Design considerations for temperature probes

Edited by the Engineeringtalk editorial team Dec 25, 2003

In every temperature probe there are multiple variables which determine the final construction and materials used.

The term "temperature probe" is used to describe a temperature sensor assembly that enables a temperature sensing element to be located at a particular position and to be electrically connected to an instrumentation system.

A typical temperature probe consists of the following constituent parts: the sensor element (in this case we are focusing on NTC thermistor elements); the probe housing and components for mechanical and environmental protection - including potting compounds and special coatings; and the electrical cabling and connectors.

Probe design always starts with a consideration of the final application which may bring many factors to bear.

In many cases, these factors can be conflicting and so good probe design often requires that reasonable compromises in performance requirements are reached.

The most relevant considerations in temperature probe design for an application are outlined below.

First is the temperature range including minimum and maximum temperatures that the probe may experience.

NTC thermistors operate within a temperature range of -55 to +300C but in designing for specific applications, narrower temperature ranges normally apply.

Second comes the temperature cycling that the probe will experience.

In some applications (such as industrial refrigeration) the temperature will cycle in a freeze-thaw environment.

In this example, a robust probe design will be required Measurement temperature range and critical temperature points.

Although the probe may be exposed to a wide temperature range (eg +50 to +100C) the critical temperature measurement range may be across a narrower range (eg +70 to +80C) or at a single temperature point (eg +75C).

Third comes the accuracy required in the application.

This can be expressed in terms of temperature accuracy, for example +/- 0.1C, or in terms of resistance accuracy, for example +/-5%.

Accuracy can also be specified at a single temperature point or over a temperature range.

Next comes the distance from location where temperature is being sensed to control instrumentation.

In many electronic temperature-sensing applications it is necessary to measure temperature at a location that is remote from a control unit or data logging equipment.

Another factor is the time response required for the temperature probe (ie how quickly should the sensor be able to respond to a change in temperature?).

Keep in mind that the definition of thermal time constant is the amount of time required for a sensor to change by 63.2% when subjected to a step function change in temperature.

For example, if a sensor is at 0C and is then plunged in a 100C.

bath, one time constant is the amount of time it takes the sensor to reach 63.2C.

The thermal mass of temperature probe relative to the system that is being measured is also important, as is the thermal contact or thermal coupling between the temperature probe and the system that is being measured.

Environmental conditions that the temperature probe and connecting wires will be exposed to or environmental ratings that it must conform to must be taken into account.

This can involve a variety of environments including immersion (partial/full), liquids, air/gas etc.

Also to be considered are the mechanical requirements for the temperature probe in terms of shock or vibration performance, and any electrical requirements in terms of noise performance, insulation or shielding.

Other factors include: the lifetime of product design (fir how long will the probe need to last?); compatibility with existing or future instrumentation systems; product qualification requirements (eg CE marking, ESA etc); price - the budgeted cost of the probe is often an issue which will affect final design; and the choice of materials.

Although every probe design does not involve such a complex consideration of issues, there are always multiple variables which determine the final construction and materials used.

With 20 years experience in probe design, Betatherm Sensors offers a wide range of standard probes along with customised probing capabilities.