Everything you ever thought of when specifying a temperature probe, plus a few more considerations to bear in mind when choosing this equipment
What is a temperature probe? 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 sensor element (in this case we are focusing on NTC thermistor elements), probe housing and components for mechanical and environmental protection (this can include potting compounds and special coatings), plus 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 are temperature range, including minimum and maximum temperatures that the probe may experience, temperature cycling, accuracy required, response time, thermal mass, and environmental conditions.
NTC thermistors operate within a temperature range of -55C to +300C but in designing for specific applications, narrower temperature ranges normally apply.
Temperature cycling that the probe will experience.
In some applications (eg 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.
While the probe may be exposed to a wide temperature range (eg +50C to +100C) the critical temperature measurement range may be across a narrower range (eg +70C to +80C) or at a single temperature point (eg +75C).
Accuracy required in the application 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.
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.
Time response required for the temperature probe.
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.
Thermal mass of temperature probe relative to the system that is being measured.
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.
This can involve a variety of environments including immersion (partial/full), liquids, air/gas, etc Mechanical requirements for the temperature probe in terms of shock or vibration performance.
Electrical requirements in terms of noise performance, insulation or shielding.
Lifetime of product design: how long will the probe need to last for? Compatibility with existing or future instrumentation systems. Product qualification requirements.
For many markets, the final probe design will need to be qualified by the end customer or achieve qualifications by an external agency - eg CE marking, ESA, etc Price: the budgeted cost of the probe is often an issue which will effect final design.
choice of materials: while 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.
