Definition
An insertion thermometer is a contact temperature measuring instrument that measures the internal or surface temperature of a temperature sensing element by inserting or immersing it directly inside the medium or object being tested. Its core design lies in the probe rod or probe structure, which is easy to install and fix, and is suitable for in-situ, continuous temperature monitoring of various substances such as solids, liquids or gases.
Principle
Plug-in thermometers operate on the principle of thermal equilibrium and thermoelectric or resistance effects. When the sensing element of the instrument, such as a thermocouple or thermal resistor, comes into contact with the medium being measured, heat exchange occurs between the two until thermal equilibrium is reached. Sensing elements convert the physical quantity of temperature sensed into an electrical signal (such as a change in thermal potential or resistance value). The electrical signal is amplified, linearized and compensated by the instrument's signal processing circuit, and finally converted into a standard temperature reading display or output. The basic relationship can be expressed by the formula of the Sebeck effect of thermocouples:
E = α(T1 - T0)
where E is the generated thermal potential, α is the Seebeck coefficient, T1To measure the end temperature, T0is the reference temperature.
Measurement method
The measurement process usually follows standardized steps. First, according to the material, temperature range and installation conditions of the tested object, select the appropriate type, probe length, diameter and material of the temperature measurement probe. During installation, ensure that the temperature measurement point is representative, that the probe insertion depth meets the specification requirements (generally no less than 8 to 10 times the diameter of the probe protection tube), and that there is sufficient contact with the medium. For in-pipe fluid measurement, the probe should be inserted counter-flow or vertically to ensure that the temperature sensing element is in the center area of the flow channel. After connecting the instrument, calibrate the zero point and range, and record the data after the reading is stable. In dynamic or high-temperature measurements, it is necessary to consider whether the thermal response time of the sensor meets the requirements of process control.
Influencing factors
Measurement accuracy is influenced by various factors. The thermal conduction error is caused by the heat dissipation or heat absorption of the probe protection tube and mounting sleeve to the environment, resulting in the temperature of the sensing element deviating from the actual medium temperature. Insufficient insertion depth will place the temperature sensing point in the temperature gradient region, introducing representative errors. The flow rate of the medium affects the convective heat transfer coefficient, and its effect on the heat exchange efficiency needs to be evaluated in the measurement of gas or liquid. The thermal inertness of the probe material will lead to a lag in thermal response, which may cause dynamic errors in the case of rapid temperature changes. Additionally, line resistance changes in electrical connections, electromagnetic interference, and reference-side compensation accuracy can also affect the final reading. Fluctuations in ambient temperature, mechanical vibration, and chemical corrosion can also affect the long-term stability of the sensor.
Applications
Plug-in thermometers are widely used in industrial process control and scientific experiment monitoring. In the food processing industry, it is used to monitor the core temperature of materials during baking, sterilization, and refrigeration. In the metallurgical industry, it is used to measure the temperature of molten metals, heat treatment furnaces and rolling processes. In the chemical field, it is suitable for temperature monitoring of reactors, pipeline transfer fluids and storage tanks. In building material testing, it is used for temperature tracking in processes such as concrete curing and asphalt paving. In environmental engineering, it can be used for composting, water and soil temperature monitoring. Its installation flexibility and direct measurement feature make it a common tool in situations where fixed-point, continuous temperature data is required.
Selection considerations
Selection should be based on a comprehensive evaluation of specific measurement needs and environmental conditions. The temperature range determines the type of sensing element, with common thermocouples suitable for a wide range and thermal resistors providing higher accuracy in the medium range. The probe material needs to be compatible with the medium being tested, considering corrosion, abrasion and temperature resistance, such as stainless steel for most applications, and superalloy or ceramic sleeve for extreme environments. The size and insertion length of the probe rod must meet the requirements of installation space and measurement depth. The process connection method (e.g., thread, flange, clamp) should match the existing equipment interface. The protection level should meet the requirements of dustproof and waterproof on site. The output signal type (e.g., analog, digital communication) needs to be compatible with the control system. In addition, thermal response time, long-term stability, ease of calibration and maintenance, and overall cost-effectiveness are also evaluated.