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Author: Subject: Protect a thermocouple?
Acetic Acid
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[*] posted on 3-9-2011 at 17:23
Protect a thermocouple?


I use a digital multimeter with temperature capabilities on a regular basis. However, I have a problem where the thermocouples are not well protected from chemicals and even the heat they are designed to measure. The plastic around the tip of the rope burns at around 250C and the metals making up the probe wouldn't stand up well to a lot of chemicals. How should one protect their temperature probes? Thanks.
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Magpie
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[*] posted on 3-9-2011 at 17:42


Buy one of these:

http://www.omega.com/ppt/pptsc.asp?ref=KMQXL_NMQXL

I buy the 1/8" diameter, 12" long type with SS sheath.




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Acetic Acid
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[*] posted on 3-9-2011 at 17:48


Expensive :O looks very worth it though
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[*] posted on 3-9-2011 at 18:26


Have you tried encapsulating the probe in glass tubing?



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Acetic Acid
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[*] posted on 3-9-2011 at 18:30


No. Do you think that will work? If I heat seal it there might be a good chance.
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[*] posted on 3-9-2011 at 18:54


Glass tubing is a great idea, or try coating it in a thin layer of RTV silicone, it should hold up in most things accept acetic acid, and acetone or alcohal type solvents. If you can find some teflon tubing, that will hold up to all but acetone or temps over 500 Deg F.



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smuv
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[*] posted on 3-9-2011 at 19:02


You can buy probes similar to what magpie mentioned from ebay for < half as much. Of course, these are Chinese probes...but should be good enough.



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[*] posted on 3-9-2011 at 19:19


Seal the end of a tube, put a small piece of hot melt glue in the end, stick your thermocouple in and melt it together

If you melt the glass onto the probe it will burn and smell bad.




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Acetic Acid
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[*] posted on 3-9-2011 at 19:26


Does a thermocouple sealed in glass tubing give accurate temperature readings?
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magnus454
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[*] posted on 3-9-2011 at 22:00


It would probably slow the response time down a little, but should still read just fine. The particular one I have (type J) is about 12" long stainless steel, 1/4" o.d. probe with epoxy glass fiber wire insulation, and is connected to a 1940's weston pyrometer in a wooden box that reads from about 50 deg F. to 600 deg F. it has about a 3 sec response time to it.



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[*] posted on 4-9-2011 at 03:40


I used glass tubing with one end sealed in flame.
If you're worried about the reading lag, you can do two things. First, you can do some flameworking and make the end seal thin walled (be sure to put a small piece of cork on the bottom; it cushions the steel).
Second, way more efficient, but somewhat more dangerous method, is filling the tube with small amount of mercury. That method is actually used in special thermometers as the mercury fills every space and makes a great heat conducting seal between the glass and the steel probe. And it doesn't dissolve steel.
Just be sure to find out how much of the probe is actually sensitive to the temperature changes, to use as less mercury as possible. It's usually less than few centimeters, so you'd need less than half mililiter.
Top of the tube should be loosely sealed with a small cork. Here's a sketch.

Of course, you realize this shouldn't be used for very high temperatures. I'd say 200-250 °C would be a wise limit.

No matter the exposed surface of mercury is very small, use ventilation if you're going to heat it.

[Edited on 4-9-2011 by Endimion17]




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Acetic Acid
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[*] posted on 4-9-2011 at 05:22


Yeah I can't really use mercury. There shouldn't be too much an issue with me waiting a few seconds for the reading to stabilize. Thanks!
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[*] posted on 4-9-2011 at 06:26


Alternatives to mercury are silicon oil if the temperature range isn't too high, small solid glass beads, or metal shot. Don't fill much above the sensitive section of the probe to keep from adding unneeded thermal mass.

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[*] posted on 4-9-2011 at 06:39


Ultra high temp epoxy would be another idea as well, 400Deg, 3000Deg, etc.;

http://www.graphitestore.com/items_list.asp/action/prod/prd_...

http://www.graphitestore.com/items_list.asp/action/prod/prd_...




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[*] posted on 4-9-2011 at 07:45


Graphite dust might be ok, but it's messy. Graphite is a relatively good heat conductor, but in a solid state. Pulverized graphite... a lot less.
Glass beads are poor conductors. Very tiny copper shot is an excellent idea, but it has to be tiny.

Here's my idea. The end of the tube is thinner than the rest.
Mercury is really the best choice and it's used in this manner for sensitive measuring equipment. In this case, response time is less than half a second.
It's a very tiny amount, and can be even less if I had proper glass tubing, so the thermal mass can be very small. It could be done with the amount used in only one thermometer.
The cork doesn't even have to be loose, and the surface of the exposed metal is negligible.
The danger is similar to putting an actual Hg thermometer in the heated vessel, and we've all done that.




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[*] posted on 5-9-2011 at 16:08


MgSo4 packed around the bare untouching (except at the end) wire into a glass pipette with the end sealed is what i do, grind the mgso4 very fine and if its really nice and dry it will flow very well and you can gently pack it down with anything appropriate like a glass rod or a plastic rod or a metal rod or best still BAMBoo skewer. Advantage of glass is it inertness and that you can see how well you've packed it and you can ensure the integrity of your circuit, ie the wires aren't touching. Poor thing is glass's poor heat transfer coeff. Stainless steel conducts heAT BEATUIFULLY but is reactive and opaque and conductive so packing the wires in is difficult.



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