Temperature readings inside copper pipe - Thermal transfer Question

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- gtslabs
  
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posted
16 years ago

Wed, Sep 12, 2007 10:45 PM

I am building a temperature data logger for galvanized water tanks. They are about 8'L x 2'H x 3'W I am using a DS18B20 to record the temperature. I need a durable thermowell so I plan on using a 24" piece of 1/2" copper pipe. I will cap the bottom, insert the sensor halfway down the pipe and spray in expanding insulating foam at the top to seal it off. I only need to record the temp once every 15 minutes so I am not looking for immediate thermal transfer readings. The temp range is 21* to 25* C typical. I can not have the sensor in direct contact with the water hense the pipe.

My question is should I fill this pipe with something to help the thermal transfer or will the air trapped inside be sufficient? I have heard of thermal grease used for heatsinks but I would need a lot to fill a 2' x 1/2" pipe. I have about 12 to make so I am looking for an economical solution.

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- N:dlzc D:aol T:com (dlzc)
  
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posted
16 years ago

Thu, Sep 13, 2007 1:32 AM

Dear gtslabs:

What kind of sensor will you be using? Anything that can be "soldered" to the endcap?

What do you expect the rate of temperature rise to be?

You only need a blob near the sensor tip. Still, quite a bit for a 1/2" pipe.

Bond the sensor mechanically to the metal of your outer sheath. Better still, use a temperature probe that has its own thermowell and is immune to your process fluid.

You can fill the balance of the pipe with your foam if you want to, or just use a couple of "Nerf balls" jammed in to hold things in place.

David A. Smith

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- Brian Whatcott
  
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posted
16 years ago

Thu, Sep 13, 2007 1:45 AM

Set a small liquid filled container out into which the thermowell will fit. Set its temperature at say 80 degC Immerse the air filled thermowell in it. Measure the time delay to reach 63% of the difference between the thermowell start temperature (say 20 degC) and the container's temperature, (80 degC) Say 38 deg C rise. Multiply this time by four. If this time is longer than about 12 minutes, fill the container with lamp oil, or some such and recheck its time constant.

Brian W