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  #1  
Old 06-11-2007, 11:12 AM
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Default COLOR vs TEMPERATURE

Wing Paint Color Temperature data.

Over the weekend, I moved everything outside and put the wings on. (Working on the cowling)

It was nice and sunny, no wind and no clouds.

I took a couple temperature data-points to get an idea what to expect.

What I found is somewhat confusing. See the temperature data on:

http://www.iflyez.com/LongEZ_Retrofit_JUN_07.shtml

The three big questions regarding the data are:

1) Although the surface temperature read 137 deg F, I was able to hold my hand on the surface. It was hot, I would guess 120, but I would not have guessed 137??

2) The Wing gray areas read about 15 deg F hotter than the gray areas on the strake. The unscientific "hand" test seemed to indicate they were both at the same temperature.

3) The night time temperature test on the wing was about 15 degrees below what it should have been, i.e. it was Reading 48 Deg F when the OAT was about 62 Def F.???

CONCLUSIONS FOR NOW

Even though the temperature data is somewhat questionable, The temperatures I observed with my "hands" were high enough that I decided to repaint the wings a little lighter color Gray. I'll redo the temperature data with the new color grays.

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  #2  
Old 06-11-2007, 11:15 AM
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Default Re: COLOR vs TEMPERATURE

Great info john - i will definitely check a sample in AZ on a 115 day to insure the colors i pick will work!
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  #3  
Old 06-11-2007, 01:09 PM
Marc Zeitlin Marc Zeitlin is offline
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Default Re: COLOR vs TEMPERATURE

Quote:
Originally Posted by Waiter View Post
What I found is somewhat confusing.
Not really.

Quote:
Originally Posted by Waiter View Post
1) Although the surface temperature read 137 deg F, I was able to hold my hand on the surface. It was hot, I would guess 120, but I would not have guessed 137??
There are two possible (and somewhat contradictory) reasons that you could FEEL one thing, while measuring another. These are somwhat complex and technical explanations, but I'll attempt it. The first possible reason is:

1) The thermal conductivity of the glass/epoxy is low. When you touch the surface, the heat at the surface of the layup is conducted into your hand, lowering the temperature directly under your skin. with low conductivity, the heat from the lower plies is conducted only slowly to your skin, and since your skin is at 96F or so, the temperature you FEEL is a lot lower than the bulk temperature of the layup. Since the non-contact laser gun does not take any heat out of the surface, it measures the REAL temperature.

2) The glass layup is thin enough so that the thermal capacity of the bulk material is very low. When you touch it with your hand, you suck enough heat out of it so that it FEELS cooler than the measurement indicates, and there is not enough energy in the rest of the material nearby to replenish the energy your hand removed.

These are semi-accurate layman's explanations - there are dynamic effects going on here, but these are the two most plausible reasons for you feeling something that seems different than what you're measuring. The measurements are correct - the feel is subjective.


Quote:
Originally Posted by Waiter View Post
2) The Wing gray areas read about 15 deg F hotter than the gray areas on the strake. The unscientific "hand" test seemed to indicate they were both at the same temperature.
See above - they're NOT at the same temp - they only feel that way because of the interaction of your hand with the surface.

Quote:
Originally Posted by Waiter View Post
3) The night time temperature test on the wing was about 15 degrees below what it should have been, i.e. it was Reading 48 Deg F when the OAT was about 62 Def F.???
No, it SHOULD have been lower that OAT. Assuming a clear sky (not overcast), which is what it looks like from the daytime pictures, then the up-facing surfaces are radiating to the night sky, which has a temperature of about 2 degrees above absolute zero. The radiation from the surface of the wings will cool them to below the ambient temperature. This is a well known phenomena.

On overcast nights, the up-facing surfaces will radiate to the clouds, which are only slightly cooler than ambient, so the surfaces don't get nearly as cold.

Quote:
Originally Posted by Waiter View Post
Even though the temperature data is somewhat questionable, The temperatures I observed with my "hands" were high enough that I decided to repaint the wings a little lighter color Gray. I'll redo the temperature data with the new color grays.
Not questionable at all - it makes sense if you understand the thermodynamics. I'm glad that you're going to lighten the color scheme - personally I think it's a big mistake to have anything even slightly dark over structural layups, like the wing/winglet joint area shown in dark grey. I'd want that white....
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Old 06-11-2007, 01:17 PM
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Default Re: COLOR vs TEMPERATURE

Marc,

Good explaination and makes perfect sense.

My brain was kind of working in this direction, but I figured someone would have a better explainition than I was coming up with.

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  #5  
Old 06-11-2007, 11:36 PM
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Lightbulb Re: COLOR vs TEMPERATURE

Interesting bit there. The non-contact temperature probes use IR (infra-red) sensors to measure IR radiation, but not all probes measure temperature accurately. I use one quite often and the color and finish of the surface will cause error. A Melanine press with shiny stainless steel drums will cook your finger in 2 seconds, but the probe shows 76 degrees, because the surface is shiny. Here is a link that explains how they work.

Your hand may be more accurate on differing surfaces, colors and textures which reflect and radiate IR.

I would use a contact type thermometer because it will measure the kind of heat that will damage the foam underneath. (If that is what you wish to measure). They work on a dynamic (sort of - thermistor) principle and are more accurate than the IRs in ALL surfaces that can be contacted. (right tool for the right job) by virtue of the fact that they use the actual heat to vary the flow of electrons in a circuit which is more "reactive" to all surfaces regardless of color or texture. Have a glob of heat sink goop to place on the material and the probe to make your readings more accurate. (the goop has gobs of silicone in it, so don't do any layups over it)
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  #6  
Old 06-12-2007, 01:13 AM
Marc Zeitlin Marc Zeitlin is offline
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Default Re: COLOR vs TEMPERATURE

Quote:
Originally Posted by Clutch Cargo View Post
I would use a contact type thermometer because it will measure the kind of heat that will damage the foam underneath......
Heat is heat. Energy is energy. The temperature of the glass/epoxy is whatever it is - there is no "kind" of heat there.

Quote:
Originally Posted by Clutch Cargo View Post
Have a glob of heat sink goop to place on the material and the probe to make your readings more accurate. (the goop has gobs of silicone in it, so don't do any layups over it)
The problem with surface contact measurements in a case like this is that (similar to the Heisenberg uncertainty principal) when you attempt to measure something, you affect what you're measuring. Because the glass is very thin (wing skins are 2 or 3 plies of 0.009" glass, or 0.027" thick at most, with some small amount of micro and paint), it has VERY little thermal mass (and as has been pointed out, relatively low thermal conductivity). I think that it's highly likely that a large glob of something on the surface, interfering with the heat absorption there and also sucking heat out of the layup, is far more likely to measure it's own temperature characteristics than that of the glass layup.

IF (and it is an if) the IR probe is using the correct emissivity settings to measure correctly for the surface in question, I think it'll give a more accurate reading, unless you're using REALLY tiny surface temperature probes (which ARE available).

You can also get temperature sensitive stick-on strips and/or coatings that will melt or change color at calibrated temperatures - this may be the most accurate method for this type of measurement.

You are correct, though, that the right tool should be used for the job - but it's not absolutely clear what the right tool is :-). John's measurements DO, however, match up with what would be expected from the characteristics of the materials at hand (including his hand) :-).
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Old 06-12-2007, 10:32 AM
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Exclamation Re: COLOR vs TEMPERATURE

[quote=Marc Zeitlin;49544]Heat is heat. Energy is energy. The temperature of the glass/epoxy is whatever it is - there is no "kind" of heat there.

The problem with surface contact measurements in a case like this is that (similar to the Heisenberg uncertainty principal) when you attempt to measure something, you affect what you're measuring. Because the glass is very thin (wing skins are 2 or 3 plies of 0.009" glass, or 0.027" thick at most, with some small amount of micro and paint), it has VERY little thermal mass (and as has been pointed out, relatively low thermal conductivity). I think that it's highly likely that a large glob of something on the surface, interfering with the heat absorption there and also sucking heat out of the layup, is far more likely to measure it's own temperature characteristics than that of the glass layup.

IF (and it is an if) the IR probe is using the correct emissivity settings to measure correctly for the surface in question, I think it'll give a more accurate reading, unless you're using REALLY tiny surface temperature probes (which ARE available).

You can also get temperature sensitive stick-on strips and/or coatings that will melt or change color at calibrated temperatures - this may be the most accurate method for this type of measurement.

You are correct, though, that the right tool should be used for the job - but it's not absolutely clear what the right tool is :-). John's measurements DO, however, match up with what would be expected from the characteristics of the materials at hand (including his hand) :-).[/quote]
That is true in the scientific "purest" sense of the term heat. " If you cant stand the 'heat', get out of the kitchen"! I guess, thermal energy and temperature are better terms, but even then there's ambient and absolute....
I was trying to point out that not all sensors read the full IR spectrum, not all of them are calibrated, and the majority of these are set to read radiated IR energy from a flat, matte surface, with color. A rounded, shiny surface, radiates IR energy in all directions, and gives a poor sample to the sensor. Thus the inaccurate readings and by a large percentage! One of the most heat sensitive areas of our body is -our faces! Close your eyes and hold the palm of your hand out in front of you about a foot away, bring it close until you feel the heat from it on your face. Now, open your eyes. Your face sensed heat less than body temp (98.6) anywhere from 88 to 93 degrees on your skin. The hotter the source, the farther away you feel it. So "face" your airplane!
Or use a patch type indicator like Marc said.
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  #8  
Old 06-12-2007, 10:47 AM
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Default Re: COLOR vs TEMPERATURE

I use this thing (RayTec) at work. About the only thing you can't measure with it is a nice shiny stainless steel pipe or tank. Other than that, it seems to do a good job, shiny surfaces seem to measure OK.

EXAMPLE: If you try and measure something by shooting through glass, guess what, you get the temperature of the glass, not the item you shooting at.

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  #9  
Old 06-12-2007, 01:21 PM
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Default Re: COLOR vs TEMPERATURE

Marc Z, (smart guy, he's forgotten more about airplanes than I learned my entire life!) sent me a good explanation on the three questions from above.

This is my interpretation of what he said!

1) The thermal mass of the two layers of fiberglass is very small. So, even thought its hot, as soon as something touches it, it cools down a little. When I put my hand on the surface, my hand absorbs the surface heat and temporarily cools it down. The RayTec None-Contact laser temperature gauge doesn't touch the surface, so it doesn't cool it down.

TEST THIS THEORY - Take a temperature reading, then place your hand on the GRAY surface for a couple seconds. Remove your hand and immediately measure the surface temperature with the RayTec. It should be significantly lower.

CONFIRMED - Using the RayTec, I found the hottest spot on the gray area that was 138 Deg F. I placed my hand on this area for about 5 seconds. When I removed my hand the temperature read 120 Deg F.

2) The blue foam under the wing area is not a good conductor of heat. The foam under the strake area conducts heat much better. Therefore, the skin temperature over the blue foam will be hotter, because the blue foam isn't absorbing heat from the skin as well as the strake foam.


3) The top of the wing is "radiating" heat faster than its able to absorb heat from the atmosphere (clear sky, there is nothing above the wing that's radiating heat). The rudders are radiating also, however, they are also absorbing heat from other sources, pole barn, trees, cars, houses, etc.

TEST THIS THEORY - Measure the temperature of the bottom of the wing with the RayTec. If radiation is the reason for the difference, than the bottom of the wing should be approximately the same as the rudders. The ground (dirt, grass, etc) is also radiating heat, this heat will be absorbed by the bottom surface of the wing.

There is nothing above the wing radiating heat, so the top surface of the wing should be colder.

I'LL TEST THIS TONIGHT AND REPORT BACK

Don't you just love this science stuff. :-)

Waiter
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  #10  
Old 06-12-2007, 01:49 PM
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Default Re: COLOR vs TEMPERATURE

Quote:
Originally Posted by Clutch Cargo View Post
Interesting bit there. The non-contact temperature probes use IR (infra-red) sensors to measure IR radiation, but not all probes measure temperature accurately. I use one quite often and the color and finish of the surface will cause error. A Melanine press with shiny stainless steel drums will cook your finger in 2 seconds, but the probe shows 76 degrees, because the surface is shiny. Here is a link that explains how they work.

Your hand may be more accurate on differing surfaces, colors and textures which reflect and radiate IR.

I would use a contact type thermometer because it will measure the kind of heat that will damage the foam underneath. (If that is what you wish to measure). They work on a dynamic (sort of - thermistor) principle and are more accurate than the IRs in ALL surfaces that can be contacted. (right tool for the right job) by virtue of the fact that they use the actual heat to vary the flow of electrons in a circuit which is more "reactive" to all surfaces regardless of color or texture. Have a glob of heat sink goop to place on the material and the probe to make your readings more accurate. (the goop has gobs of silicone in it, so don't do any layups over it)
I would just do as klutch suggested, he has measured more in a month that the average person will in a lifetime. buy heat sink glob, sorry goop (those technical words get me everytime) - you talkin a quarter inch?
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  #11  
Old 06-12-2007, 02:29 PM
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Default Re: COLOR vs TEMPERATURE

Dust, I'm not sure this would work.

I'm looking around work for the smallest "k" or "j" thermocouple I can find. I also have some RTDs floating around here someplace?

The problem as I see it:

Take a piece of black construction paper and set it out in the sun.

I bet if I use my RayTec, the paper would read 150 plus. But when I pick it up it feels like ambient air temperature. The paper really is 150, but when I touch it, my hand cools it down immediately.

I think the leads from the Thermocouple or RTD will have the same result, and "sink" the heat away from the surface they are reading.

What does all this mean, Don't know, I'm still scratching my head.

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Old 06-12-2007, 08:26 PM
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Default Re: COLOR vs TEMPERATURE

The contact temp measurements take more time. The thermal energy has to transfer and equalize.
If I were going test the effects of color and heat, I would put a layup on a piece of plastic over a flat piece of wood, the same amount of plies as the wing. Make it wide enough to put paint samples about 2" wide by 3" tall. Paint the underside flat black. Tape it to the top of an old white styrofoam cooler lid or similar. Put it where little moisture and wind can get to it, only sun. Let it sit for about 20 min. Lift it up and shoot the flat black part (the underside) with the IR gun. Aim for where the middle of each sample is painted. Record the results. Find the hottest color and take the sample to the shade, see how long it takes for it to drop to ambient.
I would doubt that you would see much over 145 F on any of those colors if the ambient was in the 90's.
The styrofoam is an insulator, so it doesn't transfer thermal energy, it bounces it back from whence it came and probably affects the plies more, because it tranfers heat better and holds heat better than the foam, but not by much. The black paint does not reflect the heat energy very well. So, in a way, you're cooking the layer between the foam and the paint. The inner core is as cool as a cucumber. The good thing is that the substrate is not a dense material and isnt a good retainer of heat, (like copper).
I got even more tests too! But I gotta go!
BTW. the laser is for aiming.
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Old 06-12-2007, 08:49 PM
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Default Re: COLOR vs TEMPERATURE

Clutch,

Makes sense.

I've got the bottom of my winglets that I cut off, I'll figure out a test for them.

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  #14  
Old 06-13-2007, 07:31 AM
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Default Re: COLOR vs TEMPERATURE

Why is the temperature on the top of the wing lower than the OAT, Did we discover a new form of Airconditioning?

In Marc's reply to my original question, he said it was a result of Thermal Radiation. OK, Marc knows his stuff, but I wanted to see first hand proof.

TEST THIS THEORY - Measure the temperature of the bottom of the wing with the RayTec. If radiation is the reason for the difference, than the bottom of the wing should be approximately the same as the rudders. The ground, dirt, grass, Trees, buildings, etc, are also radiating heat, this heat will be absorbed by the bottom surface of the wing and the rudders.

However, There is nothing above the wing radiating heat, so the top surface of the wing should be colder. (NOTE - Not entirely true, The Air above the wing is radiating heat also, but the air's thermal mass is significantly lower than the wing, so the air will absorb more heat from the wing than it radiates into the wing.)

CONFIRMED - The temperature differential on the wing, particularly, "How can the skin temperature be lower than the outside air temperature?" is a result of heat radiation.

The bottom of the wing is absorbing more heat energy (from the ground) than its radiating.

The top of the wing is radiating more energy than it absorbing (from the air above it), so its much cooler.

THE TEST - I went out about 10:30 pm (1 hour after sundown) to take some measurements with the RayTec.

When pointed straight up into a clear sky, the gauge reads 1 Deg F. As I slowly moved the RayTec down to the horizon, the temp increased to 64 Deg F.

The outside Air temperature read with a regular thermometer read 68 Degrees.

The readings on the plane were similiar to those I took a couple nights ago.

68 = OAT (measured with thermometer)
51 = TOP OF WING
65 = BOTTOM OF WING
63 = RUDDERS (both sides)
65 = Grass/Dirt

If your interested in how these things work, theres a good writeup on RayTecs web site:

http://tinyurl.com/2fql5d

Don't you just love this science stuff. :-)

Waiter
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Last edited by Waiter : 06-13-2007 at 07:46 AM.
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Old 06-13-2007, 10:54 AM
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Lightbulb Re: COLOR vs TEMPERATURE

Waiter = Canard Mythbuster!
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