Infrared Training Center

Friday, December 9, 2011

Stainless steel reflecting sky temp

This is extracted from a recent message board post:

"I'm curious as to how to adjust for stainless steel reflecting sky temp. The apparent reflected temp of the ground and surroundings is approx 50F. The bottom half of the drum is reflecting this (SP4). The top half is reflecting the sky (SP5). 

In my software, if I set the emissivity to 1, the sky temp in the picture shows -34F.
SP4 and SP5 *should* be the same temp, within a few degrees, based on the operation of the equipment. If I enter 50F as RAT for SP4, I have to enter -95F as RAT for SP5 to get the two temperatures to match.

Why is it -95F and not -34F for SP5? I want to make sure I'm understanding and looking at the image correctly. "

855088676_IR_0341-RevClick image to enlarge

 

This is an advanced application. Unless you really know what you're doing, do not attempt measuring temperatures on materials that have very low emissivities. The possibilities for significant errors are large.

Nevertheless, I will illustrate the steps I have taken to properly measure the reflected temperatures, calculate the emissivity of the steel, and of course, measure the temperature of the steel.

12-9-2011 10-22-41 AMClick image to enlarge

  1. OK, first things first. I changed the color palettes so I could more easily see exactly what is being reflected by the steel. I used the iron bow palette for this.
  2. Secondly, instead of using spots I prefer to use areas. This enables me to average out individual objects that might be reflected. I set the areas to read average temperatures.
  3. Now, since the shell of the cylinder is obviously quite warm, I prefer not to measure reflected temperatures from the shell but rather from objects that are more closely allied to the reflected temperatures. I've chosen the clear sky above for the sky reflected temperature and I have chosen what appears to be a support structure at the bottom to more closely reflect ground temperature. I set the emissivity to 1.0 on both of these areas in order to obtain an accurate reading. As you can see, the sky reflected temperature is about -39° F and the ground reflected temperature is approximately 43° F.
  4. Now that I know the reflected apparent temperatures, I can enter these into the area's for the steel.
  5. Now all we have to do is determine the emissivity of the steel. The assumption is that area's three and four are identical in temperature. That means I have to iteratively enter various values of the emissivity until the temperatures are identical. Since I know the steel is very reflective, I started with an emissivity of 0.5. Since the temperature readouts were not identical I then used a value of 0.25, etc. At a value of 0.17 I found that both temperatures read the same at approximately 170° F.
  6. So there you have it. Remember, don't try this at home; let thermography professionals perform calculations like this. One really needs to understand the physics involved to extract data like this from a thermogram. Different situations call for different tactics. I would recommend knowledge at a minimum of a level II certification to handle these types of situations.

7 comments:

  1. Wow! You've been working for 33 years in this field. It seems you have a deep-seated love for your job, and that's admirable. I bet nothing matches your passion in infrared technology. :-) What you have made here is educational. It can help physicists, chemists and even lay people understand the science of thermography.

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  2. Gary,
    You are breaking my rule of thumb using E=0.17. Anthing below E=0.6 should be coated or stickered or taped or something to bring up E until it is greater than .6. Look at the temperature over to the far left...221 F. That looks like a weldment or flange with way better emissivity value. I would bet a buck that the steel is much hotter than 170F based on that far left value. It is way too hard to eliminate all that is going on with reflective surfaces when trying to measure their temperature. E=0.17 is just too reflective!
    Signed, The old IR Goat!

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  3. I bow to the superior knowledge here but doesn't radiated heat also help to complicate the issue ?

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  4. Everything we are seeing in the image is all radiated heat, that is what an IR camera "sees". So all the calculations are based on that fact.

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  5. Dear IR Goat,

    I agree that we would all like to see high emissivity surfaces. However with a system such as this, it can be difficult and expensive to accomplish (in addition it would greatly increase the heat loss from the system resulting in higher fuel costs and/or degradation of the product quality).

    My primary intention here was a demonstration of technique that could be applied to other situations where emissivity, reflected temperature, and surface temperature are unknown, yet some conclusions or answers are sought.

    As far as the flange on the far left is concerned, I agree that the emissivity probably is higher, but I also think that the actual temperature may be higher as well due to thermal bridging.

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  6. Stuff like this just helps grow the passion, to want more

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  7. Stainless steel Usually 18-20% Chromium and about 8-10.5% nickel. It depends on the type of stainless steel you are referring to. If it is 18-8 stainless steel, then it is 18% chromium and 8% nickel (hence the 18-8). There are several other types, like the 300 series and the 400 series which have slightly different variations of chromium and nickel. If you do a search for "types of stainless steel" you will find that there are several different types.

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