filling the gap between thermowell and RTD?

[cokeguy]

What can I use to fill the 1/4" gap between the tip of an RTD and its thermowell? something that can be readily available anywhere, like grease, etc..? it is installed on an air duct, temperatures are low, 60 degC max. thanks for your advise.

 

[btrueblood]

Would you worry if it leaked out? Some heat-sink grease, like that found at Radio Shack or similar, would work. So would oil, though perhaps not as effectively. We usually leave them dry, unless there is a real need for speedy response.

 

[IRstuff]

Air ducts are generally not particularly rapid thermal response devices, so intimate thermal contact is not necessarily required.

TTFN
faq731-376

 

[Compositepro]

Rapid thermal response may not be required, but thermal lag of sensors can make control more difficult. In any case, just about any grease or oil will work including axle grease.

 

[electricpete]

A link with some advice, similar to that given above:

http://www.rtdcompany.com/technical_info/Install_rtd.cfm

Ensuring accurate performance
To ensure good performance of the sensor, the sensor must be in good thermal contact with the process. When using a sensor in conjunction with a thermowell you should use a suitable heat transfer compound to fill the air space between the sensor and the bore of the thermowell. This will improve the heat transfer between the process and the sensor, reducing the stem conduction error and improving the response time of the sensor. For moderate temperature applications (less than 250ºC) a silicone based heat transfer compound should be used. Higher temperature applications will require a graphite based material. An easy way to apply the heat transfer compound is to apply a generous amount to the tip of the sensor and insert the sensor into the thermowell, pressing the sensor firmly into the well until the tip reaches the bottom of the thermowell bore. The RTD sensor should be isolated from the sensor sheath and other metal components. Checking the insulation resistance of the sensor is an easy test to ensure the integrity of the sensor. Measure the resistance between the sensor leads, and the metal sheath of the sensor. The resistance should measure at least 100 megohms (100,000,000 ohms). Lower insulation resistances could be an indication that the sensor has suffered damage.

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(2B)+(2B)' ?

 

[DRWeig]

Check out the attached.

There are a zillion others available -- google "thermally conductive compound"

It's made for just this purpose. Be careful to apply it so that it doesn't run and leak everywhere. If your sensor goes in a well or tube, keep it pointing down or else pack/plug it.

Best to you,

Goober Dave

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[btrueblood]

Zinc oxide paste (available at any drugstore*) works ok too. Typically used for electronic heat sinks, where a non- electrically conductive paste is desired.

 

[DRWeig]

Agree, btrueblood. The Honeywell or other versions have a bunch of aluminum in there with the zinc, but zinc oxide paste is much easier to find. It's probably less expensive too.

Best to you,

Goober Dave

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[Noway2]

Ideally, when inserted, the RTD spring should be compressed slightly, so if you have a sizable gap perhaps a different RTD that is better matched to the thermowell would be in order. Otherwise, I agree with the suggestions above, use a good thermal compound or gel. It is also somewhat a misnomer that they only record at the tip as they will still be quite affected by conduction.

 

[roydm]

I have been installing RTDs in thermowells for a long time, I have never seen the need to add any form of heat transfer solution. If the probe is spring loaded so it males contact with the end of the well that's good enough. What's more important is to make sure the thermowell protruded into the flow so the well reaches process temperature.
It used to be common for a probe to reach the centerline of the pipe however if you're not carefull it's possible to have the probe vortex which is not good. Two inches into the flow stream is enough, most modern RTDs only measure with about the first 1/2 of the probe.

 

[DRWeig]

Good point for those who mentioned spring-loaded RTDs. From suppliers in the HVAC and other building services world, though, spring-loaded devices aren't available at a competitive price. Thus the RTD can't contact the well. A glob of goop solves the issue.

Best to you,

Goober Dave

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