Resistance thermometers or otherwise called Resistance Temperature Detector is used to measure temperature. We all know that temperature is a measurement of the degree of hotness or coldness. So RTDs can measure temperatures ranging from -200 to 600 degrees celsius.
RTDs are positive temperature coefficient meaning resistance is directly proportional to the temperature changes, When the temperature increases so do the resistance. The output of the RTD is measured in resistance and converted to temperatures. In industries, the temperature is measured in degrees celsius.
RTD is a passive device and has its own advantage. They can be more accurate than a thermocouple. It is made of pure metal or alloy, typically platinum, nickel, and copper. Platinum made RTDs are mostly used because of its Linear relationship between Temperature and resistance.
Among Platinum made RTDs pt100 and pt1000 are the most common types and widely used in all types of industries from ships to chemical industries.
Pt100 has a resistance value of 100 ohms at 0 degrees celsius hence the name. Pt1000 has 1000 ohms at 0 degrees celsius.
The resistance of the pt100 increases as the temperature increases. More precisely for every degree rise in temperature, the resistance of the pt100 will increase 0.4ohms.
Likewise, for pt1000 for every degree rise in temperature, the resistance of pt1000 will increase 4 ohms.
The pt100 sensors come in both wire wound and thin film constructions but pt1000 sensors come only in a thin film.
The pt100 sensors have 2-wire, 3-wire, and 4-wire configurations. But the pt1000 has only 2-wire configuration and pt1000 is more accurate than pt100.
In pt1000 the error could be 0.1 degrees celsius but in pt100 the error could be 1 degrees celsius.
So we saw the theoretical part. Now let's see how the RTD's resistance change is measured and the formula to convert it to the temperature.
Yes, the RTD will be connected to a Wheatstone's bridge to measure the resistance changes.
To convert it to the temperature this formula is used,
Rt = R0(1 + αt)
Rt - is the known resistance for which the temperature has to be calculated.
R0 - Is the resistance of the RTD at 0 degrees celsius.
α - Is the temperature coefficient.
t - Is the unknown temperature.
The formula for the temperature coefficient of pt100 is,
α = R100 - R0 / R0 * 100
R100 - is the resistance of RTD at 100 degrees celsius. For pt100 the resistance is 138.5
After substituting the value in the formula the value of alpha (temperature coefficient) will be 0.00385/degree celsius
So let's say we have the resistance value measured as 80 ohms now let's convert it to the temperature.
80 = 100(1 + 0.00385t)
After doing the math our temperature will be,
t = -51.9 degrees celsius.
Here is an image showing an RTD connected inside to a vessel taken from my mobile in an industry
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