On-site calibration of the instrument has always been a problem that plagues the instrumentation industry. Now we have introduced simple methods for everyone and hope to be helpful:
one. Preparation We know that the differential pressure transmitter is connected to the pressure guiding tube in the application. Usually, it is necessary to disassemble the pressure tube and differential pressure transmitter connector, and then connect the pressure source for calibration. This is very cumbersome, and the work and labor intensity is high. The most worrying factor is that when the joint is disassembled, the pressure guiding tube is torn off or leakage occurs. Therefore, dlr has adopted a more convenient on-site calibration method. We know that no matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valves or cocks; this calibrates the differential pressure for our site. Transmitters provide convenience. To this end, DLR has manufactured joints (also known as soothers) with the same thread as the exhaust, drain valve or cock.
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, and then connect the self-made connector in the positive pressure chamber. The location of the exhaust, drain valve or cock; while the negative pressure chamber is kept open to allow it to open to the atmosphere. The pressure source is connected with the self-made connector through the rubber hose, and check the gas seal. At the same time, the ammeter or voltmeter and the communicator are connected to the output circuit of the transmitter, and the calibration is started after the power is warmed up.
Second, the calibration of the conventional differential pressure transmitter first adjust the damping to zero state, first adjust the zero point, then increase the full-scale pressure full range, so that the output is 20mA, in the field to adjust the lecture is fast, introduce zero, A quick adjustment method for measuring range. 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. For example: the input pressure is 100Kpa, the reading is 19.900mA, the adjustable range potentiometer makes the output 19.900+(20.000-19.000)*1.25=20.025mA. The range is increased by 0.125mA, then the zero point increases 1/5*0.125= 0.025. The zero point potentiometer makes the output 20.000mA. After the zero point and full scale are adjusted properly, check the middle scale to see if it is out of tolerance and fine tune if necessary. Then perform the adjustment of the migration, linearity, and damping.
III. Calibration of the Smart Differential Pressure Transmitter It is not possible to calibrate the smart transmitter using the above 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. Since each part should be individually calibrated, the following steps can be performed during actual calibration:
1. Do a 4-20mA trim once to correct 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 re-quantification process is performed so that the analog output 4-20 mA is matched with the external signal source. The effect is exactly the same as that of the zero (Z) and range (R) switches on the transmitter housing.
Question Discussion:
Some people think that as long as the HART communicator can be used to change the smart transmitter range, zero point and span adjustment can be performed without the need to input a pressure source, but this practice cannot be called calibration. True calibration 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 when the input and output (pressure of the input transducer, A/D conversion circuit, loop current output circuit) are debugged together can it be called a true calibration.
four. After several recommendations, the exhaust valve, drain valve, and cock must be rotated back to their original positions. The raw material tape should be wrapped around and tightened to ensure that it does not leak. However, positive and negative pressure must be applied before tightening. Room exhaust, drain work.
The joint thread M processed in the figure can be processed according to the thread size of the transmitter's exhaust, drain valve or cock, because each type of transmitter is not the same, there are inch, metric.
The pressure pump of the pressure source compares the weight of the benzene, and the pressure ball of the sphygmomanometer is very light in the field. This article is transferred from http://Reprinted please leave this link!