Temperature Measurement with Thermistors

Thermistors are temperature-sensing elements that exhibit large changes in resistance proportional to small changes in temperatures; they’re typically composed of sintered semiconductors, and usually, the resistance of the thermistor decreases as temperature increases (negative temperature coefficient.)

Thermistors are made using a mixture of metals and metal oxide materials. After being mixed, they can be formed and fired into the correct shape; the thermistors can then be used in the shapes of disks, or further molded and shaped with wires and circuitry depending on the application.

The name is a portmanteau of thermal and resistor, and they can be very accurate and cost-effective temperature sensors. NTC (negative temperature coefficient) and PTC (positive temperature coefficient) are available although the former are more common.

How Do They Compare to RTDs?

NTC thermistors change their resistance in a highly nonlinear way in comparison to RTDs, that are almost linear. This might seem like a disadvantage, and is, but thermistors remain popular compared to RTDs because they have large changes in resistance per degree temperature (greater precision for the measurements), and they respond quickly to ambient temperature changes. They are highly stable, the measurements are repeatable, and they can easily be switched in and out of a circuit.

Coatings typically include:

  • Epoxy coatings for lower temperature use [typically -50 to 150 °C (-58 to 316 °F)]
  • Glass coatings for higher temperature applications [typically -50 to 300 °C (-58 to 572 °F)

The purpose of these coatings is mechanical protection (as well as that for humidity and corrosion) for the thermistor bead and wire connections. The epoxy bead-type thermistor is favored by Omega for its thermistors. They are usually supplied with very small diameter (#32AW or 0.008" diameter) solid copper or copper alloy wires.

Base Resistance

As temperature increases, the resistance of NTC thermistors drops; but so does the resistance change per degree the thermistor will provide. For this reason, the base or original resistance of the thermistor needs to be higher for higher-temperature applications, so there’s further to fall; low-temperature applications (-55 to approx 70 °C) use lower resistance thermistors (2252 to 10,000Ω). Higher temperature applications use the higher resistance thermistors (above 10,000Ω). They can be purchased at a variety of different resistances and with different calibration curves; typically the base resistance is that at 25C. Different materials provide different resistance vs. temperature curves. Some materials provide better stability while others have higher resistances so they can be fabricated into larger or smaller thermistors.

Many manufacturers list a beta constant as the gradient of this curve, which can be used to find the appropriate curve for the given material.

Thermistor Elements

Thermistor Elements

The thermistor element is the simplest form of thermistor; small and compact for applications where size is important. OMEGA sells a wide variety of thermistor elements with different form factors and curves – whatever instrument reads the element must linearize and convert the reading, however.

However, the element will not survive in rugged environments unless it’s embedded in a probe: OMEGA offers thermistor probes as metal tubes. They are much more suitable for industrial environments.

Typically, when selecting thermistors, you need to consider the base resistance, the temperature vs resistance curve, and the size or sensor package.

Common resistances include:

  • 2252Ω
  • 3000Ω
  • 5000Ω
  • 10,000Ω
  • 30,000Ω
  • 50,000Ω
  • 1 MΩ (1,000,000)

Thermistor Accuracy

OMEGA thermistors have an accuracy of ±0.1 °C or ±0.2 °C depending on the particular temperature sensor model: as accurate as many different kinds of temperature elements; however, they should only be used across a nominal range of 0 °C to 100C. Thermistors chemically stable and don't age significantly.

Use Cases

Use case is an important consideration; recall that like any temperature sensor, it can only measure the temperature that it attains. Typically you don’t want to immerse a thermistor bead directly into a process; they respond very quickly due to the thin layer of epoxy, but can be too weak for steady-state measurements. Omega offers a comprehensive line of sensors that protect the thermistor across a range of applications. Examples include:

General Purpose

Omega ON-950

General purpose sensor designs can be used in applications from electronic equipment to structures, processes and design and reliability testing applications. The design makes them easy to install and monitor. The Omega ON-950 is an example of this type of construction. A small SST housing with #8-32 threaded stud can be installed into any #8-32 threaded hole; highly compact.

Liquid Immersion Measurement

Closed tubes and specially-designed housing can protect the thermistors from corrosion while allowing them to conduct the heat from the fluid so that they can come to the temperature of the fluid; for this Omega takes care to ensure that there’s good thermal contact between the thermistor and the fluid.

Surface Sensing

The ON-409 attachable surface sensor can monitor surface temperatures; it includes a thin, round metal stamping into which the thermistor is epoxied. This is then attached to a surface to measure its temperature.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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