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Laughing Jaguar
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Problems Taking Temps with IR sensor

02/23/2011 1:45 PM

Recently, I completed the construction of a 6 foot long, 10 inch diameter mandrel with heaters inside so I might elevate the cure temperatures of composites. The intention is to speed up the cures and elevate the epoxy's transition threshold above what is normally achieved with ambient temperature cures. The exterior of the mandrel is 26 gauge galvanized sheet metal formed on a Slip Roller and secured to 10" diameter plywood bulkheads. Hi wattage incandescent bulbs inside are regulated with a 120 VAC PWM circuit. I turned on the heaters and allowed the system to "soak" for three hours and reach equilibrium. I then tested my hand held optical temperature sensor on myself and it read 98.4 F but when aimed it at the sheet metal on the mandrel, it only read 70 F. I then aimed it at an end bulkhead made from 3/4": plywood. Again I got 70 degrees F. I opened the plate covering an access port, aimed the temp sensor inside and the indicator immediately swept past 165 F! When touched, the mandrel was so hot I could not keep my hand there for fear of being burned. I removed the sheet-metal cover plate and applied some flat black, hi-temp engine paint thinking perhaps the emmissive characteristics of the bare sheet metal and wood were where the problem is. I've not tested the system with the painted parts yet. Do you have any thoughts on why an IR temp sensor would have difficulty reading the correct temps in this situation? Thanks L.J.

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Usbport
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#1

Re: Problems Taking temps with IR sensor

02/23/2011 2:27 PM

I think you are correct that the problem is the differences in the emissivities of the different materials. The IR sensor device I've used (that uses a red diode laser for aiming) would give different readings within a temp-controlled room, depending on whether I aimed it at the walls, metal pipes, or the vinyl floor.

The instruction manual said that if the user was unsure of the emissivity of the surface being measured, to place a 2" square of regular (beige) masking tape on the surface and wait a minute for the masking tape to take-on the temperature of the surface being tested. The emissivity of the masking tape is close to the emissivity of the calibration standard they use when calibrating the meter. I did that, then readings were very consistent.

Keep in mind that the IR sensor is measuring the average temp over an area. Read the owner's manual to see how large an area it 'sees'.

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Laughing Jaguar
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#2
In reply to #1

Re: Problems Taking temps with IR sensor

02/23/2011 2:35 PM

USB wrote: ". . . place a 2" square of regular (beige) masking tape on the surface. . . "

Thanks for the prompt reply and the suggestion, which makes sense!

The Laughing Jaguar

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Just an Engineer
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#3

Re: Problems Taking Temps with IR sensor

02/23/2011 9:29 PM

Does your mandrel have any internal insulation?

I did something similar once, measuringthe outside surface of a heated box. I kept getting a reading a little higher than ambient.

After thinking about it for a while, the outside surface of the box reached an equilibrium that was closer to the room ambient, while the insulated interior was very hot with the appropriate heat gradient through the surface. Also got similar results with thermocouples placed against hot surfaces until they were embedded into epoxy on the surface.

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Laughing Jaguar
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#4
In reply to #3

Re: Problems Taking Temps with IR sensor

02/23/2011 11:11 PM

You asked "Does your mandrel have any internal insulation?"

None at all, the only thing between me and the internal heat source is a thin wall (26 gauge) of galvanized sheet steel.

To insulate might provide a more uniform distribution of heat but it would occur at the expense of time to cure and the time needed to raise the final temperature of the matrix.

Like I pointed out earlier. . . . . when I touched the surface with my bare hand I pulled away instantly so I would not get burned.

Thanks

Art

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Randall
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#5

Re: Problems Taking Temps with IR sensor

02/24/2011 6:22 AM

Just an off topic point, which might help a bit with the overall project.

If you're going to be doing a lot of relative measurements then invest in a tripod from the camera shop, so that you can be certain you are always pointing at the same spot. Even if you don't think this will improve the accuracy of the readings it may speed up the process quite a lot.

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