Copper Thermowells

Being an engineer, I love DIY projects. Awhile back I decided that I wanted to take the digital path for temperature monitoring of my HERMS setup. I picked out the Maxim DS1624 IC and designed really tiny pcb's to mount it on with latchable connectors on the back. Sorry, my iPhone couldn't focus very well on it because its quite small.




The next step was figuring out a way to submerge these into hot liquids and still get an accurate temperature. I decided to design copper thermowells that I could screw into my kegs standard 1/2" npt couplings.




Finally in this picture you can see how the small sensor fits inside the cap of the thermowell. I am going to use thermal epoxy to mount the chip into the cap. Then I can place the cap back on the thermowell and seal it up using silicon adhesive. The wires then will run through the pipe to the outside of the keg and some insulation will be placed in the ends to prevent heat loss.





I have also designed a main board with a microprocessor that takes in the digital readings from eight sensors and relays them off to my PC via USB. I have finished writing the necessary code to implement the USB communication on my microprocessor and have tested it successfully. Next, I need to finish coding the serial communication with my sensors and then I can finally see how accurate they are and how quick they respond.

I think it should work great. Copper has a high thermal mass which will make it slower to react with all that copper, but like was pointed out in the other thread; we're not monitoring critical temps on the Space Shuttle.

It looks like you might have the room to thread the chip in. Could you solder the last joint and then thread the chip from the open and with the wire leads and epoxy it in place?

Nice work.

Nicely done, I like that idea a lot.

Derrin said:

It looks like you might have the room to thread the chip in. Could you solder the last joint and then thread the chip from the open and with the wire leads and epoxy it in place?

Nice work.

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What do you mean by thread the chip in?

The main reason I wanted to be able to remove the cap was to be able to replace a sensor. I figured if I used silicon epoxy then I could remove the cap rather easily and just place a new cap on that has a new sensor thermally epoxied to it.

Jared311 said:

What do you mean by thread the chip in?

The main reason I wanted to be able to remove the cap was to be able to replace a sensor. I figured if I used silicon epoxy then I could remove the cap rather easily and just place a new cap on that has a new sensor thermally epoxied to it.

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I think he means solder all joints and then push the sensor through the top and epoxy it in. Although you mentioned the possibility of replacing the sensors, so that wouldn't work out real well, although you should be able to unsolder the cap. Might be a bit messy.

Any links on the silicone epoxy you are going to use? Is it food grade?

HomebrewJeff said:

I think he means solder all joints and then push the sensor through the top and epoxy it in. Although you mentioned the possibility of replacing the sensors, so that wouldn't work out real well, although you should be able to unsolder the cap. Might be a bit messy.

Any links on the silicone epoxy you are going to use? Is it food grade?

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That's what I meant. Replaceable is good. I was thinking copper is cheap enough now that you could replace the whole thing. Either way I'm sure you'll be succesful.

Yeah that's a good point, it would be cheap to replace the copper. Then I would definitely have a water proof seal as well.

HomebrewJeff said:

Any links on the silicone epoxy you are going to use? Is it food grade?

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So I originally purchased some 100% Silicone Rubber from a random store to seal the joint on the copper tubing. I have stumbled across several websites that argue silicone rubber is food safe, but I am still worried about using it. Especially because I have no idea what kind of temperature it can hold up to.

Any suggestion on a food/temp safe material I could use to seal the copper joint??? I would prefer not to sweat the joint because I think that would be too hot for the chip and silver epoxy.

Here is a link to some food grade epoxy: McMaster-Carr, although it's a bit pricey.

I debated back and forth as well on how to seal the ends of the tube. I finally decided that I was just going to solder the tips. I used a vice to crimp the ends into a point, as much as I could. I then used the dremel to round over the edge and then finally added a bit of solder on the end. The pics below were before I did some finish sanding / filing, but you'll get the idea. I hooked up the compression fitting to the air compressor, and tested it in water at 30psi, with no leaks. That's good enough for me.




I'm going to make the wires just long enough to reach all the way to the end. I'll put a drop of epoxy on top of the chip, and then epoxy the wires on the back end of the compression fitting (outside of the kettle, so food grade won't matter). They won't go anywhere that way. I'm using DS18B20s, so they barely fit inside the 1/4" tubing, so I'm fairly confident I'll get an accurate reading.

HomebrewJeff, I have one suggestion then that might help you with your approach. Make sure you pre-coat the sides and leads of the chip with silicone so that you don't accidentally short anything out. I did that last night and it worked like a charm. This will allow you to put a good amount of epoxy on the top of the chip to ensure you get good contact.

Also, last night I applied a small amount of silicone rubber to the inside of the joint and pressed the cap tightly. Then I quickly removed any excess silicone that squeezed out of the joint using alcohol and sandpaper. I placed this in a full glass of water overnight with a control that didn't have a seal so I could compare the difference. It turns out, even without a seal only a single drop of water built up inside the thermowell while the sealed one was completely dry.

My next test is going to be boiling both in water for 60 minutes and comparing the results. I am also going to check if the water acquired any sort of off flavor due to the silicone rubber. It seems as though an extremely minimal amount of silicone is even in contact with the water.

Jared311 said:

HomebrewJeff, I have one suggestion then that might help you with your approach. Make sure you pre-coat the sides and leads of the chip with silicone so that you don't accidentally short anything out. I did that last night and it worked like a charm. This will allow you to put a good amount of epoxy on the top of the chip to ensure you get good contact.

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Thanks for the tip, I'll have to try it out. Just curious, I'm a noob to the electronics stuff, but if I didn't use the silicone, how would I short anything out?

Well, in order to utilize the copper thermowell, you are going to want to use some sort of thermal epoxy that will conduct heat efficiently. I chose to use arctic silver epoxy which is used to bond heat sinks and chips. This thermal epoxy is made of silver and so it also conduct electricity. So if it were to flow between two pins, they would short together, which means they would connect electrically.

go with some hot melt glue on the connections. Easier than silicone to cut off if you need to re-wire. Or you could just paint all exposed connections with enamel (like fingernail polish-the small brush in the bottle works great.) Then you can use the AS.

B

or just use heatshrink on each lead

After thinking about this, I guess I'm not overly concerned about heat transfer... considering how tightly these things fit in the tubing.



Considering the sensor will be several inches into the pot, I can't see that it's going to be that much different, although maybe a few seconds slower. I can live with that if it makes it so I can safely retrieve the sensor if needed. Maybe I'll try one and if the difference is significant.

edit: This picture is just showing the sensor sitting in the tube unattached. After it's soldered and the leads covered with heat shrink tape, it's much closer to the edge.

Well unless you have good thermal conduction over the surface area of the chip, I am guessing there will be both a lag and an offset which will need to be calibrated.

So I just did a boiling water test on my thermowell and I measured a temperature of 99.85C which is pretty damn close to 100C! I should point out that I didn't submerge the entire thermowell so I am pretty confident that it would have been even closer. I am planning on doing a true two point calibration once I have the sensors integrated into my kegs. Then I can just program in the coefficients and come up with a degree of uncertainty.

I also decided I am going to implement a fuzzy logic controller as opposed to the typical power integral derivative controller. I am currently working on the algorithm for a class, so I can kill two birds with one stone.

Last night I placed my thermowell in an ice bath and measured 0.4C. Things are looking good.