The true calibration of the pressure transmitter requires a standard pressure source input to the transmitter. Because the scaler (LRV, URV) is not calibrated without the use of an etalon, ignoring the input part (input transducer pressure) for output adjustment (transducer's conversion circuit) is not the correct calibration. In addition, the relationship between the pressure and differential pressure detection components and the A/D conversion circuit and the current output is not equal. The purpose of the calibration is to find out the relationship between the three. Emphasize that only the input and output (input transmitter pressure, A/D conversion circuit, loop current output circuit) can be called a true calibration.
I. Preparation
The pressure source is connected to the self-made connector through the rubber hose, the balance valve is closed, and the gas seal is checked. Then the ammeter (voltage meter) and the communicator are connected to the transmitter output circuit, and the calibration is started after the power is warmed up. 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 us with the convenience of on-site calibration of differential pressure transmitters. The pressure tube can calibrate the differential pressure transmitter. When calibrating the differential pressure transmitter, first close the positive and negative valves of the three valve group, open the balancing valve, and then loosen the exhaust, drain valve or cock vent, then use the homemade connector to replace the positive pressure chamber. The exhaust, drain valve, or cock; while the negative pressure chamber is kept open to allow it to open to the atmosphere.
Second, the calibration of conventional differential pressure transmitter
First adjust the damping to the zero state, first adjust the zero point, then increase the pressure to full-scale, so that the output is 20mA, and the adjustment in the field is fast. Here, introduce the zero and span quick adjustment method. Zero adjustment has almost no effect on fullness, but when fullness has an effect on the zero point, its effect is approximately 1/5 of the span adjustment without migration, ie 1mA of span is adjusted upwards and the zero point will move approximately 0.2mA upwards. ,vice versa.
Third, the smart differential pressure transmitter calibration
It is not possible to calibrate the smart transmitter using 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 the mechanical and electrical circuits, there is also a microprocessor chip to calculate the input data. Therefore, the adjustment is different from the conventional method.
Actually, the manufacturer's calibration of smart transmitters is also explained. For example, ABB's transmitters have calibration points such as "set range", "re-quantification range", and "fine adjustment". The "Set Range" operation is mainly performed through the digital setting of LRV.URV. The "Re-quantity Range" operation requires the transmitter to be connected to a standard pressure source and guided by a series of instructions. The transmitter senses the actual pressure directly and sets the value. The initial and final setting of the range depends directly on the actual pressure input value. However, to see that although the analog output of the transmitter is correctly related to the input value used, the digital reading of the process value shows a slightly different value, which can be calibrated by a fine-tuning item. As each part needs to be adjusted separately and must be adjusted together, the following steps can be performed during actual calibration:
1. Do a 4-20mA trim first to calibrate the internal D/A converter. Since it does not involve sensing components, no external pressure signal source is required.
2. Do a full fine-tuning again, so that the 4-20mA, digital readout matches the actual applied pressure signal, so a pressure signal source is needed.
3. Finally, a heavy quantification process was performed, and the analog output 4-20 mA was adjusted to match the applied pressure signal source. The effect was exactly the same as that of the zero (Z) and range (R) switches on the transmitter housing.
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