Characteristics and development trends of temperature measurement technology

Temperature sensor refers to a sensor that can sense temperature and convert it into a usable output signal. Temperature is a physical quantity closely related to the human living environment. It is one of the three major parameters of industrial processes (flow, pressure, and temperature), and one of the seven basic physical quantities in the International System of Units (SI). Temperature measurement is a classic and ancient topic. For a long time, a variety of temperature measuring components and sensors have been popularized in this regard. However, to this day, in order to meet the extensive requirements of various industrial sectors, scientific research, medical treatment, and household appliances, new temperature measuring components and sensors, new temperature measuring methods, new temperature measuring materials, and new market applications are still being developed. It is not easy to measure temperature accurately. If the temperature measuring component is not properly selected or the measurement method is not appropriate, satisfactory results cannot be obtained.

According to statistics from relevant departments, the sales of sensors in China in 2009 were 32.7 billion yuan, of which temperature sensors accounted for 14% of the entire sensor market, mainly used in communication electronic products, household appliances, building automation, medical equipment, instrumentation, automotive electronics and other fields.

The bimetallic thermometer TBI with hygienic clamp process connection is designed for temperature measurement in hygienic applications.
(Image source: Baumer)

Characteristics of temperature sensors

As an ideal temperature sensor, it should meet the following requirements: wide measurement range, high accuracy, good reliability, small time drift, light weight, fast response, low price, mass production, etc. However, there is no temperature sensor that meets the above conditions at the same time. Various temperature sensors should be used flexibly according to the application site. This is because different temperature sensors have different characteristics.

● Different temperature sensors have different measurement ranges and characteristics.

The temperature measurement ranges and characteristics of several important types of temperature sensors are shown in Table 1.

Table 1. Classification of temperature sensors and temperature measurement ranges

● The accuracy of temperature measurement is related to the measurement method.

According to the use of temperature sensors, they are usually divided into contact measurement and non-contact measurement. The characteristics of the two measurement methods are shown in Table 2. [page]

Table 2. Comparison of contact and non-contact temperature measurement

● Different temperature measuring elements should use different measurement circuits.

There are three commonly used measurement circuits. "Resistive temperature measuring element measurement circuit", this measurement circuit should consider eliminating nonlinear errors and the influence of thermistor wire on the measurement accuracy. "Potential temperature measuring element measurement circuit", this circuit needs to consider linearization and cold end compensation, and the signal processing circuit is more complicated than thermistor. "Current type temperature measuring element measurement circuit", semiconductor integrated temperature sensor is the most typical current type temperature measuring element. When the power supply voltage changes and the external wire changes, the output current of this circuit is basically unaffected, which is very suitable for long-distance temperature measurement. The

latest progress in temperature measurement

● Develop temperature measuring elements and temperature sensors suitable for various industrial applications.

The platinum thin film temperature sensor has a film thickness of 1μm and can be placed in a very small measurement space for temperature field distribution measurement. The response time does not exceed 1ms. The minimum diameter of the thermocouple wire is 25μm, the volume of the thermocouple is less than 1×10-4mm3, and the mass is less than 1μg. The

multi-color colorimetric temperature sensor can calculate the approximate value of the emissivity of the object being measured in real time, improving the accuracy of radiation temperature measurement.

The resonant quartz tuning fork temperature sensor has a temperature resolution of up to 0.0009℃, an accuracy of up to 0.02℃, a measurement range of up to -200～260℃, and a linearity of up to 0.1～0.05%. The

new temperature control device composed of Z-element has the characteristics of simple circuit structure, high precision, fast speed, low power consumption and low cost. The

integrated temperature sensor is made of diffused silicon technology, suitable for mass production, good consistency, and a sensitivity of 11.3/℃. The

W-Re temperature sensor is a high-temperature sensor made of W-Re thermocouple, which can detect an upper limit temperature of 2300℃ and is suitable for high-temperature measurement in reduction, inert, vacuum, nuclear radiation and other environments.

The intelligent temperature sensor uses MEMS technology to combine typical temperature measuring elements, signal conditioning circuits, and microprocessors with digital bus interfaces into a whole to form an intelligent temperature sensor system.

● Temperature measurement technology has developed from "wired" to "wireless".

Traditional temperature measurement usually uses a wired connection with a cable. However, for some occasions, such as the temperature measurement of rotating or moving objects, places where people cannot enter in harsh environments, and environments where wired connections are not suitable, with the application of intelligent temperature sensors and considering the cost of saving wiring, temperature measurement technology has begun to develop from "wired" to "wireless".

The wireless temperature measurement virtual instrument system using passive surface acoustic wave resonators introduces signal processing methods and feedback control, which reduces system costs, improves measurement accuracy and measurement distance. Combined with a general computer platform and data I/O board, it is flexibly controlled by software, and can automatically adjust measurement parameters according to different environments and measurement processes to achieve adaptive detection. When the transmission power is 100mW, the wireless detection distance is 4m, the uncertainty of repeated measurement of the resonant frequency is about 0.09kHz, and the uncertainty of the temperature measurement sensitivity at 3m is about 0.1℃.

Temperature and humidity sensor system for air conditioners This system consists of a host and an extension. The extension regularly transmits the data measured by the temperature and humidity sensors placed inside to the host wirelessly. The host converts it into an electrical signal through the output unit and sends it to the control device. Since the extension is battery powered, it can be placed anywhere and transmit the control signal wirelessly, which is very convenient.

For the data measured by the sensors installed on site, the wireless patrol detection system does not require the patrol personnel to go to the site for visual inspection or record, but through the wireless data collection system, the field sensors with wireless transmission mode are wirelessly patrolled. This detection system will be very convenient for the detection of dangerous places and high parts.

● Temperature measurement technology has achieved the development direction of "from point to line, from line to surface, from surface to inside".

Multi-core thermocouple Traditional temperature measurement is mainly based on "point" temperature measurement, but people often need to pay attention to the temperature distribution of the entire temperature field, such as the temperature distribution of the entire furnace, the temperature field in the diffusion furnace, and the temperature of each point in the warehouse. Therefore, multi-core armored thermocouples have appeared, or temperature measuring cables are used to assemble multiple thermocouples or thermal resistors along the cable to measure the "linear" temperature distribution.

Fiber-optic temperature distribution measurement technology Fiber-optic temperature distribution measurement technology is an epoch-making technology that can measure linear temperature distribution with one sensor. The basic principle of this technology is to shoot laser pulses into the optical fiber, and calculate the temperature based on the ratio of Stokes and anti-Stokes in the scattered light returned from each place. This fiber-optic temperature distribution measurement technology can measure the temperature distribution within 30 km at most. It is ideal for measuring the temperature distribution of oil wells from the ground to the depth of the underground.

Using radiation thermometers or thermal imagers to measure surface temperature distribution For the measurement and control of the surface temperature of objects, radiation thermometers and thermal imagers were mostly used in the past, such as using thermal imagers to measure the temperature distribution of the outer layer of the blast furnace in steel plants, and using infrared radiation thermometers to measure the surface temperature of rotary furnaces in the cement industry. However, using fiber-optic temperature distribution measurement technology can not only improve the measurement accuracy, but also greatly reduce costs.

Deep temperature measurement can be achieved by lengthening the thermocouple, but it is extremely inconvenient to use. A special device for deep temperature measurement can be used, which consists of a heater, a metal frame, an insulation layer, and a temperature measuring element (such as a thermistor). The principle is that on the surface of the heater, its temperature gradient can be eliminated, and its surface temperature can be measured, so as to know its deep temperature. (end)