What are the reasons for poor contact between the hot end of the thermocouple and the object being measured?

The main reasons for the poor contact between the hot end of the thermocouple and the object being measured are installation method, surface condition, mechanical factors and environmental factors. The following is a detailed introduction:
Installation method problem
Insufficient insertion depth: If the thermocouple is not inserted deep enough into the object being measured, the hot end cannot be fully immersed in the measured medium, which will result in a small contact area and insufficient heat transfer. For example, when measuring the temperature of the fluid in the pipeline, if the thermocouple is inserted too shallowly, it can only contact the fluid near the inner wall of the pipeline, and cannot accurately measure the fluid temperature at the center of the pipeline.
Improper installation position: The thermocouple is installed at the edge or corner of the object being measured. These positions may not represent the overall temperature of the object being measured and are easily affected by the external environment. For example, in a heating furnace, if the thermocouple is installed near the furnace door, the temperature near the furnace door is low due to the fast heat dissipation.
Inappropriate installation angle: The angle between the thermocouple and the surface of the object being measured is unreasonable, which will reduce the contact area between the hot end and the object being measured and affect the heat transfer efficiency. For example, when measuring the temperature of a flat object, if the inclination angle between the thermocouple and the surface of the object is too large, only part of the hot end will be in contact with the object, which will cause measurement errors.
Surface condition of the object being measured
Uneven surface: If there are unevenness, burrs or oxide scale on the surface of the object being measured, the hot end of the thermocouple cannot fit tightly with the surface of the object, and there will be air gaps. The poor thermal conductivity of air will hinder heat transfer and lead to inaccurate measurement values. For example, when measuring the temperature of a metal casting, the uneven surface of the casting will cause an air layer between the hot end of the thermocouple and the casting, affecting the temperature measurement accuracy.
There are dirt or impurities on the surface: Dirt or impurities such as oil, dust, and scale on the surface of the object being measured will form an insulating layer between the hot end of the thermocouple and the object, reducing the efficiency of heat transfer. For example, when measuring the temperature of the outer wall of the heat exchanger pipe, if there is thick scale on the surface of the pipe, the heat transfer between the hot end of the thermocouple and the pipe will be hindered, resulting in a low measured temperature.
Mechanical factors
Unsecured fixation: After the thermocouple is installed, it is not reliably fixed. Under the action of vibration, impact or thermal expansion of the object to be measured, the contact between the hot end and the object to be measured will become loose or displaced. For example, if the thermocouple is installed on a rotating device or a vibrating machine, if it is not fixed tightly, the hot end of the thermocouple will gradually deviate from its original position as the equipment runs, resulting in poor contact.
Thermal stress effect: In a high temperature environment, the thermal expansion coefficients of the thermocouple and the object to be measured are different, which will produce thermal stress. If the thermal stress is too large, it may cause a slight displacement or deformation between the hot end of the thermocouple and the object to be measured, affecting the contact effect. For example, in a ceramic kiln, the difference in thermal expansion between ceramic products and thermocouples may cause the hot end of the thermocouple to have poor contact with the ceramic products after long-term use.
Thermocouple self-problems
Deformation of the protective sleeve: The protective sleeve of the thermocouple is deformed by external force during transportation, installation or use, resulting in the hot end being unable to have good contact with the object to be measured. For example, after the protective sleeve is squeezed or collided, it may be bent or dented, causing the hot end to deviate from the correct measurement position.
Hot end damage: The hot end of the thermocouple may be damaged due to long-term exposure to high temperature or chemical corrosion, mechanical wear, etc., such as the hot end's solder joints falling off, the electrode breaking, etc., resulting in poor contact with the object being measured. For example, in a highly corrosive chemical environment, the electrode of the thermocouple hot end is easily corroded, which increases the contact resistance and affects the measurement accuracy.
Environmental factors
High temperature oxidation: In a high temperature environment, the surface of the object being measured and the hot end of the thermocouple are prone to oxidation reactions to form an oxide layer. The oxide layer has poor thermal conductivity, which will hinder heat transfer and cause poor contact. For example, in a heat treatment furnace, metal workpieces and the hot end of the thermocouple will oxidize at high temperatures, affecting the accuracy of temperature measurement.
Chemical corrosion: If there are corrosive gases, liquids, etc. in the object being measured or the surrounding environment, it will corrode the hot end of the thermocouple, destroy the contact surface between the hot end and the object being measured, and increase the contact resistance. For example, in an electroplating bath, the acidic electrolyte can corrode the thermocouple hot junction, causing increased measurement errors.