July 28, 2021 Applied Measurements Blogs Comments Off

Most temperature sensors fall into one of three categories: Thermocouples, RTDs or Thermistors. Selecting the most suitable temperature sensor for your application is about more than just determining the temperature range you wish to measure. First you need to understand the application and what is required, then you can assess the different temperature technologies and benefits of each.

What is a thermocouple temperature sensor?

A thermocouple-based temperature sensor is a versatile and cost-effective technology used in a variety of temperature measurement processes. A thermocouple is constructed of two dissimilar metals that are joined at one end creating a junction; when that junction is heated or cooled, it creates a non-linear voltage that is correlated with the temperature. In contrast to most other methods of temperature measurement, thermocouples are self-powered.

Wide temperature range
Cost effective
Fast response times
Tip sensitive

Accuracy degrades with time
Low voltage output

What is an RTD temperature sensor?

RTD stands for resistance temperature detector. This type of sensor utilises electrical resistance when measuring temperature. When the temperature changes, so too does this sensor’s resistance. When the temperature increases, the resistance increases, and when the temperature decreases, the resistance decreases as well. RTD sensors are passive, meaning they require external electronic devices to measure their resistance as they don’t produce a temperature output on their own.

Good level of accuracy when measuring temperature
Long term stability
High level of repeatability

RTDs tend to have limited temperature ranges
Can be subject to self-heating errors
Less rugged than other types
Not as cost effective as Thermocouple or Thermistor sensors

What is a thermistor temperature sensor?

Thermistor temperature sensors are widely known to be very accurate temperature sensors. There are two different types of thermistor sensors:

NTC (Negative Temperature Coefficient) thermistors use the resistance properties of ceramic/metal composites to measure temperature. An NTC’s resistance decreases while the temperature increases. NTC’s are the more commonly used thermistor, fitting a wide range of applications.

PTC (Positive Temperature Coefficient) thermistors also provide variable resistance based on temperature. Specific scenarios require a PTC thermistor over an NTC thermistor, including equipment with a near-zero reset time, extreme temperature conditions and systems that experience frequent shorts.

High temperature accuracy
High sensitivity
Small in size
Relatively low cost
Quick response time
Reasonable long-term stability
Long term repeatability

Limited temperature range
Nonlinear resistance-temperature relationship
Susceptible to moisture
Aging drift

What are the three types of temperature sensors constructed of?

Typically, RTDs are manufactured with base metals such as platinum, nickel or copper as these materials have a positive temperature coefficient that is very linear and repeatable.

Thermocouples consist of two dissimilar metals, joined together at one end. Each combination provides a different calibration with unique temperature ranges and sensor characteristics. A small voltage is produced by the two metals, which can be measured and interpreted by a control system.

Thermistors are normally constructed from metal oxides, such as cobalt, nickel, iron, manganese and titanium. The resistance change with temperature is high compared with the metallic resistances and is usually negative. The temperature characteristics are highly non-linear.

When to use each type of temperature sensor

The applications for RTD sensors are very broad, including medical, aerospace, automotive, instrumentation, motor control and HVACR. TE Connectivity’s (TE) RTD temperature sensor elements are designed to provide precise, stable measurements in extreme temperature applications. Typical operating temperature range is from – 50°C to + 600°C but special designs allow usage from – 200°C to + 1000°C.

Applications for thermocouples include everything from industrial process control to commercial ovens and heaters, home appliances, jet engine exhaust gas temperature and even some special medical applications. The two most common types of thermocouples are Type K and Type J. Type K thermocouples work in most applications because they are nickel based and have good corrosion resistance. Type J thermocouples are good for general purpose applications if moisture is not present.

Thermistor sensors work well for everyday objects and processes such as microwaves, cars, household refrigerators, and digital thermometers. Environments such as these generally do not require a wide measuring range, and being cost-effective, thermistors make the perfect sensor for these applications.

Quick compare

Accuracy Range Stability Response Time Cost
Thermocouples Accuracy of 1 degree Celsius -200 to 1800 degrees Celsius Tendency to drift over time, making them less stable over time Medium – fast response time Generally 2-3 times cheaper than its RTD counterpart
RTDs Accuracy of 0.1 degree Celsius -200 to 649 degrees Celsius Excellent stability. Generally 0.05 degrees Celsius per year Medium response time Typically the most expensive kind of temperature sensor
Thermistors Accuracy of 0.05 – 1.5 degrees Celsius -55 to 120 degrees Celsius Drift of 0.2 degrees Celsius per year Medium – fast response time The most cost effective type of temperature sensor

The bottom line

Every temperature sensor has its pros and its cons and we know that it can feel overwhelming trying to choose the best one for your application. That’s why we recommend TE sensors – they’re the highest quality on the market, so you know you’re getting the best of the best.

As industry leaders in test and measurement, Applied Measurement has been providing innovative and effective solutions for applications that involve measurement of physical parameters since 1976. We have the experience and expertise to supply the correct sensor for your application.

If you want to talk to someone about sensor solutions, contact us to discuss your requirements or application, we are always happy to help!