How-To: Pt100/RTD 1/2" NPT Thermowell

I decided to add some temperature monitors for my MT and BK in my system.

I wanted a cheap, LED temperature monitor that was panel mounted. It had to be dis-connectible since it was connecting to the MT and BK which get removed for cleaning etc. Because of this, I didn't want to have to deal with Thermocouples, specialized wire, fragile wire, and specialized connectors. The two obvious alternatives are the DS18B20 sensors and Pt100 RTD sensors. I chose the Pt100 sensors because they have small size, and are compatible with most PIDs. This also gives me a solution that I can just use XLR patch cables to connect back to my system with.

I started with the Omega 1PT100KN1515CLA for $12.00: http://www.omega.com/ppt/pptsc.asp?ref=1PT100KN_2PT100KN&ttID2=1PT100KN_2PT100KN

I used the eBay 1/2NPT Thermowells. I am sure this link will expire soon, but a search should pull them up, they were $9.99: http://cgi.ebay.com/THERMOWELL-304ss-1-2-FPT-x-2-1-2-L-x-1-2-MTP-279ER21-/200520533079?pt=LH_DefaultDomain_0&hash=item2eaff48457

The rest was made up of XLR connectors, a little shrink tube, and some electrical tape.

The sensors are tiny, so make some short lead-out wires, tack-solder them on, and wrap them in shrink tube. Then wrap the whole assembly in shrink tube. Depending on how fast of a response you want, leave more of the sensor protruding. The way I did these, I get perfectly fine response for the MT and BK. If I were doing these in a RIMS tube, I would probably choose a lower mass thermowell and find a way to get good contact inside of the thermowell. These simply float in the thermowell, but when going from 60-140 in a bucket of water, it took about a minute to get within 1° of actual.

Here are the tiny lead-out wires:


The blue shrink tube starts at about 1/16" ID, it goes over each solder joint all of the way up to the body of the Pt100. The clear covers both of those and provides some strain releif and structure by holding the actual sensor body:


The other side of my lead-out wires connects to the XLR connector. I jumper two of the 3 connector pins together in order to provide the loop-resistance measurement for the PID (White wire connects to the lower of the 3 pins, hard to see in that picture):


The XLR connector is assembled but without the rubber boot, and 2 wraps of electrcal tape are applied around the body:


This then threads directly into the 1/2" FNPT threading on the thermowell:


It all connects into a vessel like any other 1/2" MNPT threaded item would:






Enjoy.

I am about to cut the perfectly good spade connectors off my brand new PT100 so I can mount a locking XLR plug at the end that will mate with an XLR receptacle on my control box. I have two questions for you (or anyone else who knows).

- First, does it matter what kind/gauge of wire I use between the receptacle and PID in the control box and does relative wire length matter?

- Second, why did you short two of the wires? My PT100 has 3 wires, and my PID appears to have 3 terminals for connecting that probe.

First:
As long as you have a 3-wire connection, no, just keep the wire gauge fairly consistent across all 3 wires

Second:
That is how the 3rd wire works. The control can compute the loop resistance of one of the wires going to the actual RTD (It only has two connections), and that will help eliminate any error from the resistance of the wires.

For RTDs this is pretty critical since the RTD element only has 100 Ohms of resistance, the wiring can account for a fair amount of resistance relative to the element, thus skewing your measurement by a bit.

If you already have a 3-wire connection on your RTD, the short occurs closer to the element, so you cant see it. Just wire up your XLR connectors and go, no problem.

I have used these for a couple brews now, they are definitely be BEST way to measure temperature for a homebrewer. I have never had 0.1 degree accuracy that reads at exactly 32.0 degrees F in well mixed ice-slush water, and 211.6 degrees in boiling water.

I now use a RTD for my RIMS output/control, Brew Kettle and Mash-Tun sensors. I still use a K-Type for my Flash Boiler output because I want really fast response time.

Thanks for the info! Since you seem to know your way around an RTD, here's another question: The PT100 I got says that it's not liquid tight and "may leak at high pressures". Since I'm not dealing with any pressure other than the weight of the liquid, do I have to worry? I could chance it as-is, I could seal it up with JB Weld where the probe meets the threaded housing, or I could spend some more money and get a thermowell. Obviously, those possibilities are in order from cheap/quick to expensive/correct. What's the worst-case scenario for a PT100 leak? Destroy the PT100? Get inaccurate readings? Just a little wort dripping onto the counter?

I am not sure what you mean, but if it is a bare RTD... looks like ceramic or glass usually, then you want to protect it, ideally with a stainless thermowell...

The quick and easy alternative if you are just making a probe and dont need it to go through a kettle wall or something is to buy a 1' section of soft copper tube, crimp and solder the end closed and slide the RTD and wires down to the end. Then use JB Weld or epoxy on the other end to seal it up, bend it into a hook, handle or whatever.

I did this for about 50 batches!

It's not a bare probe. It's about 4" long, steel, and with a threaded base where the lead wires come out. Here is the exact item. (I'm also confused a bit because the website calls it a thermocouple but I thought PT100s were all RTDs - am I wrong or are they?)

I am all for DIY'ism, but wouldn't something like this be a lot easier?

Absolutely. And had I known what I was doing when I ordered the parts, the one I would have gotten. Honestly, the "business ends" look almost identical. Mine has a longer probe and a braided sheath protecting the wires. Otherwise, though, they look the same. That's why I wonder if what I have will actually be "liquid tight" under brewing conditions.

RTDs are great. Mine still read 0 +- 0.1C in ice slurry MANY years after setting them up. I have three wire all the way to the monitor/controller - it's a pretty long run. I used telephone wire, as it's cheap and already braided for you. I like the 3 pin connectors from omega. This is all for fermentation - I really should rig something up for the mash tun.

Mine had to be 1/2" NPT for the kettle side so i went with the same for the mash tun. I actually use that exact one for my RIMS tube.

Paul, does the XLR connector actually thread into the thermowell body? I'm looking at a thermowell body with no threads on the inside and wondering if your XLR connector is indeed threaded.

Thanks,
-Rob

Unfortunately, it doesnt, I ended up replacing the last plastic piece on the XLR with a little JB weld, which works flawlessly.

Thanks. I just ordered a 6" thermowell from brewershardware.com that doesn't have threads on the inside. I figure I'll secure it with epoxy or some other adhesive.