The working principle of pressure transmitter is as follows

The working principle of the pressure transmitter is that the two pressures of the measured medium are introduced into the high pressure chamber and the low pressure chamber, which act on the isolation diaphragms on both sides of the δ element (that is, the sensitive element) and are transmitted to both sides of the measuring diaphragm through the isolation diaphragm and the filling liquid in the element. The measuring diaphragm and the electrodes on the insulating sheets on both sides each form a capacitor. When the pressure on both sides is inconsistent, the measuring diaphragm is displaced, and the displacement is proportional to the pressure difference, so the capacitance on both sides is unequal, which is converted into a signal proportional to the pressure through oscillation and demodulation. The absolute working principle is the same as that of the differential pressure transmitter, except that the pressure in the low-pressure chamber is atmospheric pressure or vacuum. Through the above working principle, we can know that the sensor core is the core component, and the influence of these key factors, such as temperature, medium and voltage, should be fully considered in daily use.

Intelligent pressure transmitter features: super measurement performance, used for pressure, differential pressure, liquid level and flow measurement; Digital accuracy:+(-) 0.05%; Stability: 0.25% for 60 months; Range ratio: 100: 1; Measurement rate: 0.2S；; Miniaturized (2.4kg) all stainless steel flange, easy to install; Process connection is compatible with other products to achieve the best measurement; The only sensor with H alloy sheath in the world (patented technology) has achieved excellent cold and thermal stability. Adopt a 16-bit computer; Standard 4-20mA, with digital signal based on HART protocol, remote control supports upgrading to fieldbus and technology based on field control.

What are the calibration steps of pressure transmitter?

We know that the differential pressure transmitter is connected with the pressure guide pipe in application. Usually, the joint between the pressure guide pipe and the differential pressure transmitter needs to be disassembled and then connected to the pressure source for calibration. This is very troublesome, and the work and labor intensity are great. The most worrying thing is to break the pressure guide pipe or leak when disassembling the joint. We know that no matter what type of differential pressure transmitter, its positive and negative pressure chambers have exhaust and drainage valves or cocks; This provides convenience for us to calibrate the differential pressure transmitter on site, that is to say, the differential pressure transmitter can be calibrated without dismantling the pressure guide tube.

When calibrating the differential pressure transmitter, first close the positive and negative valves of the three-valve group and open the balance valve, then loosen the exhaust and drainage valves or cocks to vent, and then replace the exhaust and drainage valves or cocks connected to the positive pressure chamber with self-made connectors; While the negative pressure chamber is kept in a loose state, so that it is ventilated to the atmosphere. The pressure source is connected to the self-made connector through the rubber hose, the balance valve is closed, and the gas path sealing condition is checked. Then, the ammeter (voltmeter) and the communicator are connected to the output circuit of the transmitter, and the calibration is started after being electrified and preheated.

Calibration of conventional differential pressure transmitter

First, adjust the damping to zero, then adjust the zero point, and then add full pressure to adjust the full range, so that the output is 20mA. On-site adjustment is fast. Here, the fast adjustment method of zero point and range is introduced. When the zero point is adjusted, it has little effect on the full scale, but when the full scale is adjusted, it has an effect on the zero point. Without migration, its effect is about 1/5 of the range adjustment, that is, if the range is adjusted upward by 1mA, the zero point will move upward by about 0.2mA, and vice versa. For example, when the input full-scale pressure is 100Kpa, the reading is 19.900mA, and the output is 19.900+(20.000-19.900) * 1.25 = 20.025 Ma. When the range increases by 0.125mA, the zero point increases by 1/5 * 0.125 = 0.025. The zero-point potentiometer makes the output. Make fine adjustments if necessary. Then the adjustment of migration, linearity and damping is carried out.

Calibration of Intelligent Differential Pressure Transmitter

It is impossible to calibrate the intelligent transmitter with the above conventional methods, because it is determined by the structural principle of HART transmitter. Because between the input pressure source and the generated 4-20mA current signal, the intelligent transmitter not only has machinery and circuits, but also has the microprocessor chip to calculate the input data. Therefore, the calibration is different from the conventional method.

In fact, manufacturers also explain the calibration of intelligent transmitters. For example, ABB's transmitters have the following points for calibration: setting range, re-measuring range and fine tuning. Among them, the setting range operation mainly completes the configuration work through the digital setting of LRV.URV, while the re-measuring range operation requires that the transmitter be connected to the standard pressure source and guided by a series of instructions, and the actual pressure is directly sensed by the transmitter and the value is set. The initial and final setting of the measuring range directly depends on the real pressure input value. However, it should be noted that although the analog output of the transmitter has a correct relationship with the input value used, the digital reading of the process value will show a slightly different value, which can be calibrated by fine-tuning the term. Because each part needs to be calibrated separately and simultaneously, the actual calibration can be carried out according to the following steps.

1. First do a 4-20mA fine adjustment to correct the D/A converter inside the transmitter.

Because it does not involve sensing parts, there is no need for an external pressure signal source.

2. Make another fine adjustment in the whole process, so that the 4-20mA and digital readings are consistent with the actually applied pressure signal, so a pressure signal source is needed.

3. Finally, re-measure the range, and adjust the analog output of 4-20mA to match the external pressure signal source. Its function is exactly the same as that of the zero (z) and range (r) switches on the transmitter housing.

Some people think that the range of intelligent transmitter can be changed by using HART communicator, and the zero point and range can be adjusted without inputting pressure source, but this practice can only be called setting range. The real calibration needs a standard pressure source to input the transmitter. Because adjusting the range (LRV, URV) without using the etalon is not calibration, ignoring the input part (the pressure of the input transmitter) to adjust the output (the conversion circuit of the transmitter) is not correct calibration. Moreover, the relationship between pressure and differential pressure detection components and A/D conversion circuit and current output is not equal, and the purpose of calibration is to find out the changing relationship among them.

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