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Thermocouple vs RTD vs Thermistor | Difference between Thermocouple, RTD and Thermistor

This page compares Thermocouple vs RTD vs Thermistor and mentions difference between Thermocouple, RTD and Thermistor w.r.t. functions, advantages and disadvantages. The differences are covered in terms of advantages and disadvantages of each of these temperature sensor devices.

Thermocouple

Thermocouple

The thermocouple is a component used for measurement of temperature. They are very cost effective solution for wide range of temperature measurement with reasonable accuracy. They are used for various applications in boilers, ovens, water heaters, aircraft engines etc.

The figure-1 depicts simple thermocouple construction. As shown in the figure, there are two junctions in the thermocouple "measurement or Hot junction" and "reference or Cold junction".

Two wires of dissimilar metals which are joined together to form Hot junction. At the other end, wires are not joined and are connected with signal conditioning circuits made of copper. As earlier mentioned, this junction which is connecting metals with copper traces is known as "cold junction".

Thermocouple is a differential device as voltage generated at reference juncton depends on temperatures at both measurement junction and reference junction.

thermocouple temperature vs voltage graph

The figure-2 depicts temperature vs voltage graphs for different thermocouple types viz. type E, J, K, R and S. Depending upon use of metal wires thermocouples are available to measure temperature in the range from -200 degreeC to +2500 degreeC.

The most popular thermocouple is type-K thermocouple which consists of chromel® and Alumel® which measures temperature from -200 degreeC to +1250 degreeC.
Refer difference between thermocouple types>>.

Advantages and disadvantages of thermocouple

Following are the advantages of thermocouple:
• They support wide temperature range from -200 degreeC to +2500 degreeC depending upon metal wires used in the construction.
• They can be used in hazardous environments as they are rugged devices and also are immune to shock and vibration.
•  They respond very rapidly to temperature changes as they are small in size and have lower thermal capacity. They respond in few hundred milliseconds.
• They do not require any external power and hence are prone to self heating. Hence they are safe compare to all the other types viz. RTD/thermistor.
• They are simple.
• They are available in wide variety of physical forms.

Following are the disadvantages of thermocouple:
• They are least stable and least expensive.
• Substantial signal conditioning is needed to convert voltage into usable temperature reading.
• The measurement is only as accurate as reference junction temperature measurement, and is approx. within 1 degreeC to 2 degreeC.
• As it is made of dissimilar metals, corrosion results in certain environments and hence this results into deterioration in the accuracy of measurement. Due to this thermocouples need care, maintenance and protection
• Thermocouple measurement at microvolt level will be a problem as noise due to stray electric and magnetic fields will create problem. Due to this, wires are twisted and shielded cables are used to reduce magnetic and electric fields respectively.

RTD | Resistance Temperature Detector

RTD temperature vs resistance curve

As mentioned RTD stands for Resistance Temperature Detector or sensor. RTDs are passive devices usually made of platinum wire whose resistance varies with temperature. platinum RTD is the most common and accurate temperature sensor. It measures temperature from -200 degreeC to +850 degreeC. It has measurement accuracy of +/-0.02 degreeC.

The figure-3 depicts resistance versus temperature relationships for different RTD materials viz. nickel, Balco, copper and platinum.
X-axis: Temperature
Y-axis: normalized resistance w.r.t. resistance at 0 degreeC or 32 degreeF.

Advantages and disadvantages of RTD

Following are the advantages of RTD:
• It has reasonable linearity.
• Higher accuracy
• Lower drift.
• They are most stable among all the temperature sensor devices.
• suitable for precision applications.
• They are more linear compare to thermocouples.

Following are the disadvantages of RTD:
• It requires signal conditioning.
• Cost of RTDs are higher.
• They are slow in operation.
• Need current source for operation.
• They need four wires in measurement.
• The effects known as self heating and lead wire resistance can cause errors in the measurements carried out by RTDs.

Thermistor

thermistor

The Thermistors are devices whose resistance changes with temperature variation. They are referred as temperature dependent resistors. Depending upon different materials in the construction of thermistors there are two types of thermistors viz. NTC (i.e. Negative Temperature Coefficient) thermistor and PTC (Positive Temperature Coefficient) thermistor. In NTC themistor resistance is inversely proportional to the temperature and hence resistance decreases when temperature increases. In PTC thermistor resistance is directly proportional to the temperature and hence resistance increases when temperature increases and viceversa. Most of the thermistors are of NTC type.

They are available in temperature range from -100 degreeC to +150 degreeC. They are used for various applications in batteries, coolants, engines, freezers and incubators for over-temperature shutdown purposes and for monitoring and control of temperatures.
thermistor basics and manufacturers>>

Advantages and disadvantages of thermistor

Following are the advantages of thermistor:
• They are inexpensive.
• They are available in small packages.
• They provide higher output.
• They are fast in operation.
• They need two wire ohms measurement.

Following are the disadvantages of thermistor:
• They achieve higher precision but in limited temperature range.
• They are nonlinear and their nonlinearity can be addressed using software or hardware circuits.
• Thermistors are resistive devices and hence requires excitation current to read the voltage across their terminals. This results into effect known as self heating. In order to limit the self heating error, care must be taken to limit the sensing current to lower value.
• They are nonlinear in operation.
• They are fragile.
• As mentioned they require current source for their operation.

Comparison between thermocouple, RTD and thermistor

comparison between thermocouple vs RTD vs thermistor

The above figure depicts voltage/resistance versus temperature curve in a single plot to compare thermistor, RTD and thermocouple (TC) as shown.

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