Message Board Thread - "Insulating coating"

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Insulating coating ROP 10/22/2009
Hello, we have a question about insulating coatings. We have done some testing on insulating coatings in order to replace regular thermal insulation in our plants (chemical industry). Testing was done on small coated steel plates (thickness of plate was 5mm and coating was 3mm of thickness). The plate was then put on a hot plate at 100 °C for 10 minutes. When looking with the infrared camera (emissivity was put at 1), an average tempurature was measured of about 80 °C (on a steel plate without coating in the same conditions, average temp. was 96 °C). When we measure the temperature with a contact thermometer on the coated plate while on the hot plate , we measure about 62 °C. This seems more correct because we were able to touch it without burning ourselfs. My question is, what is the problem with the measurement done with the infrared camera?
Some explanation on the picture : picture is showing the coated plate on the hot plate. White and black part is paint we have put on to make it easy to compare with not coated plates on which we put the same paint. The coating itself contains very small particules with incapsulated air. Thanks in advance
 
Re:Insulating coating manuel-thermoimagen 10/22/2009
It is not clear enough your description of objets conditions to me. E value set to 1 is for very high emissivity or opaque bodys. Can you please tell me if the "non coating plate" have shiny or mirror finish? what was the temp readings for the "non coating plate" with A) contact thermometer and with b) infrared camera?.. may i ask if you have taken any infrared course?
 
Re:Insulating coating ROP 10/23/2009
On the picture you can see the test on the non coated plate above and the coated plate below (plates are steel). Thickness of non coated plate is 5 mm (coated plate is 8 mm) put on the same hot plate during the same time. The non coated plate was painted with the same black paint used on the coated plate in order to make comparison possible. Temperature readings on the non coated plated (as you can maybe see on the picture) were 95 °C with the camera and 89 °C with the contact thermometer and respectively 82 and 67 °C on the coated plate.
I have put E value at 1 to make it easy for comparison and if I lower the E value, my temp. readings will be higher, for example if I put the E value at 0.8 on the non coated plate, I will find an average of 109 °C.
Hopefully, this explanation makes it clearer to you and others and yes I have taken a level I course.
PS I have even my doubts on the readings with the contact thermometer on the coated plate since we are able to touch the plate (during a certain time) without burning ourselves and that with a reading of 67 °C??
 
Re:Insulating coating Bob Berry 10/23/2009
As you can see, contact temperature measurements are not as simple as people think. Contact temperature measurements work on the principle of thermal equilibrium. They will only display the correct temperature for the sample if they have reached equilibrium with it. This depends on the mass of both objects, the time in contact and how good the contact is. Firstly, if your sample is in transient, then it is very unlikely that the thermometer will display the correct temperature.
I think it is likely that the particles in the paint are reducing the contact with the thermometer and affecting the accuracy of the thermometer measurement. And I think the same thing happens when you touch the painted sample.
I have looked at so called insulated paints before and my conclusion (along with many others) is that they don’t work. It simply is not possible to get enough insulation into a paint to make any real difference. Ask the manufacturer for a verified U or R value for the paint, I bet they can’t give you one. They might offer a U value equivalent, but this does not really exist, a material either has a U value or it does not.
Finally, you must use the correct emissivity for the paint and not a value of 1.
 
Re:Insulating coating ROP 10/23/2009
Thank you Bob. A agree on the difficulties of contact thermometers. But still, what is the problem with the measurement of the infrared camera which gives us 82 °C as an average temperature? If I put an emmissivity lower then 1, it will be even higher and then it should really be impossible to touch it or do I oversee something obvious to explain this.
 
Re:Insulating coating Bob Berry 10/23/2009
ROP wrote:
ou Bob. A agree on the difficulties of contact thermometers. But still, what is the problem with the measurement of the infrared camera which gives us 82 °C as an average temperature? If I put an emmissivity lower then 1, it will be even higher and then it should really be impossible to touch it or do I oversee something obvious to explain this.
What was your reflected apparent temperature set at? How did you measure this? Was there a spot reflection in the image?
 
Re:Insulating coating ROP 10/23/2009
Bob Berry wrote:
What was your reflected apparent temperature set at? How did you measure this? Was there a spot reflection in the image?
No spot reflection around the object. measurements were performed in our lab and reflected apparent temperature was set at 24 °C.
 
Re:Insulating coating JKEngineer 10/24/2009
A few comments:

I suggest that you not rely on a fixed time, 10 minutes, to initiate your measurements, but use an extended time, monitored with the camera for example, to reach steady state. This will then reflect a non-transient condition, as Bob points out.

The ability to touch a hot surface is governed not only by its temperature, but also by the thermal diffusivity of the surface and material. Highly diffusive materials are harder to touch at a given temperature than non-diffusive materials. The rate of heat transfer into your hand is a factor in your ability to touch the surface and this is affected by the temperature, the mass, and the thermal diffusivity. For example, you can take a piece of aluminum foil out of an oven at 350F, for example, and touch it without being burned because it has such low mass and cools sufficiently on contact that it does not actually heat your hand significantly.

You must measure and use the emissivity of each of the surfaces. If you are taking your readings on the overapplication of black paint then your emissivity is consistant across the samples. However, many of the "insulative paints" on the market depend on reduced emissivity to affect their radiative transfer, not on reduced thermal conductivity to reduce their conductive transfer. You are therefore running the risk of destroying what you are trying to measure in the method by which you are measuring it.

If you want help in redesigning your testing, or an outside agent to perform the testing for you, please contact me.

Jack
 
Re:Insulating coating JKEngineer 10/24/2009
Sorry, I never remember which bbs has signature files built in and which does not.
Jack
Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

Bronx, NY 10463

718-884-6644
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Skype: JKEngineer

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Re:Insulating coating Mansor 11/11/2009
Hello ROP,
I propose to do the measurements after removing the plate from the hot plate. The problem could be due to heat transmission, the additional energy the camera receives but the contact thermometer cant see it.
Mansor.
 
Re:Insulating coating Phanikumar 11/14/2009
Dear Rop

I think you should change the emissivity settings of the camera while taking readings on bare steel and insulated surfaces. As of my knowledge emissivity for steel varies 0.8 to 0.9 and insulated surfaces around 0.15. Second thing is IR thermometers if they have the vaiable emissivity settings then only you get good temperature readings. Contact thermometer is good enough, it works on the thermocouple principle .
 


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