DIY Sensor Probes for RANCO Controllers

So I wanted to use my RANCO ETC controller to control my electric HLT, but I realized that the probe that came with the unit was coated with a crappy plastic. I thought about housing the probe in a thermowell, but the stock probe is essentially already a thermowell for a 30 kohm thermistor. The response of this probe didn’t seem all that good and so I set out to make some custom probes to use with this controller. I thought I would chronicle the process here for the benefit of other homebrewers who may be using this common controller.

I ordered the following components from Digi-Key.ca (all costs are in $CDN):

Part Part # Unit Cost
30 kohm thermistor, 30mm NTC CP3-1004-ND $0.887 ea
Male phone plug, 2.5 mm 490-4655-ND $0.507 ea
Right Angle Panel Jack, 2.5 mm CP-2502-ND $1.34 ea

They have flat rate shipping ($8.00) which makes things relatively easy. I believe the US site for Digi-Key also offers the same deal. I also purchased some ¼” brass tubing from Metal Supermarkets to make the probes. I chose brass over stainless to get quicker response.

First, I cut the tubing to the lengths that I wanted for my probes. I then crimped one end of the tube using a pair of vise grips:
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I then used a stationary belt sander to round off the crimped ends:
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Once all of the probes were roughly rounded off as above, I proceeded to solder the ends closed. I used some very fine grit sandpaper to smooth out the probe tips and the probe:
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Once the probes were done, I proceeded to solder the thermistors. I decided to use cat5 cable that I had lying around. These types of thermistors only require 2 wires, so I only needed 2 of the 4 twisted pairs in the cat5 cable. I twisted the pairs together and soldered them to the end of the thermistors:
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I then covered the thermistor connections with shrink wrap tubing:
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I pushed the thermistor into the brass tubing until it hit the end. I then filled the open end of the sensor tubing with JBWeld using a disposable syringe:
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Once the ends of the probe were filled, I put some shrink wrap and electrical tape over the end to seal it all up. On the other end of the wires, I soldered a male 2.5mm plug for making the connection to the RANCO or to a DMM:
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The last step was to add a female plug connector to the RANCO control unit so that I could use different probes for different applications. The internals of the RANCO controller before I made any modifications looked as follows:
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The stock probe has two wires coming in from the thermistor sensor. Thermistors are not a directional device and therefore it doesn’t matter which wire is connected to which side of the thermistor. I cut the existing 2 wires to make the connections for a plug:
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I drilled a hole in the case to house the female panel mount jack. I soldered the 2 wires to the jack and mounted it to the case:
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I also added a similar female plug to my digital multimeter (DMM) so that I could also take temperature readings without the RANCO unit:
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http://rancosensors.shutterfly.com/40

All in all, this setup has worked excellent for me. I have calibrated all of my probes using 2 accurate thermometers from my lab, and the factory curves were very accurate. I have made a 16” probe to reach into my HLT thru the lid rather than make a hole in the side of the vessel. I have also made 2 probes to reach thru the lid of my MLT so that I can measure the temperature of my mash without lifting the lid. I hope this info helps. Any questions, post away.

Cheers,

Greg

I'm impressed with the project but just as a PSA to others, the Johnson A419 already has a stainless probe.

Very nice!

Once the probes were done, I proceeded to solder the thermistors. I decided to use cat5 cable that I had lying around. These types of thermistors only require 2 wires, so I only needed 2 of the 4 twisted pairs in the cat5 cable. I twisted the pairs together and soldered them to the end of the thermistors:

Click to expand...

Curious why you used a pair for each leg instead of a single pair carrying both legs?

Derrin said:

Very nice!



Curious why you used a pair for each leg instead of a single pair carrying both legs?

Click to expand...

I decided to twist them together so that resistance from the wiring would be minimized/reduced. To be honest, the next probes I make will not use cat5 cable. I would likely use a simpler 2 wire setup (22-24 gauge wire) to make it easier to feed the wire into the probe end. With the cat5, you have alot of extra wires not being used, which makes it needlessly bulky. In my case, I already had it and so it was cheap.

gotcha, that makes sense. I bought a bunch of DS18B20's, LM34's and 1K thermistors and I've been planning to add built sensors to my line. I was going to use 2-wire with drain cable for all of them. Just wanted to make sure you didn't know something I didn't! My understanding is splitting the signal between pairs will nullify any suppression the TP provides. You might have been better off wiring the blue and green to one leg and the stripes to the other. It probably doesn't matter in the least in this environment though.

I know this is an older post but I had a somewhat related question: Do you think that an RJ11 male plug could be wired to a ranco probe, and then wire the female plug into the ranco unit so that the probes could be swapped out?

The_Dtrain said:

I know this is an older post but I had a somewhat related question: Do you think that an RJ11 male plug could be wired to a ranco probe, and then wire the female plug into the ranco unit so that the probes could be swapped out?

Click to expand...

I can't see why not. The main thing to watch for is that you want to minimize the resistance added by the various connections and the wiring since this will affect the output of the thermistor. Otherwise, I would think it should work.

Cheers,

Greg

Anyone know what thermistor would be needed for the Johnson A419?
Looks lite it is a PTC thermistor with a resistance of 1035 ohm at 25C, but that's all I can make out.
http://cgproducts.johnsoncontrols.com/MET_PDF/125186.PDF