A temperature sensor is a sensor that senses temperature and converts it into a usable output signal. Temperature sensors are the core component of temperature measuring instruments and come in a wide variety of types. They can be broadly classified into two categories based on their measurement method: contact and non-contact. Based on the characteristics of their sensor materials and electronic components, they are further divided into resistance temperature detectors (RTDs) and thermocouples.
Contact
A contact temperature sensor, also known as a thermometer, has its sensing part in good contact with the object being measured.
Thermometers achieve thermal equilibrium through conduction or convection, allowing the thermometer reading to directly represent the temperature of the object being measured.
Non-contact
Its sensing element does not contact the object being measured, hence it is also known as a non-contact temperature measuring instrument. This type of instrument can be used to measure the surface temperature of moving objects, small targets, and objects with small heat capacity or rapid (transient) temperature changes. It can also be used to measure the temperature distribution of a temperature field.
The most commonly used non-contact temperature measuring instruments are based on the fundamental law of blackbody radiation and are called radiation temperature measuring instruments.
Advantages of non-contact temperature measurement: The upper limit of measurement is not limited by the temperature resistance of the sensing element, so there is no limit to the highest measurable temperature in principle. For high temperatures above 1800℃, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation thermometry has gradually expanded from visible light to infrared light, and it has been used for temperatures below 700℃ up to room temperature, with very high resolution.
Working principle
Sensor designed based on the principle of metal expansion
Metals expand when the ambient temperature changes, and sensors can convert this response into signals in different ways.
Bimetallic strip sensor
A bimetallic strip consists of two metals with different coefficients of thermal expansion bonded together. As the temperature changes, material A expands more than the other metal, causing the strip to bend. The curvature of the bend can be converted into an output signal.
Bimetallic rod and metal tube sensor
As the temperature rises, the length of the metal tube (material A) increases, while the length of the non-expanding steel rod (metal B) does not increase. Thus, the linear expansion of the metal tube due to the change in position can be transmitted. Conversely, this linear expansion can be converted into an output signal.
Sensors designed based on the deformation curves of liquids and gases
When the temperature changes, both liquids and gases will change in volume accordingly.
Various types of structures can convert this expansion change into a position change, thus producing a position change output (potentiometer, sensing deviation, baffle, etc.).
Resistance sensing
The electrical resistance of a metal changes with temperature.
For different metals, the change in resistance is different for every degree of temperature change, and the resistance value can be directly used as an output signal.
There are two types of resistance changes.
Positive temperature coefficient
Temperature increases = resistance increases
Temperature decrease = resistance decrease
Negative temperature coefficient
Temperature increases = resistance decreases
Temperature decrease = resistance increase
Thermocouple sensing
A thermocouple consists of two metal wires of different materials welded together at their ends. By measuring the ambient temperature of the unheated portion, the temperature of the heated point can be accurately determined. Because it requires two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivities also vary. The sensitivity of a thermocouple refers to the change in output potential difference when the temperature of the heated point changes by 1°C. For most thermocouples supported by metallic materials, this value is approximately between 5 and 40 microvolts per degree Celsius (µV/°C).
Because the sensitivity of thermocouple temperature sensors is independent of the material's thickness, temperature sensors can be made from very fine materials. Furthermore, due to the excellent ductility of the metal materials used to make thermocouples, these tiny temperature-sensing elements have extremely high response speeds, enabling them to measure rapidly changing processes.
Selection method
To perform reliable temperature measurements, the first step is to select the correct temperature instrument, also known as a temperature sensor. Thermocouples, thermistors, platinum resistance thermometers (RTDs), and temperature ICs are among the most commonly used temperature sensors in testing.
The following is an introduction to the characteristics of two types of temperature instruments: thermocouples and thermistors.
1. Thermocouple
Thermocouples are the most commonly used temperature sensors in temperature measurement. Their main advantages are a wide temperature range and adaptability to various atmospheric environments.
Moreover, it is sturdy, inexpensive, requires no power supply, and is the cheapest option. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated, a potential difference exists in the thermocouple circuit. The temperature can be calculated by measuring the potential difference.
However, the relationship between voltage and temperature is non-linear. Because of this non-linear relationship, a second measurement is needed for the reference temperature (Tref). The voltage-temperature conversion is then processed internally by the test equipment's software or hardware to ultimately obtain the thermocouple temperature (Tx). Both the Agilent 34970A and 34980A data acquisition units have built-in measurement and processing capabilities.
In short, thermocouples are the simplest and most versatile temperature sensors, but they are not suitable for high-precision measurements and applications.
2. Thermistor
Thermistors are made of semiconductor materials, most of which have a negative temperature coefficient, meaning their resistance decreases as temperature increases.
Temperature changes cause significant resistance shifts, making it the most sensitive temperature sensor. However, thermistors have extremely poor linearity and are highly dependent on the manufacturing process. Manufacturers do not provide standardized thermistor profiles.
Thermistors are extremely small and respond quickly to temperature changes. However, they require a current source, and their small size makes them extremely sensitive to self-heating errors.
Thermistors measure absolute temperature over two wires, offering good accuracy, but they are more expensive than thermocouples and have a shorter measurable temperature range. A common thermistor has a resistance of 5kΩ at 25°C, with each 1°C temperature change causing a 200Ω resistance change. Note that the 10Ω lead resistance introduces only a negligible 0.05°C error. It is ideal for current control applications requiring rapid and sensitive temperature measurement. Its small size is advantageous for space-constrained applications, but self-heating errors must be carefully considered.
Thermistors also have their own measurement techniques. Their small size is an advantage; they stabilize quickly and do not create a thermal load. However, this also makes them quite fragile, as high current can cause them to heat up. Because a thermistor is a resistive device, any current source will generate heat due to power. Power equals the product of the square of the current and the resistance. Therefore, a small current source must be used. Exposing a thermistor to high heat will result in permanent damage.
This introduction to the two types of temperature instruments aims to provide some assistance to everyone in their work and studies.
Selection Notes
1. Does the temperature of the object being measured need to be recorded, alarmed, and automatically controlled? Does it need to be measured and transmitted over long distances?
2. Temperature measurement range and accuracy requirements;
3. Is the size of the temperature sensing element appropriate?
4. In situations where the temperature of the object being measured changes over time, can the hysteresis of the temperature sensing element meet the temperature measurement requirements?
5. Whether the environmental conditions of the object being measured will damage the temperature sensing element;
6. Price guarantee and ease of use.
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