1、 1 Heating temperature and pressure test Thermistors are inexpensive, easily-obtainable temperature sensors. They are easy to use and adaptable. Circuits with thermistors can have reasonable outout voltages - not the millivolt outputs thermocouples have. Because of these qualities, thermistors are w
2、idely used for simple temperature measurements. Theyre not used for high temperatures, but in the temperature ranges where they work they are widely used. Thermistors are temperature sensitive resistors. All resistors vary with temperature, but thermistors are constructed of semiconductor material w
3、ith a resistivity that is especially sensitive to temperature. However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature. Thats due to the properties of the semiconductor material that the thermistor is made from. For some, that may be counter
4、intuitive, but it is correct. Here is a graph of resistance as a function of temperature for a typical thermistor. Notice how the resistance drops from 100 kW, to a very small value in a range around room temperature. Not only is the resistance change in the opposite direction from what you expect,
5、but the magnitude of the percentage resistance change is substantial. Temperature Sensor - The Thermocouple You are at: Elements - Sensors - Thermocouples Return to Table of Contents A thermocouple is a junction formed from two dissimilar metals. Actually, it is a pair of junctions. One at a referen
6、ce temperature (like 0 oC) and the other junction at the temperature to be measured. A temperature difference will cause a voltage to be developed that is temperature dependent. (That voltage is caused by something called the Seebeck effect.) Thermocouples are widely used for temperature measurement
7、 because they are inexpensive, rugged and reliable, and they can be used over a wide temperature range. 2 In particular, other temperature sensors (like thermistors and LM35 sensors) are useful around room temperature, but the thermocouple can The Thermocouple Why Use thermocouples To Measure Temper
8、ature? They are inexpensive. They are rugged and reliable. They can be used over a wide temperature range. What Does A Thermocouple Look Like? Here it is. Note the two wires (of two different metals) joined in the junction. What does a thermocouple do? How does it work? The junction of two dissimila
9、r metals produces a temperature dependent voltage. For a better description of how it works, click here. How Do You Use A Thermocouple? You measure the voltage the thermocouple produces, and convert that voltage to a temperature reading. It may be best to do the conversion digitally because the conv
10、ersion can be fairly nonlinear. Things You Need To Know About Thermocouples A junction between two dissimilar metals produces a voltage. In the thermocouple, the sensing junction - produces a voltage that depends upon temperature. Where the thermocouple connects to instrumentation - copper wires? -
11、you have two more junctions and they also produce a temperature dependent voltage. Those junctions are shown inside the yellow oval. When you use a thermocouple, you need to ensure that the connections are at some standard temperature, or you need to use an electronically compensated system that tak
12、es those voltages into account. If your thermocouple is connected to a data acquisition system, then chances are good that you have an electronically compensated system. Once we obtain a reading from a voltmeter, the measured voltage has to be converted to temperature. The temperature is usually exp
13、ressed as a polynomial function of the measured voltage. Sometimes it is possible to get a decent linear approximation over a limited temperature range. There are two ways to convert the measured voltage to a temperature reading. Measure the voltage and let the operator do the calculations. Use the
14、measured voltage as an input to a conversion circuit - either analog or digital. Let us look at some 3 other types of base-metal thermocouples. Type T thermocouples are widely used as are type K and Type N. Type K (Ni-Cr/Ni-Al) thermocouples are also widely used in the industry. It has high thermopo
15、wer and good resistance to oxidation. The operating temperature range of a Type K thermocouple is from -269 oC to +1260 oC. However, this thermocouple performs rather poorly in reducing atmospheres. Type T (Cu/Cu-Ni) thermocouples can be used in oxidizing of inert atmospheres over the temperature ra
16、nge of -250 oC to +850 oC. In reducing or mildly oxidizing environments, it is possible to use the thermocouple up to nearly +1000 oC. Type N (Nicrosil/Nisil) thermocouples are designed to be used in industrial environments of temperatures up to +1200 oC. A polynomial equation used to convert thermo
17、couple voltage to temperature (oC) over a wide range of temperatures. We can write the polynomial as: The coefficients, an are tabulated in many places. Here are the NBS polynomial coefficients for a type K thermocouple. (Source: T. J. Quinn, Temperature , Academic Press Inc.,1990) Type K Polynomial
18、 Coefficients n an 0 0.226584602 1 24152.10900 2 67233.4248 3 2210340.682 4 -860963914.9 5 4.83506x1010 6 -1.18452x1012 7 1.38690x1013 8 -6.33708x1013 What If The Surrounding Temperature Exceeds Limits? There are really no thermocouples that can withstand oxidizing atmospheres for temperatures above
19、 the upper limit of the platinum-rhodium type thermocouples. We cannot, therefore, measure temperature in such high temperature conditions. Other options for measuring extremely high temperatures are radiation or the noise pyrometer. For non-oxidizing atmospheres, tungsten-rhenium based thermocouples shows good performance up to +2750 oC. They can be used, for a short period, in temperatures up to +3000 oC. The selection of the types of thermocouple used for low temperature sensing is primarily based on materials of a thermocouple. In addition, thermopower at low temperatue is
