On-site calibration method for differential pressure transmitters - Solutions - Huaqiang

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

TP-IP4220CZ6 5V 0.35PF breakdown voltage 6V SOT

Photocoupler

Differential pressure transmitters are widely used in factories. To ensure their normal operation and accuracy, regular inspection and calibration are necessary. A method of performing field calibration without removing the pressure guiding tube is now described.
First, preparations:
We know that the differential pressure transmitter is connected to the pressure guiding tube in the application. Generally, the connector of the pressure guiding tube and the differential pressure transmitter needs to be disassembled, and then the pressure source is connected for calibration. This is very cumbersome, and the work and labor intensity is great. The most worrying thing is to break the pressure guiding tube or cause leakage when disassembling the joint. We know that no matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valve or cock; this is convenient for us to calibrate the differential pressure transmitter on site, that is to say, it is not necessary to remove the guide. The differential pressure transmitter can be calibrated with a pressure tube. When calibrating the differential pressure transmitter, first close the positive and negative valves of the three-valve group, open the balance valve, and 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; while the negative pressure chamber remains loose, allowing it to pass through 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 sealing condition is checked, and then the ammeter (voltmeter) and the hand-held device are connected to the output circuit of the transmitter, and the calibration is started after the power is preheated.
Second, the calibration of the conventional differential pressure transmitter:
First adjust the damping to zero state, first adjust the zero point, then adjust the full scale pressure to full scale, so that the output is 20mA, the on-site adjustment is fast, here introduces the zero adjustment of the zero point and range. Zero adjustment has almost no effect on fullness, but it has an effect on zero when adjusting fullness. When there is no migration, the effect is about 1/5 of the range adjustment amount, that is, the range is adjusted upward by 1 mA, and the zero point will move upward by about 0.2 mA. ,vice versa. For example: input full-scale pressure is 100Kpa, the reading is 19.900mA, and the range 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. Zero point potentiometer makes the output 20.000 mA. After the zero point and full scale are adjusted normally, check the middle scales to see if it is out of tolerance. If necessary, fine tune it. Then carry out the migration, linear, and damping adjustment work.
3. Calibration of Intelligent Differential Pressure Transmitter Calibration of the smart transmitter by the above conventional method is not acceptable, as this is determined by the structural principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20 mA current signal, in addition to the mechanical and electrical circuits, there is also the operation of the microprocessor chip on the input data. Therefore, the adjustment is different from the conventional method. ;V???_7@)m7v#F In fact, the manufacturer also has a description of the calibration of the smart transmitter. For example, ABB's transmitter has the following functions: “set range”, “heavy quantitation”, "fine tuning" points. The “set range” operation is mainly through the digital setting of LRV.URV to complete the configuration work, while the “re-quantity” operation requires the transmitter to be connected to the standard pressure source, guided by a series of instructions, The transmitter directly senses the actual pressure and sets the value. The initial and final settings of the range are directly dependent on the actual pressure input value. However, it should be noted that although the analog output of the transmitter is correctly related to the input value used, the digital reading of the process value will display a slightly different value, which can be calibrated by fine-tuning the item. Since the parts must be adjusted separately and must be adjusted, the actual steps can be followed by the following steps:
1. A 4-20 mA trim is done first to correct the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure source is required.
2. Perform a full fine-tuning to match the 4-20 mA, digital reading to the actual applied pressure signal, thus requiring a source of pressure.
3. Finally, the heavy-quantity process is performed, and the analog output 4-20 mA is matched with the applied pressure signal source by the adjustment, and the action is exactly the same as the zero-setting (Z) and the range-adjusting (R) switch on the transmitter casing.
Discussion of the problem:
Some people think that the HART Communicator can change the range of the smart transmitter, and can adjust the zero point and the range without inputting the pressure source. However, this method cannot be called calibration. "Set the range". The true calibration requires a standard pressure source to be input to the transmitter. Since the range (LRV, URV) is not calibrated without the use of a standard, ignoring the input section (pressure input to the transmitter) for output regulation (transmitter conversion circuit) is not the correct calibration. Furthermore, the relationship between the pressure and differential pressure detecting components, the A/D conversion circuit, and the current output is not equal. The purpose of the calibration is to find the relationship between the three. One point to emphasize: Only the input and output (input transmitter pressure, A / D conversion circuit, loop current output circuit) can be called a true calibration.
4. Suggestions After the adjustment work, the exhaust, drain valve or cock should be rotated back to the original position, and the raw material belt should be wrapped. It should be tightened to ensure no leakage, but it should be positive before tightening. Exhaust and drain operation of the negative pressure chamber. At this time, the process pressure can be used to perform a simple static pressure error check of the transmitter.