RTDs vs Thermocouples

Working Principles and When to Use Each

This article will focus on the qualities of RTDs vs Thermocouples and their typical applications and principles.

Measuring temperatures in industrial engineering is a key part of monitoring the operation of the various mechanical, electrical, and electronic systems, determining their performance, and evaluating their health.

Likewise, in chemical processes, temperature and heat control may be crucial in achieving the desired end product. Additionally, the same applies to welding, heat treatment processes, industrial ovens used for plastic shrinking, and a wide range of other possible applications. That said, measuring temperatures is neuralgic in a wide range of engineering operations, and doing so with accuracy and precision is equally important.

While there are many types of thermometers that can be used for measuring temperatures including the familiar “mercury” thermometers, the bimetallic, and the vapor pressure thermometers, those that are most commonly and widely used in the industrial environment are the resistance temperature detectors (RTD) and the thermocouple devices. Both of these thermometer types have their respective advantages and drawbacks, and they are both suitable and valuable for a distinctively different set of applications.

Working Principle of Resistance Thermometers (RTD)

The RTD temperature sensor is based on the temperature dependence of the electric resistance of metals. As the temperature increases for metal, so does its electrical resistance. Of course, there’s an intrinsic coefficient of resistance for all materials, and a positive value for this coefficient makes specific materials better than others for the measurement element role. With nickel and platinum, for example, there is near-perfect linearity that is introduced by their respective coefficients, resulting in high accuracy and precision across repeated measurements. Moreover, nickel and platinum are generally chemically stable and extremely resistant to corrosion.

Drawing of a Coil in Tube RTD.  Read the article for a breakdown of RTDs vs Thermocouples.
Coil-element Platinum Coil In Tube RTD. Source: Burns Engineering. Licensed Under CC-BY-SA-3.0
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Industrial Automation Terms You Should Know

Industrial Automation panel with digital technology and LEDs
Industrial Automation covers a lot of ground.

Here’s a list of industrial automation terms you may need to be defined as you’re looking at our extensive catalog of parts. 



The commonly available electric power supplied, an AC generator and is distributed in single or three-phase forms. AC current changes its direction of flow (cycles).


A motor (see motor definition) operating on AC current that flows in either direction (AC current). There are two general types: induction, and Synchronous.


The amount of steel (iron) in the stator and rotor of a motor. Usually, the amount of active iron is increased or decreased by lengthening or shortening the rotor and stator (they are generally the same length).


The space between the rotating (rotor) and stationary (stator) member in an electric motor.


Used on motors with blowers to measure the difference in pressure across the filter so as to detect a clogged filter.


A device used in air hooded motors to detect the temperature of the exhausted air. When used in this manner an air temperature switch will detect blockage in the cooling air system or long-term motor overload.


The atmospheric altitude (height above sea level) at which the motor will be operating; NEMA standards call for an altitude not exceeding 3,300 ft. (1,000 meters). As the altitude increases above 3,300 ft. and the air density decreases, the air stability to cool the motor decreases – for higher altitudes, higher grades of insulation or a motor derating are required. DC motors require special brushes for high altitudes.


The temperature of the surrounding cooling medium, such as gas or liquid, which comes into contact with the heated parts of the motor. The cooling medium is usually the air surrounding the motor. The standard NEMA rating for ambient temperature is not to exceed 40ƒC.


An anti-friction bearing is a bearing utilizing rolling elements between the stationary and rotating assemblies.


The portion of the magnetic structure of a DC or universal motor which rotates


Rated full load armature circuit current.


The armature inductance in milli-henries (saturated).


The current that flows in the armature winding of a DC motor tends to produce magnetic flux in addition to that produced by the field current. This effect, which reduces the torque capacity, is called armature reaction and can affect the commutation and the magnitude of the motor’s generated voltage.


The armature resistance is measured in ohms at 25ƒ C. (cold)


The force or loads that are applied to the motor shaft in a direction parallel to the axis of the shaft. (Such as from a fan or pump)