# Difference Between Thermistor and RTD

Edited by Diffzy | Updated on: April 30, 2023

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## What is temperature?

Temperature is the physical quantity that expresses the coolness or the hotness. It is the measurement of any thermal quantity present in all the matter. It is the flow of energy present in any matter when it comes into contact with another body. However, temperature shall not be confused with heat.

Though people have always known about temperature even when there were no tools to measure it, there was always this certain knowledge about a quantity of measurement known as temperature. We know that fire is hot and ice is cold. Our ancestors knew what the temperature was they just didn’t know how to accurately calculate it. When we feel the first signs of fever, we always check our wrists or our foreheads to know about the estimate temperature to know if we are sick or not.

## History of the measurement of Temperature

As we talked about this earlier, the people in the past who did not have the right instruments to measure the temperature, worked mostly through their intuition. It was a basic concept where people knew exactly what the difference between hot and cold was. Though what they did not know was how to measure the temperature. Like we cannot be sure about the body’s temperature after touching the forehead of a person. Have you ever heard a doctor say that you have 101 degrees temperature after he has just touched your forehead or your wrist? No, that is because this method does not tell us the accurate body temperature.

Galileo invented the first thermometer in 1592, or that is what the records tell us. It was an air thermometer which had a glass bulb attached to a long tube. The tube would be dipped in water and the bulb was warmed, making the air inside expand, some of which even escaped. But as the temperature inside the tube rose, it made the water rise higher on the tube giving us a rough idea of what the temperature actually was. This type of thermometer is sensitive and is affected by the atmospheric pressure.

Through the researches we have found about thirty five different temperature scales by the beginning of the eighteenth century AD. In the year 1714, Daniel Gabriel Fahrenheit invented the alcohol and the mercury thermometers. To make the mercury thermometer, a capillary tube was used which was filled with mercury and the mercury was heated which made all of the air escape from the tube and then the tube was sealed trapping the mercury inside and using it to measure the temperatures. The sealing of the thermometer makes it resistant to the atmospheric pressure. Mercury’s freezing point is -39 degrees hence it cannot measure temperature below that, whereas alcohol’s freezing temperature is -113 degree Celsius allowing him to record temperatures up to that point.

The common Flemish thermometers were divided up to twelve points which was later subdivided by Fahrenheit into ninety six points giving his thermometers more accuracy. In the late eighteenth century, Andres Celsius divided the scale into hundred points as he believed that it would be more advantageous to use more common calibration references. He chose to use 100 degrees as the freezing point and 0 degrees as the boiling point which was reversed later for obvious reasons.

William Thomson proposed the existence of an absolute zero. Sir William Hershel was the person who found that light scatters once it is passed through a prism, and detected an increase in temperature as he moved a black thermometer across the spectrum. He realized that the heat increased at and beyond the red in the region. He then performed the same experiment on the other means of light in the nineteenth century and found out the same results. T J Seedbeck in the year 1821 discovered that heat travelled through the matters, he conducted the experiment by heating a mental and choosing another metal to be unheated, but when the two dissimilar metals came in contact the heat travelled from metal into the other metal and used these constants to compute the total amount of current flowing.  In the same year Sir Humphery Davy discovered that all temperatures have a positive temperature coefficient of resistanc(RTD).

Semiconductors such as Thermistor were discovered in the 20th century.

## What are the scales of measurement of the temperature?

There are three main scales of recording the temperature, namely, Celsius, Fahrenheit and Kevin. They can be converted from one scale to another using the formulas to change the degree of temperature. Apart from these three, the Rankine scale is also another degree to measure the temperatures.

## What are the different tools to measure the temperature?

Although we might know that a thermometer is the most common and the most used means to measure the temperature, we are to note that a thermometer cannot record greater degrees of temperature. Other devices such as :

• Thermocouples; (widely used in science and industry)
• Thermistors; (a type of a resistor)
• Resistance temperature detector (RTD); (sensors used to measure the temperature)
• Pyrometer; (used to measure the temperature of distant materials)
• Langmuir probes; (used to determine the electron temperature)
• Infrared thermometer; (infers temperature from a portion of the thermal radiation)

These are some of the very common and other devices used to measure temperature.

## What is a Thermistor?

A thermistor is a thermal resistor used for the measurement of the temperature which is made up of semiconductor material because of which it has both negative and positive temperature coefficient. The temperature increases with the increase in resistance and it decreases with the decrease in resistance. It makes the temperature inversely proportional to the resistance.

It is used to measure a small range of temperature from -55 degree Celsius to +114 degree Celsius. It is a device used to measure the temperature of home appliances such as the fridge or the AC or the television. It produces the results faster and is highly sensitive and also an expensive measuring device.

The term is derived from the combination of thermal and resistor which gives us a thermistor. A thermistor has two types namely a Negative Temperature Coefficient (NTC) and a Positive Temperature Coefficient (PTC). With an NTC if the temperature increases, the resistance decreases. This is also one of the most commonly used thermistors. However, in a PTC if the temperature rises, the resistance increases and if the temperature decreases then the resistance will also decrease. The PTC is generally used as a fuse.

These are accurate and easily found and easy to use at a given temperature. Although they fail to measure extremely low or extremely high temperatures, it works the best at a short range of temperatures. These are very suitable for when a precise temperature is being measured. It is most commonly used in the ovens or fridges or any other devices that heat up or cool down.

A thermistor does not read the temperature, it instead changes according to the resistance of the appliance it is used for. Unlike other sensors, it is nonlinear, which means when graphed down, it does not form a straight line but instead forms a curve. It is also the best way to control and measure the heating and cooling of the devices in which it is used. The thermistors are very sensitive and react to every small change in the temperature. They are used well for a small and specific temperature within 50 degrees of ambient. Because they can adjust into minute increments they provide the greatest overall system stability. It can be fixed in or on the appliance which needs the monitoring of the temperature. It measuressolidsd, liquids and gases.

## What are the Resistance Temperature Detectors (RTD)?

Resistance temperature detectors also known as resistance thermometers are devices made to accurately resist changes in the variability of the temperature. The increase in the temperature makes the resistance increase as well. It is a passive device which cannot produce an output of its own. An additional external device is used to measure the resistance by passing a small electrical current through the sensor to generate a voltage. It is often made up of pure metals like platinum, nickel, or copper.

The response time from the RTD is slow and is not very accurate. However, it is generally used to measure high temperatures up to 850 degrees Celsius. While computing the reading of the RTD we get a liner graph. It is not very sensitive and is quite huge but despite that it is cheap. The resistivity level of the Resistance Temperature Detector is very high and this is what is being regulated with the help of this device. It not only regulates the resistivity with the increase of the temperature but also has a low hysteresis effect.

With the amount of work done by an RTD, it is generally used in the industries for measuring a large number of temperatures.

Platinum is generally used in the RTD because it is capable of holding onto high temperatures and it is also a very stable pure metal. The metals are used in the RTD because of their high–temperature coefficient which means that the temperature of the metal increases with a rise in the temperature and this property makes the metals an ideal material for the RTD. Many RTD consists of a long wire wrapped around a glass or a ceramic core but other constructions might also be used and because of its fragility it is stored in protective probes. RTDs that have higher accuracy and repeatability are replacing the thermocouples in the industry applications that use below 600 degrees Celsius.

The use of platinum in RTD was suggested by Sir William Siemens because he studied that it is a noble metal and has one of the most stable resistance–temperature relationships over the largest temperature range. Nikel elements have a limited temperature range because the change in resistance per degree of the temperature change becomes nonlinear when the temperature exceeds 300 degrees Celsius or 572 degrees Fahrenheit. Copper has a very liner temperature – resistance relationship but it oxidizes at moderate temperatures and cannot be used over 150 degrees Celsius or 302 degrees Fahrenheit. Thus, we conclude that platinum is the best metal to be used for an RTD, while the others are useful too but there have to be certain precautionary measures required while using copper or nickel.

## Conclusion

Although the RTD and the thermistor are both temperature sensing devices both have very different functions in relation. While RTD is used for measuring high temperatures up to 850 degrees Celsius and is used in the industries, the thermistor is used for a fixed temperature from -55 degrees Celsius up to +114 degrees Celsius, which are usually measured in the home appliances. The resistance provided by the RTD is lesser than the resistance provided by the thermistor and an RTD is less expensive than a thermistor. An RTD gives slow results in comparison to the immediate results of the thermistor.  A thermistor is a highly accurate device due to its negative temperature coefficient which allows it to record even the smallest of temperature changes whereas the RTD may not be that accurate. The thermistor is made up of semiconductor materials and the RTD is made up of either platinum or nickel or copper. The characteristic graph of a thermistor is nonlinear and the characteristic graph of the RTD is linear.

## References

• https://in.omega.com/prodinfo/thermistor.html
• https://www.ametherm.com/thermistor/what-is-a-thermistor/
• https://www.te.com/usa-en/industries/sensor-solutions/insights/understanding-rtds.html
• https://www.electrical4u.com/resistance-temperature-detector-or-rtd-construction-and-working-principle/

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"Difference Between Thermistor and RTD." Diffzy.com, 2024. Mon. 10 Jun. 2024. <https://www.diffzy.com/article/difference-between-thermistor-and-rtd-726>.

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