Why you Can’t Measure Air Temperature with a Laser IR Gun and a Simple, Effective Alternative

While Laser IR Thermometers are an extremely common tool, they are entirely ineffective for measuring the outlet temperature of an air heater. To understand why, we must first understand how this tool works. Laser IR Thermometers measure the surface temperature of an object by measuring the thermal energy emitted by the target. Knowing the amount of thermal energy discharged and the emissivity of an object’s surface, the object's temperature can be determined by the device.

When measuring a heater’s output air temperature, the largest issue with these tools is that they measure surface temperatures. As the heated air is transparent, the measurement will always be the surface temperature of a nozzle or a component of the heater housing; and frequently it will be an exterior surface. These items will always be cooler than the heated air, often by a significantly larger margin than the user would expect. To accurately measure the output temperature of an air heater you must measure air temperature and laser IR thermometers are incapable of doing so.

The above paragraph covers the most important reason these devices are ineffective for measuring air temperature, below are some quick points on why the reading can be even further from the true air temperature:

• Measuring an External Surface of the Nozzle/Pipe/Heater – It is often difficult or impossible to aim the IR device at an internal surface that is in direct contact with the hot air and as a compromise the device is aimed at an external surface. In this case the surface temperature is a product of: convective heat transfer from the hot air to the interior surface, conductive heat transfer through the thickness of the nozzle/pipe/heater component, and convective heat transfer from the ambient air to the external surface of the object. The result is a surface temperature that is well below the hot air temperature.
  
  
• Emissivity Mismatch – As mentioned earlier, the device uses the measured thermal energy discharge and the emissivity of an object’s surface to determine temperature. Many devices assume a common emissivity value of 0.9. However reflective surfaces, in general, have much lower values of emissivity. Most components at the outlet of an air heater will be made with stainless steel and will be at least somewhat reflective. As a result, their emissivity will be lower than the general use value and this will result in a miss-reading of even the surface temperature! While there are more sophisticated tools that allow for adjustment of the emissivity value, they require the user to know the correct value for the surface they are measuring, and even then they are still only measuring a surface temperature.
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• Measuring an Internal Surface of the Nozzle/Pipe/Heater – Even when an internal surface is targeted with the IR device, that temperature will still be lower than actual air temperature. That surface is being heated by the hot air, the hot air is the hottest component. Air flowing through a nozzle/pipe is usually closer to laminar than it is to turbulent and there is usually not much impingement, which will yield a lower heat transfer efficiency. If the nozzle/pipe is not effectively insulated, the ambient temperature of the surrounding environment will reduce the temperature of that internal surface as well.

What to Use Instead

To effectively measure the output temperature from an air heater, STANMECH recommends a physical probe be used. The most common device is a K Type thermocouple[1], this is what STANMECH uses for all air temperature measurements in our lab and in the field. When using the thermocouple, place the tip of the probe directly into the hot air stream (preferably in a way that allows for good impingement of air against the tip) and let it rest until the measurement on your reader reaches a steady state. Make certain the tip of the probe is does not make contact with any nozzle, pipe, or heater surfaces; it should only be in contact with the output air flow. This method will provide an accurate measurement of the output temperature from your heater.

​Figure 1: Laser IR Thermometer providing a measurement of 195°C while the K Type Thermocouple provides the correct air temperature measurement of 626°C, a difference of over 400°C!

Having trouble with your tool? Contact your Technical Sales Representative for help troubleshooting.

​[1] The K Type thermocouple is only the probe component, you will also need a readout device that takes the signal from the thermocouple and outputs it to a digital display.