Message Board Thread - "Calculate temperature with different emissivity"

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Calculate temperature with different emissivity RonFrend 11/4/2004
I have developed a spreadsheet for quantifying heat loss through building fabric using temperatures from from a thermal imager and actual air temperature and wind speed. My problem lies in that I pick up the temperatures from the thermal image but that is all at the same emissivity and I am looking at brick, glass and plaster surfaces. I know I could take lots of images or simply change the emissivity in the program but I would like a simple calculation to put in the spreadsheet so that I can take whatever temperature the image gives me and adjust those temperatures using different emissivities.
Does anyone know the calc I could use?

Thanks in advance,
Ron Frend
 
Re:Calculate temperature with different emissivity Gary Orlove 11/5/2004
I would caution you on trying to quantify heat loss on buildings from just a thermal image. I worked extensively with this 24 years ago and the results were less than impressive. The basic problem is that building surfaces are in a constantly changing and dynamic heat transfer situation. The solar loading, wind speed, air temperature, and sky radiation (having a clear sky or clouds can change heat transfer significantly) continually change during a diurnal cycle. On top of that you have cycles in the internal temperatures of the building caused by the heating/cooling systems. Remember that a building envelope contains many different materials, each with their own thermal conductivities and thermal diffusivities. Is the temperature of a building envelope surface at any one point in time (e.g. at thermal image) indicative of the heat loss situation at that time? Generally no, as the envelope reacts to its environment with varying amounts of time lag based on the construction.

Thermography is great for finding areas of poor thermal insulation, moisture, and air leakage, in other words to locate building problem areas. But in order to gain a better understanding of envelope thermal performance, I think you need to have thermal information every hour for several days. This data could be gathered by your thermal camera and or thermocouples as is practical. Now to your specific questions:

1. What type of thermal imager and IR image software are you using? Depending on the tools at your disposal, you may be able to produce a matrix for Excel that is composed of the corrected temperatures for your model. Have you measured the emissivities of these building materials using the infrared camera you own? Emissivities from handbooks can have major errors.

2. Once you know the emissivities, you can have the IR software calculate the corrected temperatures. If you want to do this yourself, you will need the calibration curve of the camera. The basic formula IR cameras use to measure temperatures is shown below:

Etot = e x E(Tobj) + (1-e) x E(Tref)

Where Etot = The total radiation received by your IR camera
e = emissivity of your target surface
E(Tobj) = The radiation emitted by your target surface if it was a blackbody at the target temperature
E(Tref) = The radiation emitted by surrounding surfaces that reflect off your target surface back into the camera

What your need to solve for is Tobj knowing the other parameters. The calibration curve for your camera gives you the relationship between E (radiant power) and T (temperature). As you can see, this can be a somewhat complex subject.

Gary Orlove
Infrared Training Center
 


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