Pt100 Temperature Sensor Working Principle

In practical terms, the sensing device is a very small measuring element which is based on one of two technologies. For higher volume, lower cost and general-purpose industrial applications it is normal to use a Flat Film Sensing Element.

This type of element has a ceramic substrate onto which a very fine Platinum grid is deposited. This grid is trimmed by laser to give the characteristics demanded by the standards.

Each element is calibration checked and graded according to the tolerance band into which it falls. Flat film elements are manufactured in an automated system which gives high production rates and they are used in high volume automotive and white goods markets too.

The construction of the element means it is highly robust despite being very small and can withstand higher levels of shock and vibration than other types. Surface mount devices are also now available using this sensing technology.

Flat film sensing elements are extremely versatile devices but do suffer some drawbacks. They can operate over a wide temperature range, generally -70 to +500°C.

They are only available in the lower tolerance bands of Class B and Class A. Some are available as 1/3 DIN. As the accuracy increases there are limits placed on the temperature range of the detector.

The last disadvantage only applies in applications where the temperature is cycling between two extremes.

Because the Platinum is fused to the ceramic substrate and there are other layers in the construction of the element, the conducting grid is not able to expand and contract freely, it is constrained by the other layers. This can cause stresses within the wire which affect the resistance and therefore the accuracy.

The second type of sensing device is the wire-wound sensing element. This type of detector is manufactured in far smaller volumes by hand. Despite their higher cost, they provide the highest possible levels of reliability, particularly with respect to long term drift.

The sensing element is based on a multi-bore ceramic insulator. A hand made helical coil of Platinum wire is placed in one or more of the bores and carefully trimmed to provide the correct characteristics. The benefit of this design is that the Platinum wire is simply supported.

This means it can expand and contract with temperature changes which avoids a build-up of stresses within the wire, therefore, reducing the potential for long term drift. Because the wire is simply supported this type of sensing element is not suitable for applications where there is a high shock of vibration loads on the sensor.

The main advantage of a wire-wound sensing element is the ability to work at a higher temperature as standard, generally up to 650°C and they are also available with tolerances to the highest level. They are also the only option for sensors required to operate to cryogenic temperatures of -200°C.

With both types of sensing element, the construction of the finished temperature probe generally follows very similar methods and techniques. In general, a Pt100 should be housed in a metal housing or sheath to provide protection from the outside environment.

The housing is usually 316 stainless steel as this is an excellent general-purpose material for the majority of applications. It is however possible to use alternatives such as 310 stainless steel or Inconel if necessary.

In some cases, it is permissible to use an exposed or ventilated element for the measurement of dry gases. The exposed sensor should never be exposed to water or conducting liquids.

The sensing element itself is joined to an extension wire or cable in order to provide the resistance output to a point where it is measurable.

Process Parameters always weld extension wires to the sensing elements as this provides a higher temperature join that soldering and gives the lowest possible resistance between the cable and detector.

The sensor is then potted into the housing using various materials depending on the temperature range required and the other environmental conditions. We can provide constructions for very low temperatures down to -200°C all the way up to 650°C.

In general, sensors are either terminated in a cable, a terminal housing or a connector. Process Parameters offers all these options.