Thermistor Puzzle

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Given all this free time, I'm fixing stuff around the house.

I have a Brookstone Grill Alert. Stick a probe into a chunk of meat, plug  
the cable (2.5mm phone plug) into the box and it happily monitors my steak  
on the barbecue. Except it stopped working with unreasonably high readings  
(150F reading at room temp). Wisdom on the 'Net says that the probe is  
probably broken. But the replacement part (Brookstone) is no longer  
available. No problem. I'll plug my decade resistance box into the grill  
alert, start cranking in some resistance values and figure out the  
thermistor curve. Then find another probe.

It appears to expect an NTC thermistor with 180K ohms giving an 80 F reading  
and 60K ohms reading 125F. But then I pick up the old (broken?) probe and  
measure it. At room temp (65F) it has a 220K resistance. Holding the probe  
in my hands drops its resistance to 140K. Seems to be OK. I wiggle the cord,  
expecting an intermittent short (low resistance giving a high temp reading).  
Nope. It's fine. Maybe it's something wrong with the plug/jack interface. So  
I open the box, unsolder a lead from the jack and measure the probe  
resistance plugged in at the back of the jack. It reads 220K. Hook it all  
back up and it's still wonky (probe gives a 150F reading sitting on my  
desk). But if I plug my decade box in to the grill alert and dial it to 220K  
ohms, the thing reads room temp.

The Grill Alert appears to just not like that probe. What have I missed?

Paul Hovnanian     mailto:
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Re: Thermistor Puzzle
My guesses, officially classified as S.W.A.G.:

a) Clean the battery compartment, and replace the battery with a fresh one.
 Analogy: An audio speaker has a resistance that may be measured by a VOM,  
but in use, it has impedance that varies by input frequency. Could it be th
at the probe does the same under actual use? If so, available current will  
be critical for accurate readings. And these things live in a harsh environ

b) Cold-solder or a component is intermittent. Again, these things live in  
a harsh environment, and something could have worked loose. Fine when sitti
ng nicely on a bench, not so fine in use.  

c) Try the probe in a pot of water. Bring it gently to a boil. It will neve
r get above 212F, but you should be able to follow the change in resistance
 as the water heats, and it should be as gradual as the heating. The differ
ence between a test at fixed points, and a test of behavior over actual ran
ge-of-use conditions.  

I would start with c) first, as it will be the most immediately revealing.

Best of luck with it!

Peter Wieck
Melrose Park, PA  

Re: Thermistor Puzzle
On Monday, June 1, 2020 at 8:12:45 PM UTC-7, Paul Hovnanian P.E. wrote:
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It might be that there's some oscillation at the sense amplifier; the impedance of
the probe wiring might be the issue.

If so, good news; that means it's not gonna need a new probe.   Bad news,
you have an oscillating consumer device to debug.

Re: Thermistor Puzzle

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Could it possibly be a semiconductor probe that uses the forward voltage
drop of a diode?

~ Adrian Tuddenham ~
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Re: Thermistor Puzzle
Adrian Tuddenham wrote:

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I checked the probe with both polarities of my ohmmeter. Also used the diode  
test function to look for this.

The thing is that the temperature the device displays seems to work well  
with a resistance decade box. And all the parasitics that introduces.

Paul Hovnanian     mailto:
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Re: Thermistor Puzzle
contamination inside the probe, subject to position of probe (vertical / horizontal, which jiggling the *wire* might not have changed). Grind off the crimp at its neck or whatever holds the wire, pull out sensor.

Re: Thermistor Puzzle

I chased down the curve of a microwave cooking probe.  The way I did it depended on the temperatures of interest.

most temps
Stuck good and bad probes in a potatoe
Used peanut oil and the stove

Low temps
Stuck good and bad probes in a potatoe
Water and added ice cubes

I got an excellent curve match.  It also turned out, that a random probe I purchased had the same curve.

The bad probe actually had an intermittent contact.  The actual readout was broken at the time.  

Re: Thermistor Puzzle
Most failures are from wire fatigue, either at the neck of the probe or at the neck of the plug.  Reasonably easy to fix, never really lasts though.

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