How to pick the right temperature sensor
Release time：2020-02-21 Clicks：717
How to pick the right temperature sensor
A sensor that uses various physical properties of a material to change with temperature and converts temperature into electricity. The physical properties of these regular changes are mainly physical. The temperature sensor is the core part of the temperature measuring instrument. According to the measurement method, it can be divided into two types: contact and non-contact. According to the characteristics of the sensor material and electronic components, it can be divided into two types: thermal resistance and thermocouple. If you want to make reliable temperature measurements, you need to choose the right temperature sensor for your application. Thermocouples, thermistors, platinum resistance (RTD), and temperature ICs are the most commonly used temperature sensors in testing.
1 Thermocouple
Thermocouple is the most commonly used sensor in temperature measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is sturdy and low-cost, without the need for power supply, especially the cheapest. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end, as shown in Figure 1. When one end of a thermocouple is heated, there is a potential difference in the thermocouple circuit. The measured potential difference can be used to calculate the temperature.
However, the voltage and temperature have a non-linear relationship as shown in Figure 2. Since the voltage and temperature are non-linear, the temperature needs to be measured a second time for the reference temperature (Tref). The instrument handles the voltage-temperature conversion internally to finally obtain the thermocouple temperature (Tx). The Agilent 34970A and 34980A data collectors have built-in measurement capabilities.
In short, thermocouples are the simplest and most versatile temperature sensor, but thermocouples are not suitable for high-precision applications.
2 Thermistor
Thermistors are made of semiconductor materials, and most of them have a negative temperature coefficient, that is, the resistance value decreases with increasing temperature. Temperature changes cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of the thermistor is extremely poor, and has a great relationship with the production process. The manufacturer cannot give a standardized thermistor curve.
Thermistors are very small and respond quickly to temperature changes. However, the thermistor requires a current source, and its small size also makes it extremely sensitive to self-heating errors.
Thermistor measures absolute temperature on two lines with better accuracy, but it is more expensive than a thermocouple and the measurable temperature range is smaller than a thermocouple. A common thermistor has a resistance value of 5kΩ at 25 ° C, and each 1 ° C temperature change causes a 200Ω resistance change. Note that the lead resistance of 10Ω only causes a negligible 0.05 ° C error. It is ideal for current control applications that require fast and sensitive temperature measurements. Small size is advantageous for applications with space requirements, but care must be taken to prevent self-heating errors.
2.1 Measurement Techniques
小 The small size of the thermistor is an advantage, it can be quickly stabilized without causing thermal load. However, it is also very weak, and large currents can cause self-heating. Since the thermistor is a resistive device, any current source will generate heat on it due to power. Power is equal to the product of the square of the current and the resistance. So use a small current source. If the thermistor is exposed to high heat, it will cause permanent damage.
3 Platinum resistance temperature sensor
Similar to thermistors, the platinum resistance temperature sensor (RTD) is also a thermistor made of platinum. When calculating the RTD temperature by measuring the voltage, the digital multimeter measures the voltage generated by the current source with a known current source. This voltage is the voltage drop across the two leads (Vlead) plus the voltage across the RTD (Vtemp). For example, the resistance of a common RTD is 100Ω, and only 0.385Ω changes in resistance per 1 ° C. If each lead has a 10Ω resistance, it will cause a measurement error of 26 ° C, which is unacceptable. Therefore, 4-wire ohmic measurements should be made on the RTD.
RTD is the most accurate and stable temperature sensor, its linearity is better than thermocouple and thermistor. But RTDs are also the slowest and most expensive temperature sensors. Therefore, RTDs are best suited for applications where precision is critical, and speed and price are not critical.
3.1 Measurement Techniques
· Using a 5mA current source will cause a temperature measurement error of 2.5 ° C due to self-heating. It is therefore extremely important to minimize self-heating errors.
· 4-wire measurement is more accurate, but requires twice as many leads and twice as many switches.
4 Temperature IC
Temperature integrated circuit (IC) is a digital temperature sensor that has a very linear voltage / current-temperature relationship. Some IC sensors even have digital outputs that represent temperature and can be read directly by the microprocessor.
4.1 Two types of temperature ICs with the following temperature relationship
· Voltage IC: 10 mV / K.
· Current IC: 1μA / K.
The temperature IC's output is a very linear voltage, ° C. The actual voltage is Kelvin, so the output at 1 ° C at room temperature is about 3V. The temperature IC requires an external power supply. Usually the temperature IC is embedded in the circuit and not used for detection.
缺点 The disadvantage of the temperature IC is that the temperature range is very limited, and it also has the same problems of self-heating, ruggedness and requiring external power. In summary, the temperature IC provides a way to produce an easy-to-read reading that is proportional to temperature. It's cheap, but also limited by configuration and speed.
4.2 Measurement Techniques
温度 Temperature ICs are bulky, so they change slowly and may cause thermal loads.
· Use the temperature IC for applications near room temperature. This is its most popular application. Although the measurement range is limited, high temperatures of 150 ° C can also be measured.
5 Conclusion
We have discussed the advantages and disadvantages of various commonly used temperature sensors. If you understand the necessary trade-offs and carefully select the right sensor for your application, you can avoid common pitfalls and achieve reliable temperature measurements.
