Temperature Sensor

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How to Choose The Correct Temperature Sensor?

The use of various physical properties of materials with temperature variation of the sensor that converts temperature to electricity. These show regular changes of the physical nature of the main body. Temperature sensor is a core part of the temperature measuring instruments, which is a wide variety. By measuring method it can be divided into two major categories: contact and non-contact. According to characteristics of the sensor materials and electronic components, sensor are divided into two types of RTD and thermocouple. If you want to make reliable temperature measurements, you need for your application to select the correct temperature sensor. Thermocouple, thermistor, platinum resistance (RTD) and the temperature is the most commonly used IC temperature sensor in the test。

Thermocouple

Thermocouple is the most commonly used sensor in the temperature measurement. Its main advantage is a wide temperature range and adapt to the atmospheric environment, and strong, low price, without power, especially the cheapest. Thermocouple connected at one end by the two different metal lines form, when heated at one end of the thermocouple, thermocouple circuit potential difference there. Potential difference measurements can be used to calculate the temperature.

However, the nonlinear relationship between voltage and temperature is shown as below. And because of this, the need for the reference temperature (Tref) for the second measurement, and the use of test equipment software and / or hardware within the instrument handle voltage - temperature changes, to finally get the thermocouple temperature (Tx). Agilent34970A and 34980A data acquisition system equipped with an internal measure of computing power.

In short, the thermocouple is the simplest and most common temperature sensors, but the thermocouple is not suitable for precision applications.

Thermistor

Thermistor is a
semiconductor
semiconductor material; most of them are negative temperature coefficient, i.e. resistance decreases with increasing temperature. Temperature changes will cause a large resistance to change, so it is the most sensitive temperature sensor. But poor linearity of the thermistor, and a great relationship with the production process. Manufacturers to not standardized thermistor curves.

Thermistor is very small; the response to temperature change is also fast. But need to use the thermistor current source, the Small Size also makes it extremely sensitive to the self-heating errors.

Thermal resistance measurement in the two lines is the absolute temperature and better accuracy, but it is expensive even than the heat can be measured is less than the thermocouple temperature range. A common thermistor resistance at 25 ℃ to 5kΩ, per 1 ℃ temperature change caused by changes in the resistance 200Ω. Note 10Ω lead resistance caused only a negligible error of 0.05 ℃. It is ideal for fast and sensitive measurement of current Temperature Control applications. Small size for applications requiring a space is beneficial, but must be taken to avoid self-heating errors.

Measurement Techniques

Thermistor is small is an advantage, it can quickly stabilize and will not cause heat load. But also it not enough strong, high current will cause self-heating. As the thermistor is a resistive device, any current source will be in on it because of power caused by heat. Power equal to the square of the current and the resistance of the product. Therefore, to use a small current source. If the thermistor is exposed to heat, will result in permanent damage.

Platinum Resistance Temperature Sensor

Similar to the thermistor, platinum resistance temperature detector (RTD) is made of heat-sensitive platinum resistance. When the voltage is calculated by measuring the RTD temperature, digital multimeter measuring current source with a known source of the current generated by voltage. The voltage of two lead (Vlead) plus the voltage drop on the voltage on the RTD (Vtemp). For example, commonly used RTD resistance of 100Ω, per 1 ℃ produced only 0.385Ω resistance changes. 10Ω per lead if there is resistance, it will result in 26 ℃ for measurement error, which is unacceptable. It should be 4-wire Ohm RTD measurement.

RTD is the most accurate and stable temperature sensor, and its linearity over the thermocouple and thermistor. But also the slowest and most expensive RTD temperature sensor. Therefore, the most suitable for precision RTD strict requirements, speed and price is not a key application.

Temperature IC

Temperature integrated circuit (IC) is a digital temperature sensor, it has a very linear voltage / current - temperature relationship. Some even have representatives of the temperature sensor IC, and the microprocessor can be read directly form the digital output.

Two temperature Ics have the following temperature relationship:

1. Voltage IC: 10 mV / K.

2. Current IC: 1μA / K.

The output temperature IC output is very linear voltage / ℃. Voltage is actually produced / Kelvin, so when the room temperature 1 ℃ output of about 3V. Temperature IC requires external Power Supply. Temperature of IC is usually embedded in the circuit and not used for detection.

Temperature IC drawback is the limited range of temperature, there are also the same self-heating is not strong and needs external power supply problems. In short, the IC provides temperature that is proportional to the temperature readings readable way. It is cheap, but also by the configuration and the speed limit.

Measurement Techniques:

• Temperature IC bulky, so it changes slowly, and may result in heat load.

• Take the temperature IC to the close room temperature occasion; this is its most popular applications. Although the range is limited, but can also measure the temperature of 150 ℃.

We have discussed the various types of commonly used temperature sensors the advantages and disadvantages. If you understand the necessary balance for your application carefully select the right sensor, you can avoid common shortcomings and reliable temperature measurement

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