of the gas line is checked. Then the ammeter (voltmeter) and handheld device are connected to the output circuit of the transmitter, and the calibration is started after power on and preheating. 2. For calibration of conventional differential pressure transmitter, first adjust the damping to zero state, first adjust the zero point, then add full pressure and adjust the full range so that the output is 20mA. On-site adjustment is about fast. Here we introduce the quick adjustment method of zero point and range. There is almost no impact on the full scale when adjusting the zero point, but it has an impact on the zero point when adjusting the full scale. Without migration, the impact is about 1/5 of the range adjustment amount, that is, if the range is adjusted upward by 1mA, the zero point will move upward by about 0.2mA, and vice versa. For example: the input full-scale pressure is 100Kpa, the reading is 19.900mA, adjust the range potentiometer so that the output is 19.900+(20.000-19.900) * 1.25=20.025mA. If the range increases by 0.125mA, the zero point increases by 1/5 * 0.125=0.025. Adjust the zero point potentiometer so that the output is 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 adjust migration, linearity, and damping. 3. Calibration of intelligent differential pressure transmitter It is not possible to calibrate the intelligent transmitter using the above conventional method, because this is determined by the structural principle of the HART transmitter. Because between the input pressure source and the generated 4-20mA current signal, the smart transmitter, in addition to machinery and circuits, also has a microprocessor chip to calculate the input data. Therefore, the adjustment is different from the conventional method. In fact, manufacturers also have instructions for the calibration of smart transmitters. For example, for ABB transmitters, calibration can be divided into: "setting the range", "re-ranging" and "fine-tuning". The "set range" operation mainly completes 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, and the transmitter directly senses the actual pressure and sets the value. The initial and final settings of the measuring range depend directly on the actual pressure input value. But see that although the analog output of the transmitter has the correct relationship to the input value used, the digital readout of the process value will show a slightly different value, which can be calibrated with the trimmer. Since each part needs to be adjusted individually or jointly, the actual calibration can be carried out according to the following steps: 1. First make a 4-20mA fine-tuning to calibrate the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required. 2． Make full fine-tuning again to make the 4-20mA and digital reading match the actual applied pressure signal, so a pressure signal source is required. 3． Finally, perform re-ranging and adjust the analog output 4-20mA to match the external pressure signal source. Its function is exactly the same as the zero-adjustment (Z) and range-adjustment (R) switches on the transmitter housing. Discussion: Some people think that the range of the smart transmitter can be changed by using the HART handheld communicator, and the zero point and range can be adjusted without inputting the pressure source. However, this approach cannot be called calibration, but can only be called "setting the range". True calibration requires a standard pressure source to be input into the transmitter. Because adjusting the range (LRV, URV) without using a standard is not calibration, and ignoring the input part (pressure input to the transmitter) to adjust the output (conversion circuit of the transmitter) is not correct calibration. Furthermore, the relationship between the pressure and differential pressure detection components, the A/D conversion circuit, and the current output are not equal. The purpose of calibration is to find out the changing relationship between the three. One point to emphasize: Only when the input and output (input transmitter pressure, A/D conversion circuit, loop current output circuit) are debugged together can it be called calibration in the true sense. Four. Some suggestions: After the adjustment work is completed, the exhaust and drain valves or cocks should be screwed back to their original positions, and the raw material tape should be wrapped around them. They should be tightened to ensure no leakage. However, the exhaust and drain of the positive and negative pressure chambers should be carried out before tightening. At this time, the process pressure can also be used to perform a simple transmitter static pressure error check. The joint thread M processed in the picture can be processed according to the thread specifications of the transmitter's exhaust, drain valve or cock, because each type of transmitter is different, including imperial and metric. The pressure pump used to generate the pressure source is relatively bulky, but the pressure bulb of the sphygmomanometer used on site is very lightweight.