The platinum-rhodium thermocouple is a traditional temperature measuring element. It has stable thermoelectric performance, strong oxidation resistance, and is suitable for continuous use in oxidizing and inert atmospheres. The long-term operating temperature is 1600°C, and the short-term operating temperature is 1800°C.
I. How to diagnose faults caused by the platinum-rhodium thermocouple input?
1、If the wiring is correct according to the instrument wiring diagram, but after the instrument is powered on, the upper digital display shows a negative value or similar, it indicates that the "+" and "-" terminals of the thermocouple connected to the instrument are reversed. Simply swap them.
2、If the wiring is correct and the instrument is running, but the temperature displayed on the upper digital display differs from the actual measured temperature by 30°C–60°C, or even more, it indicates that the instrument's indexing type does not match the thermocouple's indexing type. Based on the relationship between temperature (°C) and millivolt (mV) values for thermocouple indexing types B, S, K, E, etc., at the same temperature (°C), the millivolt output (mV) is smallest for type B, second smallest for type S, larger for type K, and largest for type E. Use this principle to diagnose.
3、After wiring correctly according to the instrument wiring diagram and powering on, the instrument first displays the thermocouple indexing type, then the instrument range, then the lower digital display shows the set temperature, and the upper digital display shows the measured temperature. If the upper digital display does not show the temperature of the heated body but shows "OVER", "0000", "000", or similar, it indicates a fault at the instrument input. Perform the following tests:
a) Remove the thermocouple from the instrument's thermocouple input terminals, then short-circuit the input terminals with any wire. When powered on, if the upper digital display shows approximately room temperature, it indicates an open circuit in the thermocouple internal wiring. Replace the thermocouple with one of the same type. If the same abnormal reading persists, it indicates that the instrument's input was damaged during transport; replace the instrument.
b) Remove the suspected faulty thermocouple and replace it with a known good thermocouple of the same indexing type taken from a neighboring instrument that is operating normally. After powering on, if the previously faulty instrument's upper display now shows the temperature of the heated body, it indicates an open circuit in the original thermocouple; replace it. If the same abnormal reading persists, the instrument input is damaged; replace the instrument.
c) Remove the suspected faulty thermocouple from the instrument. Set a multimeter to the resistance (R) *1 range and measure across the two ends of the thermocouple. If the multimeter shows a very high resistance, it indicates an open circuit inside the thermocouple; replace it. If there is a certain resistance value, then the instrument input is likely faulty; replace the instrument [4].
II. How to prevent interference during temperature measurement?
When using the reference junction grounding method, one end of the platinum-rhodium thermocouple (or compensation lead) output is grounded through a sufficiently large capacitor (the larger the capacitance, the better, if conditions permit). The measuring junction grounding method involves grounding the thermocouple's measuring junction by leading a metal wire from the measuring junction to ground. This method effectively prevents leakage interference at high temperatures. When selecting the metal wire, choose one that is high-temperature resistant and harmless to the thermocouple electrodes.
During the use of thermocouples, many situations can interfere with measurement accuracy. This is a very tricky issue. When using thermocouples, preventive measures against such interference must be taken. As industrial production levels improve, the usage rate of thermocouples is very high; therefore, the measurement accuracy of thermocouples is a matter of great concern.
III. How to handle interference when it occurs during thermocouple use?
Shielding method: Run the thermocouple compensation leads through iron pipes or other metal shielding to shield them. This prevents electromagnetic interference and high-voltage electric field interference. When using this method, the iron pipe and shielding should be properly grounded, and the compensation leads should be twisted together.
Isolation method: Install the thermocouple in a suspended manner, preventing it from contacting the refractory bricks of the furnace wall. Also, isolate the thermocouple from its support using insulating materials. This method effectively prevents high-temperature leakage interference.
Grounding method: This method involves grounding the measurement circuit to divert interference to the ground, thereby ensuring measurement accuracy. There are two forms: first, grounding the thermocouple reference junction; second, grounding the thermocouple measuring junction.
Depending on the specific temperature measurement environment and operating conditions, the choice of which method to use for prevention ultimately aims to minimize interference and achieve the most ideal measurement accuracy. Therefore, during the prevention process, careful attention must be paid to how best to eliminate the relevant interference.
