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A temperature measurement is one of the most common measurements. There are many different sensors available for this. Some of the most common ones are thermocouple (K, J type), thermistor (PTC, NTC), thermal resistance (RTD) and others. Let's look at the two most common types.
Thermocouple
First is the thermocouple, where we connect two wires together. Due to the difference in material properties, we can measure a small voltage (in a region of milivolts) on the open ends of the wires. We need to connect two different materials on the hot point as well as on the cold point, and then the voltage is almost proportional to the temperature difference between those two points. The cold point should have a known temperature. For this purpose it is necessary to measure the temperature at the cold point (as all Dewetron modules do). This is called CJC or cold junction compensation. The voltage is also not linear with temperature and therefore we need to perform a linearization, either in the hardware or in the software. The accuracy of such measurement with "home built" sensors is usually not better than 1 deg C, but we can increase this by using thermocouple sensors.
The temperature range of thermocouples is huge. Depending on the type we can measure up to 1800 deg C. There are different types available, but the most commonly used one is K type, which has a range from -200 to 1200 deg C, while type E is better suited for low temperatures. Type N has similar properties as K type, but better stability and better resistance to high temperature oxidation.
B, R and S include platinum, and are therefore more expensive, but they offer better high temperature stability and are used for these applications.
T type consists of copper and constantan. Since they are both non-magnetic, they are often used to take measurements for generators with strong magnetic fields.
RTD
RTD stands for resistive temperature detectors. The theory behind this sensor is that the resistance of the sensor changes with temperature. These sensors are more expensive than thermocouples, and can only go up to 600 deg C (800 in some versions), but they are more accurate. The best known RTD is the Pt100. The name tells us that the base material is Platinum and nominal resistance is 100 Ohms at 0 deg C. The accuracy is better than thermocouples - below 0.3 deg C.
This tutorial will now demonstrate how temperature measurements can be performed in DEWESoft. On the next page, we will even be invited to a cup of tea!
