Inexpensive pre-fab thermistor probe?

April 8, 2009 5:17 PM

I need a new thermistor probe for this temperature controller. The manufacturer sells these for $37 shipped. Is there a less-expensive solution?

I use the controller for sous-vide cooking, which means the probe is always submerged in hot water, which maybe leads to frequent failures. I know from the manufacturer that it's a thermistor with 5K ohm resistance at 25C. It has a stereo miniplug "jack" to connect it to the controller. They say it's 3.5mm. I suppose I could get the parts and solder my own but I am almost certainly too lazy. Does anyone else make these? Or have any suggestions on getting them to last longer?

posted by hsawtelle to food & drink (6 comments total)

Unfortunately you'll need to find someone who makes replacements for that exact model, or take the chance that the industry has standardized on a particular type, because you're not fully constrained yet on the specifications -- to completely specify you'll need to know if it's NTC or PTC (negative/positive temperature coefficient, or, does the resistance go up or down when the temperature goes up?) along with its B-value (very roughly, a measure of how quickly resistance changes w/ temperature). Chances are it's NTC (especially since Digikey doesn't list 5k PTC as being available at all), but without matching the B value the calibration will be different.

I could of course be wrong, and you could try looking for a generic temperature probe with a phono plug and hope it's calibrated right. There seem to be a lot of manufacturers, though quick searching didn't find anything cheaper that what you're being asked to pay for the correct replacement.

As far as making the next one last, for me what usually wrecks these things isn't the immersion in hot water, but either mechanical fatigue, especially at the joints in the cabling, or washing technique -- the caustic soap used in dishwashers is probably right out for something like this.
posted by range at 6:29 PM on April 8

I deduced that it's NTC - I forgot to mention that. No clue on B-value. Thanks for the info, range. I broke down and ordered a $37 one (they gave me a discount because it was just out of warranty, of course. I would still like to figure this out for when the replacement fails. I may be able to deduce the B-value using a multimeter and known temps?
posted by hsawtelle at 6:42 PM on April 8

Yeah, you should be able to get 2 data points from ice water and boiling water and deduce the B-value from that. The unfortunate part is that very few manufacturers of "finished" sensors (eg, with phono plugs, that big metal bit that transfers heat to the thermistor, etc) sell them sorted by B, they just say "replacement probe for Fluke model XXX" because nobody is (normally) crazy enough to care about that sort of thing. It does, however, give you the info to buy the right component, so you could build your own.

On the other hand, at $37 the time to build your own will very quickly fail any sort of cost-benefit analysis (as in, how many dogs would I have to walk / cups of coffee would I have to pour / quarts of blood would I have to donate to make $37), so if you build it you should at least know that you're doing it to learn how to make a temperature probe and for whatever the geeky equivalent is to machismo (I say this in full support of this plan; I have plenty of $10 toys around here that took me $500 in labor to build).
posted by range at 8:12 PM on April 8

I will also try to reverse engineer the broken one - maybe I can get a part number off of the thermistor sensor or something. $37 is tough to beat time-value wise, but if these things break every 4 months it is going to be annoying, especially since the controller only cost $130.

If I get annoyed/motivated enough I could always build my own controller (which is what I originally set out to do, and in the process I found this one) designed around a cheap off-the-shelf probe.
posted by hsawtelle at 10:17 PM on April 8

I have seen some BBQ temperature probes at local stores, and the metal probe part is about 12 inches long with a curved hook at the end where the wire comes out. This sounds like a much better design than the one you linked to, where it appears that the whole probe and wire are submerged.

Maybe you could find a fat old-school cooking thermometer, cut the dial off the top, and push your probe down to the bottom of it? There really seems to be some room for improvement there.
posted by MonsieurBon at 11:25 PM on April 8

Digikey has a bunch . Checking prices, none of the ones they have in stock were more than a few bux. Most were under a dollar.

Characterize the new one you have using a crushed ice bath and boiling water for data points, or examine the broken one to get a part number and buy some replacements. It'll be a decent and fun thing to do yourself.

As far as making your own controller, it's not all that difficult. The packaging of the one you have is fairly decent, however, and I'm not sure you'd get much mileage out of that project, unless you just like to roll your own electronics.

O, and you might want to check out Omega Engineering Thermistor Probes. They probably won't have anything cheaper, but they sure have a lot of choices. I usually go there first for temperture measurement stuff.
posted by FauxScot at 12:59 AM on April 9