Surface temperatures are notoriously difficult to take accurately. Traditionally thermocouples and in some cases RTD’s have been used with a number of ways of fixing them to the surface. Some examples would be:-
Hand held design with a spring loaded copper disc.
Hand held design with a coiled spring thermocouple junction.
Magnet thermocouple or RTD.
Self-adhesive silicone rubber patch thermocouple or RTD.
Washer thermocouple or RTD.
Jubilee clip style sensor specifically for pipe measurement.
All of the designs have been supplied for many years and indeed as a temperature sensor manufacturer ourselves, we regularly supply all of them to customers.
There are however some limitations with taking surface measurements in this way:-
The sensor itself is being influenced not only but the surface that you wish to measure, but also by the surroundings. The surroundings are almost certainly at a different temperature even in very close proximity to the sensor and this can give an incorrect reading.
The sensor will only have a single point of contact with the surface, particularly when considering components with a curved surface such as pipes. This will lead not only to a possibly incorrect reading, but the response time will be very slow.
Some designs of sensor, in particular the magnetic types, have a very large thermal mass. When clamped onto a surface, the magnet can draw heat energy away from the area you wish to measure causing an incorrect measurement. This is particularly true with objects with low thermal mass themselves such as the thin metal wall of a vessel.
In addition to a potentially incorrect reading using a magnetic type thermocouple, the response time will also be adversely affected due to the thermal mass, making the sensor very slow to respond.
In terms of contact sensors we are always happy to work with customers to ensure that the design of the sensor we supply best matches their needs and overcome as best as possible the pitfalls of surface temperature measurement.
There is however an alternative…
Instead of using a traditional contact sensor, it is now widely recognised that the use of non-contact infrared temperature sensors can give extremely reliable measurements of surfaces. Indeed that is what they are designed to do. The advantages of an infrared sensor are as follows:
- By not being in contact with the surface, the sensor itself does not influence the temperature of the object itself.
- When correctly aimed, an infrared sensor will measure the temperature of the surface only. It cannot measure the temperature of surrounding air and is therefore not influenced by it.
- Infrared sensors all have a very fast response time. Where a thermocouple may have a response time of many seconds, an infrared sensor will respond very quickly. 150 milliseconds is normal, 1 millisecond is possible depending on the sensor chosen.
- Because the sensor can be mounted remotely to the object, in some cases many metres away, it immediately becomes less susceptible to conditions within the process be they high ambient temperature, high vibration and so on. In many cases there can be an expectation an infrared sensor will have a longer life than a contact sensor in the same application.
- Integration with existing process control equipment is simple and straightforward. Infrared sensors have industry standard outputs such as 4-20mA, 0-10V and in many case thermocouple outputs too making them a direct replacement for many contact probes.
As the UK distributor for Optris GmbH, the German manufacturer of infrared temperature sensors and thermal imaging cameras, we are best placed to assist you with your non-contact temperature measurement requirements.
There are some technical considerations to be taken into account when selecting an infrared sensor but we can guide you through this should you need any help.