What is a Temperature Sensor? (RTD, Thermocouple, Thermistor)

In your daily life and different branches of industries, there are many cases in which it’s required to know the temperature of the environment, inside a reactor, winding of an electrical machine and so on. This aim will be achieved by using a Temperature Sensor.

Temperature Sensor Technologies

Over the years we have invented different technologies to measure the temperature in different specific applications. But why different technologies?

This is because of different applications which make one technique better than the others for each specific case and there’s no unique way recommended for all applications.

Most likely you’ve heard about “RTD”, “Thermocouple”, “Thermistor”, “Semiconductor” type elements and so on, which will be addressed here.

Temperature Transmitter

Before I go into details of this subject, let’s see what a “Temperature Sensor” (Temperature Transducer) is and what does a “Temperature Transmitter” mean.
Generally, a sensor or transducer is a physical device which is capable of transforming one type of process variable to my favorite signal type.

To elaborate on this generalized sentence, let me give you an example.

Temperature, pressure, flow, etc., are some process variables and actually, they are physical characteristics of our real world.
With modern technology and because of tremendous advances in Electrical Engineering in the past century, we like to transform every measurable process value into an electrical signal and a temperature sensor is a device which will transform the Temperature into an electrical signal, no matter how tiny the amount of this signal might be!

So far I took a big “First Step” which was the transformation of “Temperature” into “Electrical Signal”.

Based on different sensor technologies, this signal may have different ranges and for industrial applications, I need to have my signals limited to some universally accepted electrical “signal-ranges”.

Some of these globally accepted electrical signal-ranges are:

– Four to twenty milliamps (4-20 mA)

– One to five volts (1-5 V)

– Zero to ten volts (0-10 V)

A “Temperature Transmitter” is a device which transforms the tiny output of a “Temperature Transducer” to one of these standard signal ranges.

Resistance Temperature Detectors (RTD)

Now let’s get back to different “Temperature Transducer” technologies.

RTD or “Resistance Temperature Detector” is a device the resistance of which varies with the temperature.

Since it is a “passive” device, an external electrical current should be applied to it and then the voltage drop across it can be measured. This voltage is a good indication of the temperature.

When referring to such a device as “passive”, it means that the device needs external current (or voltage) source.

To state the obvious, big amount of external current can cause power dissipation in the resistor of RTD and lead to excess heat, so to avoid this type of error, the current should be kept at a minimum level.

This wiring diagram shows the simplest application of an RTD, called “two-wire” configuration.

More accurate reading calls for 3-wire or 4-wire configurations.

In reality, the distance between the temperature sensing point and measuring system calls for wiring and since the real wiring has its own resistance, some measurement error sneaks in hereby!

Three-Wire (3-wire) and Four-Wire (4-wire) solutions are developed to remove this error.

One of the most common RTDs is “PT100” which consists of a thin film of Platinum on a plastic film and shows a resistance of 100Ω at 32°F.

Its resistance varies with temperature and it can typically measure temperatures from -330 to 1560°F.

The relationship between resistance and temperature of PT100 is relatively linear.

PT100 is just an example of platinum RTDs and in the industry you may find different RTD types suitable for various applications such as Copper, Nickel, Nickel-Iron, etc.

Thermistors

Thermistors are temperature-dependent resistors and are widely used in industrial purposes, such as:

– Over-current protection

– Self-regulating heating elements

– Inrush current limiters

Thermistors can be NTC or PTC.
In NTC (Negative Temperature Coefficient) thermistors, resistance decreases as temperature rises. NTC’s are commonly used as inrush current limiters.

With PTC (Positive Temperature Coefficient) thermistors, resistance increases as temperature increases. PTC thermistors are commonly used as overcurrent protection and in resettable fuses.

Thermocouples

A thermocouple or simply “TC” is comprised of a couple of specific dissimilar wires joined together, forming the “sensing point” or “junction”.

Based on physical characteristics called “Thermoelectric Effect”, when this junction is placed at different temperatures, different millivolt signals are generated which can be interpreted as an indication of the temperature.

In comparison with RTDs, Thermocouples are self-powered and require no external excitation current source.

Thermocouples are commonly used for furnaces, Gas Turbine combustion chamber, high-temperature exhaust ducts, etc.

The main restriction of Thermocouples is the “accuracy” which doesn’t make it the best solution for precise applications.

Also, Thermocouples need a reference measurement point called “Cold Junction”.

The thermocouple junction is often exposed to extreme environments, while the cold junction is often mounted near the instrument location.

Based on “range” of temperature measurement, “sensitivity” and some other factors in each application, different types of Thermocouples are available, for example E, J, K, M, N, T and so on.
For instance, Type “J” is made up of “Iron-Constantan” combination with a range of −40°F to +1380°F and sensitivity of about 27.8 µV/°F .

while Type “K” (Chromel-Alumel) is one of the most common general-purpose thermocouples with a sensitivity of approximately 22.8 µV/°F.

Type K is inexpensive and a wide variety of probes are available in its −330°F to +2460°F operating range.

Since the functionality of thermocouples is based on Thermoelectric Effect in different types of conductors, when the location of a thermocouple is far from the “measuring instrument” (e.g. electronic transmitter), the proper type of conductors should be used for extension purpose. Otherwise, the tiny signal generated by thermocouple will be added with some error at the point where thermocouple wires are connected to the extension wire!

Semiconductor Temperature Sensors

“Semiconductor Temperature Sensor” is based on the fact that the junction voltage across a p-n combination of semiconductors, like a diode junction or “base-emitter” junction of regular transistors, is a function of temperature.

This technology is vastly used in electronic devices and IC technologies.

Linear characteristic, small size, and low cost are advantages of this technology, but it should be noted that the limited range of around -40°F to 248°F makes it suitable for specific applications.

Also alongside the technical bonus-malus comparison, sometimes cost is the key factor in selecting the proper device, as they say: “Money Talks!”

Comparison between Temperature Sensors

To wrap up this video, the comparison between different types of temperature sensor technologies is a multi-facet task.

For example, if “accuracy” is considered as the key performance indicator, usually RTD’s are better than Thermocouples; approximately 10 times more accurate.

From the “sensitivity” point of view, while both RTDs and Thermocouples respond quickly to temperature changes, at similar costs, thermocouples are often faster.

If I have to measure electronic PCB and/or IC temperature, silicon-based types are the best choices.

Also alongside the technical bonus-malus comparison, sometimes cost is the key factor in selecting the proper device, as they say: “Money Talks!”

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The RealPars Team

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