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Old 10-17-14, 01:47 AM   #1
strider3700
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Default my temp sensor idea for inside of collector panels

I started on the road to building my own solar DHW heating system almost 5 years ago. Life got in the way but I've got back at it lately. It's going to be a copper/aluminum fin hizer design. Somewhere on the board there is a thread or two about an arduino based differential controller I was building. I've been using it for awhile to measure temperatures around the house and I wanted to use it to measure things for the solar setup. The sensors for that are DS18b20. I ordered a bunch off of ebay and they work great but are not water proof/resistant at all.

The lack of water resistance worries me so I've been mulling over ideas for a year or two. In the panel I want to measure income water temp outgoing water temp and panel temp in probably a couple of places. I wasn't content with just attaching the sensor to the outside of the pipe. I wanted as close to direct water readings as possible.

I knew that I didn't want to solder anything with a sensor in it directly to the copper so when i build the hizer I added 1/2" pex to copper adapters with the intention of crimping on a small chunk of pex with the sensor inside it "somehow" and getting the wiring back out. all without causing a leak.

Tonight I think I came up with something that will work. I took a standard end cap for the pex.


Then I filled the inside with melted leadfree solder. this was just a standard roll left over from soldering the hizer together.


next I flipped it over and driledl a hole in the top making sure I didn't go all the way through.


here's one of the sensors


and here's how it fits inside


I can't find the cat3 I've been using to build these so I can't complete tonight but the next step is to solder the 3 wires on to the sensor and wrap in heat shrink. Insert it into the plug. Carefully add a couple of drops of oil to transfer the heat to the sensor better. then use a few drops of silicone to seal the oil and sensor in. That will be the hardest part as any oil on the wire or cap will prevent a seal but the good news is a leak isn't the end of the world like a water leak would be. it just means a slightly less accurate reading. Really the oil should make such a little difference just filling the entire opening with silicone should work.

Anyways when I get some more wire I'll take a few more pictures but I think this will work well. My only concern is about air getting trapped in the system next to the sensors as they're both in high points. that may require some minor changes to the collector or maybe adding an air valve or release of some sort makes sense. At my current pace I'll have the answer before the end of the decade

I think for in the tank I'll do a similar idea but with the sensors inside of some 1/2" copper with a cap on one end and massive silicone plug on the other. That's what I've used outside for temp monitoring for 4+ years and they still work fine although that's a much easier environment then submerged in the tank.

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Old 10-17-14, 11:06 AM   #2
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You can get the sensors in a waterproof version just incase you didn't know. I use one in my Kegerator to track temp in a container of liquid.

Linky

For example

Last edited by celblazer; 10-17-14 at 11:08 AM..
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Old 10-17-14, 11:39 AM   #3
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yeah I've seen those and can build similar but the question of how to get them into the inside of the collector pipes without creating something likely to leak was my big issue.

Those will work fine for inside of the tank.
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Old 10-17-14, 06:49 PM   #4
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I think you guys are barking up the wrong tree with this sensor. The overwhelming majority of sensors used in solar are normal 2 wire 1k or 10k thermistors and they are cheap and accurate.

I have made 100s of them and potted them in 3/8" copper tube and then it is inserted in to a 1/2" copper tube which is soldered to the top header of the panel. Works great and is easy to do.

If you want to use the sensor you have, you will have to get some silicon insulated wire for the first 6 inches away from the panel, otherwise you will melt the insulation.
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Old 10-17-14, 08:28 PM   #5
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hmm I hadn't thought about the actual wire melting. I'll have to set up a test and see what happens with my mini panel stagnating. The data sheet suggests it will melt.
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Old 10-17-14, 08:42 PM   #6
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The controller companies often supply two different sensor wire colours. One is designed for high heat and the other one is for the tank which is lower temp. The sensor is the same but the wire insulation is different. I have seen lots of melted sensors which were switched.
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Old 10-17-14, 11:58 PM   #7
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Default Pictures speak thousands of words.







EDIT: Pictures posted with my phone on the go.

Top photo: Cheap ebay or amazon sensor, 1/4" OD waterproof stainless end, mates right up to MPT "icemaker" reducer (available anywhere plumbing dept).

Middle photo: Easy-peasy DIY sensor exploded view. Sensor tubing available at brewers' supply shops everywhere for $1 and up. Measure, solder, fit, insulate, assemble, done. Make custom sensors you just can't buy (at least not under $40).

Bottom photo: How the TO-92 sensors fit in icemaker tubing, 1/4" OD. If you buy straight sections, the sensor will slide right in. Coiled tubing not so well. Cheaper than stainless and easier to solder or braze for permanent mounting. Will slide inside a 1/4" copper or steel pipe plumbed as a thermowell.

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Old 10-18-14, 06:35 AM   #8
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If you do it the way Jeff did, make sure it is not a long run of wire. That phone cable is probably only 22g and a long run will change your resistance as well as power draw, and I think it is rated for 60C. There is 3 wire "belden" cable that is also shielded and has a higher temp rating.
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Old 10-18-14, 01:28 PM   #9
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Quote:
Originally Posted by Mikesolar View Post
If you do it the way Jeff did, make sure it is not a long run of wire. That phone cable is probably only 22g and a long run will change your resistance as well as power draw, and I think it is rated for 60C. There is 3 wire "belden" cable that is also shielded and has a higher temp rating.
If he's using DS18b20 (1-wire temp sensor), and it looks like he is, the temperature conversion is done inside the chip itself, and a digital pulse packet is sent back down the cable, which is immune to variation of value due to resistance differences. If a pulse packet returns that has been mangled, it is 'refused'. There are specifications regarding how long the sensor wiring can be (approaching 100 meters) and the topology (all sensors should be in a serial layout). In my own house I'm running six 1-wire sensors, I have broken the layout rules by using three branches (bad, bad, bad layout), but I have kept the runs pretty short, and everything is running just great. As I was adding the last branch and it's sensor, I could tell that I was at the very edge of data integrity. So far (2-years later) all is well.

The 1-wire devices can run just using data and ground wires, which is very finicky, or they can use a three-wire setup (power, ground, data), which is very forgiving and easy to implement. I would not have been able to do my wacky three-branch system had I not used the three-wire setup.

Once you are in "Arduino-land", there is almost no difference in difficulty between setting up analog sensors and digital sensors. Cost differences are tiny. Accuracy is very high.

-AC
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Old 10-20-14, 12:50 AM   #10
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there is a limit to how far you can go with the cat3 and ds18b20s in powered mode but it's a long ways. I hit it before trying a 100 meter run to see if it would work I believe. Parasite mode I hit issues fairly quickly trying to get 5 or 6 sensors running on a 50' long run.

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