Working mechanism and troubleshooting methods of pressure transmitter of pressure calibrator

1. Working mechanism of pressure transmitter of pressure calibrator

1. In the initial stage of research, the capacitive transmitter of 1851 series pressure calibrator has a variable capacitance sensor component in its body, which is called "δ" chamber. The sensor is a completely closed component. We know that the process pressure and differential pressure are transmitted to the sensing diaphragm through the isolation diaphragm and the filling liquid silicone oil to cause displacement. When the capacitance difference between the sensing diaphragm and the two capacitor plates is converted by the electronic components into a two-wire output electrical signal of (4-20) mA. This is the mechanism.

2. And we know that the electronic amplifier circuit is composed of a demodulator, an oscillator, an oscillation control amplifier, a current detector, a current control amplifier, a current limit controller, a reference voltage, a voltage regulator, etc. Through them, the capacitance signal is detected to control the oscillation frequency, and then converted into a current output. Such a process can drive its work.

2. Assembly and test of pressure calibrator sensor

When the pressure calibrator sensor fails, we generally cannot repair it on site, but only replace it. If we do not find any damage to the isolation diaphragm or oil leakage, the following steps can be used to detect the pressure calibrator sensor assembly:
1. Carefully unplug the sensor assembly lead socket from the plug socket.

2. Check the forward and reverse bias of the internal diode circuit: one loop is the red line and the yellow line, and the other loop is the green line and the blue line. The mechanism is shown in the figure below.

Figure 1 Pressure calibrator sensor assembly test mechanism diagram

Now we use the positive pole of the multimeter to connect the red line and the negative pole to the yellow line. The sum of the resistance values ​​of the series circuit D1, D2, D5, D6, and R4 should be close to or equal to the sum of the resistance values ​​of the series circuit D3, D4, D7, D8, and R5 of the green and blue lines.

3. Check the resistance of the sensor assembly housing and these four wires, that is, check the resistance between the capacitor plate and the sensing diaphragm connected to the housing. The resistance value should be greater than 10MΩ.

3. Pressure transmitter failure operation of the pressure calibrator

The fault parts of the 1851 pressure calibrator capacitive transmitter are generally divided into two categories: sensor failure and electronic detection and amplification conversion failure. The sensor is generally not prone to problems. If it is normal when tested according to the above method, there is generally no need to doubt it. The circuit part should be tested in particular. The main fault symptoms of the 1851 pressure calibrator capacitive transmitter are as follows:

1. Excessive output

When there is no pressure (differential pressure), the transmitter output current (mA) is too large, sometimes exceeding the range, and adjusting the zero point and range potentiometer does not work. This fault occurs in many circuit parts, and the circuit's back end, that is, the current control amplifier to the current control output part, is often damaged. Its structure is shown in Figure 2. [page]

Figure 2 Detailed diagram of circuit damage caused by excessive surgical output

IC3 is a voltage amplifier that is converted to mA output by V17. Use the DC voltage block of the multimeter to measure the potential value of IC3's 3rd pin, adjust the range or zero point, and the voltage of 3rd pin should change, indicating that the front pole circuit is normal. At the same time, measure the output of 6th pin and there should be an amplified voltage signal output. In this way, it can be judged whether IC3 and the peripheral bias circuit are normal. If the voltage of 6th pin is always very high (close to the power supply voltage), it can be judged that IC3 is damaged and replaced with the same model LM308. If IC3 is normal, use a multimeter to measure whether V17 and V18 are broken down. In actual detection, it is often encountered that V17 and V18 are broken down, resulting in excessive output.

2. The output is too small or there is no output

Check whether the voltage applied to both ends of the transformer is normal; whether the voltage across the integrated operational amplifier is normal; determine whether the oscillation circuit is oscillating; whether the range, zero point, and potentiometer adjustment voltage change and whether the 15Ω range load resistor is damaged. Use the method of detecting excessive output to detect the rear pole circuit and determine whether the rear pole circuit is normal.

3. Unstable output

Check whether the transformer has intermittent short circuit, open circuit and multi-point grounding; check whether the voltage applied to the transformer is stable and normal; whether the voltage stabilizing circuit is normal; check each voltage stabilizing diode and test whether the oscillation frequency is stable; whether the circuit board has cold soldering.

4. Output nonlinearity

This type of fault occurs relatively frequently and is difficult to detect. The deterioration of parameters in various parts of the circuit may cause this type of fault. However, it mainly occurs in the nonlinear adjustment circuit part. The circuit is shown in Figure 3.

Figure 3 Circuit diagram of output nonlinear fault

Use the resistance block of a multimeter to check whether diodes V9, V10, linear potentiometer R24, resistors R23, R22, and C3, C6, and C4 are normal, and adjust the output of R24 to see if there is a corresponding change. Damage to the above components often causes poor linearity.

The 1851 series pressure calibrator capacitive pressure and differential pressure transmitters have a variety of faults in long-term operation. When dealing with faults, careful and detailed observation and analysis should be conducted to determine the general location of the fault and the steps to solve it, and then debugging and testing should be carried out, so that twice the result with half the effort can be achieved. In addition, the user unit is reminded that the instrument must be calibrated and qualified after maintenance before it can be put into normal use. At this moment, we have a very thorough and detailed understanding of the working mechanism and fault operation of the pressure transmitter of the pressure calibrator.