Difference Between RTD and Thermistor

Both RTD and Thermistor are used to detect the change in temperature. However, the crucial difference between RTD and Thermistor lies in the material of their construction. RTD is constructed with materials that are pure metals whereas thermistors are fabricated with semiconductor materials of ceramic or polymers.

Also the two shows variation according to the temperature range provided. As RTD provides wider temperature range in comparison to Thermistor.

Let us proceed towards the contents to be discussed under this article and then we will understand other major differences between the RTD and thermistor.

Content: RTD Vs Thermistor

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Advantages
  5. Disadvantages
  6. Conclusion

Comparison Chart

ParameterRTDThermistor
Fabrication materialPure metals (for eg platinum, tungsten etc)Semiconductors (for eg manganese, cobalt etc)
Symbolic representationsymbol of rtdsymbol of thermistor
Response timeSlow (about 1 to 50 s)Fast (about 0.12 to 10 s)
Temperature rangeLarge (between -100 to 650°C)Small (between -15 to 60°C)
SensitivityLowHigh
CostHighLow
Resistance versus temperature characteristicLinearNon-linear
SizeLargeSmall
Repeatability / ReproducibilityHighLow
Cable length permissibleSmallGreater than RTD because of providing errors to acceptable limits.
AccuracyRTD generates less accurate resultsThermistor produces highly accurate results.
RecalibrationEasyIts recalibration is somewhat tough as compared to RTD
ApplicationsIt is widely used in industrial applicationsIt is widely used in home appliances

Definition of RTD

RTD is an abbreviation used for resistance temperature detector. It is also known as a resistance thermometer. RTD is a device that is used to determine the temperature in terms of resistance change. As RTD utilizes the property of change in resistance with the corresponding change in temperature.

Also, a small difference in temperature is also measured by the RTD.

It is such a device that does not hold the property of producing its own supply. Thus, it needs a voltage source in order to perform the circuit simulation.

Now, the question arises how an RTD is used to determine temperature change?

Suppose some voltage is applied to the sensing element of the RTD. Then, it causes the molecules of the metal to dislodge and collide with each other. With the increase in applied voltage, the probability of collision of molecules somewhat increases.

Thus, their movement somewhat disturbs and this causes the resistance of the device to get increased. Thereby, raising the temperature corresponding to RTD.

The materials that are used as resistance elements in RTD must possess a high-temperature coefficient of resistance along with high resistivity and linearity. Also, it must show stability towards electrical characteristics of the metal.

Platinum is majorly used in the construction of RTD. However, nickel and copper are also used in its construction.

Definition of Thermistor

The thermistor is a device that is composed of the word thermal and resistor. As its resistance changes with the variation in temperature associated with it.

These are basically those resistors that show thermally sensitive nature.

These are composed of semiconductor materials or can be manufactured with the combination of oxides of metal like manganese, cobalt etc.

disc form of thermistor

These exhibits non-linear characteristic of resistance vs temperature. Its resistance at a given temperature T is given by:

eq1

: α and β are the two constants that entirely depends on the material used for construction as well as construction technique involved.

Thermistors possess both positive and negative temperature coefficient of resistance. But, most of the time it possesses a negative temperature coefficient of resistance. As only in case of few thermistors resistance increases with the increase in temperature.

Now, a question strikes our mind that how thermistor possess negative temperature coefficient of resistance?

Suppose some temperature is provided to the material, this causes the carrier concentration to increase at a rapid rate. This causes to generate more current through the device. Resultantly, the resistance of the device decreases. Up to a temperature of around 150 °C, a large decrease in resistance is noticed.

But, it is to be noted in this case, that with the rise in current, more power gets dissipated through the device. This, in turn, increases the temperature associated with the device.

Hence, we can say that resistance also depends on the power dissipation factor of the device.

Key Differences Between RTD and Thermistor

  1. The factor that generates a key difference between RTD and thermistor is the response time provided by the device. As the response time for thermistor is better as compared to RTD. This is so because thermistor generates faster results as compared to RTD.
  2. The relation between change in resistance with respect to temperature is linear in the case of RTD. While thermistor exhibits non-linear characteristics between change in resistance with respect to temperature.
  3. The temperature range provided by the RTD lies between -100 to 650 °C. However, in the case of the thermistor, the temperature range is -15 to 60 °C.
  4. The materials used for the construction of RTD are basically platinum, copper and tungsten etc. While, the materials used for the fabrication of thermistor are manganese, cobalt, aluminium etc.
  5. The size of the thermistor is somewhat smaller than that of the RTD as large size of RTD sometimes proves its disadvantageous factor.
  6. The length of cable attached to the device also affects the resistance of the device in case of temperature change. But, thermistors show a better response than RTD. So, the application of long cable wires in case of thermistors does not affect the overall performance of the system.
  7. RTD exhibits positive temperature coefficient of resistance, that means its resistance increases with the change in temperature. However, thermistor majorly possesses a negative temperature coefficient of resistance, this simply means that its resistance decreases with the corresponding change in temperature.
  8. The recalibration in case of RTD is somewhat easier than that of the thermistor.
  9. RTD offers low hysteresis while thermistors offer high hysteresis.
  10. RTD provides less change in resistance with temperature. However, thermistor gives large but non-linear change in resistance with respect to temperature.

Advantages

RTD

  1. The replacement of the sensing element is not difficult.
  2. They are also used to measure the small differential temperature.
  3. It possesses a high degree of repeatability.

Thermistor

  1. It is small in size.
  2. The thermistor is a low-cost device.
  3. It is a highly sensitive device.

Disadvantages

RTD

  1. RTD requires a voltage source along with bridge circuit.
  2. It is a high-cost temperature detector.
  3. It is large in size.

Thermistor

  1. It is not suitable for a large temperature range.
  2. It shows non-linear resistance versus temperature characteristic.

Conclusion

So, from the above discussion, we can conclude that though RTD and thermistor both show a change in resistance with respect to temperature. But, thermistors provide more accurate results as compared to RTD. Also, thermistors are more immune to noise effects as compared to RTDs.

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