Pressure transmitters have always played a key role in controlling industrial processes and pressure changes. In fact, calibration of pressure transmitters requires the use of a standard pressure source input transducer.
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 calibrate 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.
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. 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.
It is not acceptable to calibrate the smart transmitter using the conventional method described above, 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.
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” and “fine adjustment”. 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 calibrately 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 calibrate 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.
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.