Abstract : This paper describes the common faults that occur in the operation of thermocouple test furnace temperature controller. At the same time, the causes of these faults and how to eliminate the faults are analyzed. The maintenance and maintenance methods of the equipment are also proposed.

The Dushanzi Ethylene Plant has a total of 2 sets of thermocouple verification furnaces, which mainly use the DWT-702 precision temperature automatic controller to control the temperature and verify that the thermocouple is constant at the desired temperature point when the thermocouple is calibrated. Especially for the verification of precious metal thermocouples, the temperature field requirements are accurate and constant at the verification point. The temperature change in the furnace during the measurement process shall not exceed 0.2°C per minute, and the temperature change in the furnace during the entire measurement process shall not exceed 0.5°C. This depends on the temperature control of the temperature controller.

1 Principle

DWT-702 precision temperature automatic control instrument is mainly composed of millivolt, millivolt amplifier, PID regulator and thyristor trigger. The adjustment system block diagram is shown in Figure 1.

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The temperature of the furnace is measured with a thermocouple and the set temperature value is given by a millivolt valuer. If the thermocouple's thermoelectric potential deviates from the value of the determinator's output millivolt, it means that the furnace temperature deviates from the given value. This deviation is amplified by the microvolt and sent to the PID regulator. The thyristor trigger is then used to control the deviation. The silicon actuator adjusts the heating power of the furnace wire accordingly, thereby quickly eliminating this deviation and achieving the purpose of automatic temperature control.

2 millivolt setter

The millivolt-measuring device is a high-precision DC millivolt signal source, which can accurately give a DC millivolt signal. The DC offset signal after being compared with the thermocouple signal is input to a lower-level microvolt amplifier.

The millivolt valuer consists of a DC regulated power supply, a digital-to-analog dial-type switch, and a resistor network. The setter has 5 decimal digits in total. Dial the digital dial to give the required millivolt value. After the instrument is powered on, connect the digital voltmeter to the “+” and “-” terminals of the output to change the dial number. The difference between the millivolt value of the output of the digital calibrator and the millivolt value measured by the digital voltmeter should be Within the allowable error. The normal allowable error is generally less than ± 0.04%, that is, the thermocouple with indexing number K, the maximum setting error of the millivolt setter should be less than ± 24μV; the thermocouple with indexing number R, the maximum value of millivolt. The setting error should be less than ±8μV. Otherwise, it indicates that the setter has a large set error and should be overhauled.

When the millivolt setting error of the instrument is large or unstable, can you check whether there is a partial short circuit inside a certain dial switch, whether the output resistance is inadvertently welded or deteriorated, and whether the DC regulated power supply of the millivolt determiner is normal? If all parts of the regulated power supply are normal and the millivolt setting is not stable, it can be further determined whether the voltage temperature coefficient of the standard regulator tube 2DW7C is within the allowable range. The normal value should be less than ±5×10-6/°C. In addition, it can be further checked whether the dial switches are in good contact with each other, whether there is an imaginary welding phenomenon between the connecting wires.

3 microvolt amplifier

As shown in Figure 2, the DC offset signal from the millivolt reference and the thermocouple is modulated by a low-pass filter to a chopper composed of field effect transistors to be an AC signal, and then amplified by AC, phase-sensitive amplification and demodulation. The DC signal is output to the PID regulator. In addition, the multivibrator supplies the chopper and the phase-sensitivity amplifier, and the power is modulated and demodulated with a frequency of 250 Hz.

Because of adopting the form of "modulation-AC amplification-demodulation", the modulation part of it has also been selected as a low-noise, stable-field-effect transistor. Therefore, the amplifier not only has higher magnification, but also has high sensitivity and long-term drift. Small and strong anti-interference ability.

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When the measured temperature is lower than the set value, the signal voltage applied to the amplifier is positive and negative, the deviation table indicates negative deviation; when the measured temperature is higher than the set value, the voltage signal applied to the amplifier is positive, negative, and negative. Deviation indicates positive deviation. Measure the output of the microvolt amplifier with a multimeter or a digital voltmeter. For ±(0.2~0.3)mV deviation input, there should be a voltage output of about ±5 V, otherwise the amplifier will not work properly. Common faults in the use of the amplifier: the signal does not go in; the deviation table does not move (the amplifier has no output); the resistance to dryness is reduced.

When the signal does not go in, you can first check whether the part related to the signal is normal. For example, thin film cables, connectors, connectors, etc. It is also possible to add a signal using the standard voltage and current generator at the "+" and "-" terminals of the amplifier. Under normal conditions, the deviation table should indicate. If the deviation table still does not indicate, you can check whether the MOSFET modulation part is normal, whether the input and output transformer leads are broken or bad; whether the triacs of the AC amplification part have breakdown or broken legs; if the capacitance is damaged; whether the deviation table itself is Damaged; whether the multivibrator stops.

When it is found that the ability of the amplifier to withstand dryness is reduced, it can be checked whether the insulation resistance of the original secondary coil of the power transformer and the coils of the amplifier input and output transformers meets the requirements; whether the capacitance C204 of the modulation part is being welded or leaked.

When you find that the amplifier amplification is reduced, you can first check whether the DWT-202-2K amplifier output potentiometer is spinning too small. The potentiometer's normal position should be rotated to the maximum; finally check the amplifying part of each transistor whether there is performance deterioration or poor contact phenomenon; input transformer output lead is broken or bad contact.

4 PID regulator

The PID regulator consists of a high-impedance DC amplifier, a power amplifier, a PID feedback network, and a regulated power supply, as shown in Figure 3. It receives the DC signal output by the micro-volt amplifier and gives the PID regulation law, so that the DC signal is input to the thyristor trigger according to a certain regulation law, so as to achieve the purpose of controlling the heating power of the furnace wire.

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Common faults in the use of PID regulators: The output is maximum when there is no signal input, and there is no output when there is a signal input.

When there is no signal input and the output is maximum, check the resistance and capacitance of the control circuit for damage or poor contact; whether the operational amplifier and its U/I converter transistor are damaged or have poor contact.

When there is a signal input and no output, it can be checked whether the control loop components are damaged or bad, whether the U/I converter triode is damaged or bad, whether the impedance converter and the operational amplifier are damaged, and whether the regulator output voltage meter is in good condition.

5 thyristor trigger

After the instrument is powered on, if the “manual-automatic” switch is turned to the manual position, adjust the manual knob to make the regulator output maximum (10 V), observe the pulse output with the oscilloscope, then there should be about 15 V amplitude The value of the phase shift shall not be less than 160 degrees; the average value of the pulse output of the lowest voltage of the alternating current of the multimeter may also be used. Normally, there should be an indication value of about 0.5 V. Otherwise, there is a problem with the trigger and should be overhauled.

For triggers with no pulse signal output, check whether the single-junction transistor is damaged or the performance is deteriorating; whether the other transistor breaks down or breaks the foot; whether the pulse transformer lead wire is broken or falsely welded; whether the diode or the capacitor is protected against burn-through; Whether the film cable is damaged; whether the manual potentiometer is in good contact; if the 20Ω protection current limiting resistor is burned out, etc.

When the regulator has no output and the thyristrists compete, the diode of the control circuit can be checked for damage, and the thyristor can be punctured.

6 Maintenance and care

In order to prolong the service life of the thermocouple test oven temperature controller and ensure the reliability of the thermocouple standard, necessary maintenance and maintenance measures should be taken. First of all, the equipment should be specifically designated for maintenance. The operating personnel should be limited to those who hold the thermocouple verification certificate. Second, if the temperature control time is too long, the PID parameters should be adjusted in time. Do a good job in verification and maintenance records, and timely maintenance in the event of a failure to ensure the normal use of the equipment.

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