1. Principle of thermocouple measurement
We measure temperature based on thermoelectric effect, which consists of two ends conductors or semiconductors with different compositions forming a loop. When the two ends have temperature gradient, there will be fluent circuit in the loop. At the same time, there will be an electric force between the ends.
2. Reasons of temperature deviation
(1) . Unstable thermoelectric characteristics
After using for a long time, affected by the environment, thermocouple will have certain changes, which means there will be a certain deviation between the measured temperature and the real temperature.
(2) . Measuring points
The choice of measuring points is significant, including location and depth. The measuring points should be avoided to be affected by strong sources of electromagnetic field interference and not too close to heating services. The material of sheath also affects the measuring accuracy. Metal sheath has good thermal conductivity, which needs to be inserted deeper to avoid heat loss.
(3) . Disturbances during measuring process
Thermocouple temperature measurements can be subject to interference from the external environment, such as electromagnetic fields, vibration, or radiation
(4) . Deviation due to thermal inertia
Due to delay of measuring, the measured temperature fluctuation will be less than real temperature fluctuation. The large measurement hysteresis, the smaller the amplitude of the thermocpuple fluctuation and the greater the difference from the real furnace temperature. When using thermocouple with long thermal response time to measure or control temperature, the temperature of equipment is almost unaffected by the fluctuation, but the real environment temperature will have a serious fluctuation. To get more specific results, we should choose thermocouple with small diameter sheath, thin hot electrode and quick hot response time. If we need fast and accurate measuring results, bare wire thermocouple can be applied. But it can be easily contaminated and destroyed. We need to change it timely.
(5) . Contamination & corrosion of hot electrode
Thermocouple wire is thin, soft and easily deformed. When producing the wire, temperature or other reasons will result in uneven thickness of the wire or serious coiling. In addition, the repeated processing for the thermocouple will make thermocouple distorted or lose homogeneity. Therefore, wire should be pulled straight before measuring. When it’s contaminated or oxidized, the surface will turn black and dark. The greater the degree of contamination on the surface, the greater change of hot electrode characteristics and the greater the deviation.
(6) . Deviation caused by improper compensation
Thermocouple temperature measurement principle indicates that when the temperature of cold end of thermocouple is stable, the thermopower can correspond to the measured temperature 1:1. In real use, the cold end of thermocouple will be exposed in the air and affected by the environment temperature.
(7) . Inner Contamination
Junction box cannot guarantee no dust and conductor liquid, so we need clean the box regularly.
(8) . Response time
Only when the temperature of measured object and the measurement element are the same, and the temperature of element is stable. At the same time the two reach thermal equilibrium, which is the basic principle of contact measurement method. The measured object and the measurement element need a certain time to reach out the same temperature. The length of the time mainly decided by thermal response time.
The thicker the thermocouple electrodes , walls and sheaths, the more inert the thermocouple will be, which will increase the stability of thermocouple and have a higher thermal conductivity time. Differently, it will also affect response time. Therefore, we need choose proper heating rate and equilibrium time based on the type of thermocouple.
3. Solution
(1) . Regular inspection
Due to frequent use, the conjunction of each switch is easily contaminated, leading to inaccurate measuring results after a long period time of use. Therefore, you need clean the conjunction regularly.
(2) . Selection of temperature measurement point
Thermocouple should be placed as close as to the measured points. When measuring fluid, the thermocouple should be placed in center of the fluid. To avoid heat transfer along the thermocouple and prevent the measuring data affected by the sheath, thermocouple should be put into the measuring fluid. To minimize deviation, you need assure the stability of measured environment. Don’t put the measuring ends around corners.
(3) . Suitable material of sheath & insulation
Temperature measuring needs bare thermocouple to improve the response time. In using, we still use protection tubes. The choice of sheath has a huge affect on stability of thermocouple. The material of sheath is more important, which is crucial factor for temperature deviation.
(4) . Compensation
In order to reduce or eliminate deviation caused by temperature change of cold ends. It’s necessary to take cold ends compensation measures. When connecting thermocouple and cold end temperature compensator, you should pay attention to the polarity, compensator should be checked regularly, replace the wires if necessary.
Attention:
A. You cannot use ordinary wire instead of thermocouple compensation wire, the ordinary one can reduce the accuracy of measuring.
B. When connecting compensation wire with thermocouple and measuring instruments, you need pay attention to polarity for the wire and thermocouple.
C. When using compensation wire, the thermal power and the thermocouple should be consistent in temperature zone.
(5) . Anti-interference measures
A. Select thermocouple with insulation and shielding, applying PVC or PTFE, and wrap the sheath tightly around the insulation. We can strand thermocouple wire, put a magnetic filter on test devices to achieve better anti-interference.
B. Ground testing ends or reference ends of thermocouple, and the measured object also need to be grounded. When choosing grounded wire, we need select metal wire which has high resistance and harmless to the thermocouple.
C. Suspend thermocouple in the air to avoid any contact with other objects. It can prevent electric leakage and high temperature interference.
For more complicated issues and special applications, it is recommended to consult professional engineers, and we can provide more specific suggestions and solutions for you.
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