Definition
A probe thermometer is a portable or stationary measuring instrument that obtains its temperature value by directly touching or inserting a temperature sensing element (probe) inside the object or medium to be measured. Its core is to use the sensor at the front end of the probe for direct heat exchange and measurement, which is suitable for a variety of industrial and scientific research scenarios that require accurate measurement of point temperature.
Principle
The working principle of a probe thermometer is based on a sensor that converts a thermal signal into a readable electrical signal. Common sensor types are thermocouples and thermal resistors. According to the Seebeck effect, the thermocouple is connected by two conductors of different materials at the tip of the probe to form a loop, and when there is a temperature difference between the measuring end and the reference end, a thermal electromotive force will be generated in the loop, and the temperature can be calculated by measuring the EMF. Thermal resistance is based on the characteristics of the resistance value of a conductor or semiconductor with temperature, usually using platinum, copper and other materials to determine the temperature by measuring the change in its resistance value. The relationship can be expressed as:
For platinum thermal resistance, the relationship between resistance and temperature is approximately R in the range of 0 to 850°Ct = R0(1 + At + Bt2), where Rtis the resistance at temperature t, R0is the resistance at 0°C, and A and B are the material constants.
Measurement method
The measurement method is mainly based on the contact between the probe and the measured object. For solid surfaces, flat or light pressure is usually used to ensure that the probe measuring surface is in sufficient contact with the measured surface to reduce thermal resistance. For semi-solid or viscous materials, the probe can be inserted into it and kept stable. For liquid measurements, the probe should be immersed to a sufficient depth and avoided being close to the vessel wall to ensure that the measurement is representative. During the measurement process, it is necessary to wait for the value to be stable, which is called the response time, and its length depends on the probe structure, the type of sensor, and the thermal characteristics of the object being measured.
Influencing factors
Measurement accuracy is influenced by various factors. The probe should be inserted to the minimum depth recommended by the manufacturer to ensure that the sensor is adequately aware of the temperature of the medium being measured. Contact pressure and contact area can affect heat conduction efficiency, and poor contact can lead to low readings. The thermal conductivity of the object being measured is a critical factor, and for materials with low thermal conductivity, longer thermal equilibration times are required. Ambient temperature can affect the meter's electronic components and reference compensation, and modern instruments usually have automatic ambient temperature compensation. In addition, the response time of the probe, electromagnetic interference, and the long-term stability and calibration status of the probe itself can affect the reliability of the measurement results.
Applications
Probe thermometers play a role in numerous fields. In food processing, it is used to monitor core temperatures during cooking, refrigeration and storage. In agriculture and storage, it is used to detect the internal temperature of grain, tobacco and other accumulations. In materials science laboratories, it is used to monitor the curing process, melt temperature, or reactor temperature. In electronics manufacturing, it is used to detect local temperature rise in circuit boards or components. In environmental monitoring, it can be used to measure soil and compost temperature. The common feature is the need to obtain precise temperatures inside objects or at specific points.
Selection considerations
Selection is a process of comprehensive trade-offs. First, it is necessary to clarify the temperature range and accuracy requirements, which determine the choice of sensor type, such as thermocouples for a wider temperature range, while platinum thermal resistors may have better stability in the common range. The characteristics of the medium being measured, such as corrosiveness, hardness, or pressure, are also considered, which determine the material and structure of the probe sheath, such as stainless steel, Hastelloy or ceramic. The response time requirements determine the size and structure of the probe, and fine-tipped probes have a faster response but may have lower mechanical strength. In addition, the instrument's display capabilities, data logging and output interfaces, degree of protection, and ease of calibration and maintenance need to be considered. The final selection should be based on the needs and constraints of the specific application scenario.