How to select the right type of temperature sensor

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Selecting the right type of sensor is critical to the success of applications requiring temperature measurement. In this article, Labfacility offers some advice, based on the requirements of the automotive industry.

How to select the right type of temperature sensorThe significance of engine management, fuel efficiency, exhaust emissions control, systems protection and safety place continual demands on the reliability and accuracy of the sensors utilised to monitor, measure and facilitate control of the vehicle.

As with any process or application, the choice of sensor type is determined by many factors including operational conditions. And sensor types used for development applications will not necessarily be those used in production vehicles.

Non-mechanical temperature sensors are just one family of a wide variety of sensors and transducers deployed in a vehicle to monitor the whole range of appropriate parameters. Any or various of the temperature sensors described here are used according to the requirements of the many applications and temperature ranges encountered; for example, brakes and catalytic converters exhibit elevated temperatures requiring thermocouples and/or infrared sensors, whereas cooling and electrical systems operate at much lower temperatures.

Non-mechanical temperature sensors

A thermistor is a special type of metallic oxide resistor that displays a significant change in resistance with temperature. Two versions are available, the most common being the NTC (negative temperature coefficient) thermistor in which the resistance decreases with increasing temperature, and the PTC (positive temperature coefficient) thermistor in which the resistance increases with increasing temperature.

The temperature range is generally –50 to +250degC with a sensitivity of around 5 per cent per degree with an accuracy of +/-0.1 to +/-0.3degC over a limited range.

Thermistors are small, rugged sensors suitable for high-vibration applications.

A metal film Resistance Thermometer Detector (RTD) comprises a thin (1 micron) film of platinum on a ceramic substrate. The film is laser trimmed to achieve a precise Ro (resistance value at 0degC) and then encapsulated. Ro values are either 100ohms or 1000ohms.

The wide temperature range is generally -250 to +850degC, depending on the construction method specified, with a sensitivity of 0.385ohms/degree for Ro 100 or 3.85ohms/degree for Ro1000. Accuracy is +/-0.1 to +/-0.5degC as standard, but an order of five times better is readily available.

RTDs are small, rugged sensors that can be used in moderate vibration applications.

A thermocouple utilises a thermoelement (a junction of two specified dissimilar metals) to generate a small voltage that increases non-linearly with increasing temperature. A wide range of different types is available to suit different temperature ranges and accuracy demands. The junction and the thermoelement wires are insulated and protected in a suitable bead or miniature sheath to suit the application.

The very wide temperature range extends from -200 to +1750degC, depending on the type specified. Type K, for example, covers -200 to +1200degC and is a popular choice. The sensitivity is around 40uV/degree and accuracy is +/-2.5degC as standard; higher accuracy versions are available.

Thermocouples, especially in mineral insulated construction, are very rugged and vibration resistant. The probe can be bent or formed to suit the installation.

The infrared (IR) non-contact, test thermometer can be a self-contained, handheld device with an integral display or can comprise a remote sensor for a fixed installation. The sensor is usually a thermopile with associated optics to remotely receive infrared radiation from the heat source from which the temperature is obtained.

Typical applications include cooling systems, engine operation, a/c performance, catalytic converter operation and brakes. The significant advantage is that contact with the target is not required, simply line-of-sight access. Such temperature measurement is safe and convenient aided by a laser point sight.

Temperature ranges are typically -30 to +500degC with an accuracy of 2-3degC. The useful range is typically 2m or so.

A silicon bandgap diode temperature sensor operates over a limited range and is used in electronic equipment. In many cases the device is incorporated in an integrated circuit. It operates on the principle that the forward voltage varies with temperature. It is rarely practical as separate device, however, and applications are limited to the location of the associated circuitry.

Measurement and control

Output signals from each of the various temperature and other sensors and transducers are applied to suitably configured input ports on the host measurement and/or control circuits. These comprise on-board or external data acquisition and logging units (DAUs) or integral (to the vehicle) systems. Interfacing can be hard-wired and/or remote, wireless (telemetry) based, usually a combination of both.

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