Let's learn the correct method of the differential pressure transmitter.
1. Preparation work
The pressure source is connected with the self made joint through the rubber pipe, and the balance valve is closed, and the sealing condition of the gas path is checked. Then the ammeter (voltmeter) and the manual operation device are connected to the transmitter output circuit, and then the calibration is started after the electricity is preheated. We know that no matter what type of differential pressure transmitter, the positive and negative pressure chamber has exhaust, drain valve or cock, which provides convenience for our on-site calibrating differential pressure transmitter, that is to say, the differential pressure transmitter can be calibrated without removing the pressure pipe. The calibration of the pressure transducer, the positive and negative valve three valve closed, open balance valve, then unscrew exhaust, exhaust valve or cock vent, and then used a homemade joint instead of exhaust, connected to the positive pressure chamber of the exhaust valve or cock; while the negative pressure chamber is maintained to loosen state. The air.
Two. The calibration of the conventional differential pressure transmitter
First, the damping is adjusted to zero state. First, zero point is adjusted first, then the full pressure is adjusted to the full range. The output is 20mA, which is fast in the field adjustment. Here we introduce the quick adjustment method of zero point and range. Zero adjustment has almost no effect on the full scale, but when the adjustment is full, it has an effect on the zeroes. When it is not migrating, its influence is about 1/5 of the range adjustment amount, that is, the range up adjustment of 1mA, the zero point will move upward about 0.2mA, and vice versa.
Three, calibration of intelligent differential pressure transmitter
It is not possible to calibrate the intelligent transmitter with the conventional methods mentioned above, because it is determined by the structure principle of the HART transmitter. Because the intelligent transmitter operates between input voltage source and generated 4-20mA current signal, besides machinery and circuit, and micro processor chip operates on input data. Therefore, the adjustment is different from the conventional method. 1. first do a 4-20mA tuning to correct the D/A converter inside the transmitter, because it does not involve sensing components, and no external pressure signal source is required. 2. do a whole process of fine tuning to make 4-20mA and digital readings consistent with the actual pressure signals, so the pressure signal source is needed. 3., at last, the re determined range is adjusted to make the analog output 4-20mA coincide with the external pressure signal source. Its action is exactly the same as that of the Z on the transmitter housing and the R switch.
Some people believe that as long as the HART hand-held device can change the range of the intelligent transmitter, and adjust the zero point and range, and do not need to input the pressure source, this method can not be called calibration, it can only be called "set range". The real calibration is to use a standard pressure source to enter the transmitter. Because the calibration range (LRV, URV) is not calibrated without using the standard device, ignoring the input part (input transmitter pressure) to carry out the output regulation (transmitter conversion circuit) is not the correct calibration. Furthermore, the relationship between pressure and differential pressure detection components is not equal to the A/D conversion circuit and current output. The purpose of calibration is to determine the relationship between the three. It is emphasized that only debugging the input and output (input transmitter pressure, A/D conversion circuit, loop current output circuit) is a real sense of calibration.