These thermometers work on the principle that all bodies give off infrared radiation in proportion to their temperature. Using infrared detectors this temperature can be measured; however there are some problems in applying this technology to the everyday operations of industry. The accuracy of these measurements will depend on the following.
This relates to the ability of a body to absorb or reflect incident radiation. As a rule black bodies absorb all incident radiation so that all emitting radiations is a result of their own body heat. They have an emissivity factor of 1.0. However shiny surfaces reflect a lot of incident radiation and therefore need special compensation i.e. emissivity factor less than 1.0. This is not an exact science. It is not uncommon to get readings in a fridge that pick up your own body heat reflected from the shiny surfaces.
Infrared detectors have internal temperature sensors which compensate for changes in ambient temperatures. Check the manufacturer’s specifications to make sure that they can operate at your ambient temperatures.
The surface being measured is described by the base of a cone whose tip is at the sensor and whose diameter varies with the distance between them. This “cone angle” is given as a ratio and is specified by the manufacturer. A 6:1 ratio for example means a distance of 6cm will measure a spot of 1cm. It is important that the target you are measuring is bigger than the spot size.
Continuously holding the sensor over a product can cause errors through the heating or cooling effects of incident radiation on the sensor. Take the reading, remove the instrument from the product & then repeat the reading.
Many entry level Infrared devices have a preset emissivity for example 0.95 or 0.97. These instruments amplify the objects temperature to allow for the lower preset emissivity value.
However not only does the instrument amplify the target temperature they also amplify any radiant energy from walls & ceilings that has fallen on the target. This radiant energy is detected by the IR instrument & is also amplified. These errors can be reduced if you aim the IR thermometer at a small gap between similar items however it is best to use an infrared instrument that can have its emissivity set to 1 to eliminate the amplification of radiation errors altogether.
While it is good practice to check your IR instrument with an ice point at regular intervals – this procedure will be meaningless if you do not understand the impact the emissivity has on your results.
Ice has an emissivity of 0.999. If your instrument is set to emissivity 0.999 then your ice point measurement should read 0.0 °C.
If however your instrument has an emissivity setting of 0.95 then your ice point should read -1.2 °C.
If your instrument has an emissivity setting of 0.97 then your ice point should read -0.7 °C.
Please note if your instrument has been calibrated by an IANZ laboratory you will be provided with corrections that you can also apply to your readings to increase your accuracy.
Your instrument is set to E=0.97
At 0 °C the correction for your instrument according to its IANZ Calibration Test Certificate is -0.4 °C
You do an ice point and the instrument display reads -0.6 °C
Therefore the corrected instrument reading is (-0.6) + (-0.4) = -1.0 °C
Although you may think the ice in your ice-point should read 0.0 °C this is not the case because the IR thermometer (IRT) is set to 0.97 and not 0.999. By theoretical calculation we know that at E=0.97 the ice point should read -0.7 °C. (Reference; MSL TG22)
Therefore has the ice point shown that my instrument is OK – yes it has - my instrument is reading -0.3 °C colder than it should but that is well within the stated accuracy of the manufacturer’s specification.
Points to note:
Only when the IRT and the target have the same emissivity will the IRT ‘see’ the same temperature.
When an Emissivity is set to <1 any readings will be amplified.
Above ambient temperature the IRT displayed readings will be hotter, while below ambient the displayed IRT readings will be colder.
IR thermometers offer excellent ease of use and are hygienic however if in doubt or your process is critical, check your IR measurement with a contact temperature probe.
Measurements will be more accurate if you hold your IR thermometer with a steady hand.
Also care for your instrument do not drop it or get it wet.
Have your instrument IANZ certified annually to ensure it is accurate & working correctly. Call Teltherm +64(0)9 633 0040