Faulty Readings on Thermometer

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CookAS
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Joined: Thu Jul 19, 2018 6:53 pm

Faulty Readings on Thermometer

Post by CookAS » Tue Jul 24, 2018 5:51 pm

I read in other forums many cases where an electronic thermometer goes faulty with inaccurate readings. I had this problem and found the cause.
Problem: Our Weber Style 6741 Wireless thermometer
WeberStyle6741.jpg
WeberStyle6741.jpg (25.04 KiB) Viewed 334 times
produced temperature readings higher than actual which caused it to detect the end of cooking ahead of time and produced lean outcomes. After an investigation and testing this was found to be caused by moisture within the probes.
The temperature sensing element inside the metal tube is a bead-type NTC thermistor whose resistance falls from around 100k ohm at ambient to a few thousand ohms at 100 degrees C. The metal tube is lined with fibreglass fabric winch acts to protect the thermistor from instantaneous direct heat like happens if the probe falls into the fire for a short time or there is a flare-up. But it also acts very effectively as a wick, so if you drop it into the sink it very effectively conducts soapy water down to the probe tip and holds it there. The soapy water sits across the bare wires exiting the thermistor bead, lowering its resistance value and this is read as a higher temperature than actual.
How to avoid? The obvious is not to dunk the probe. This is hinted at in the manual under Care and Usage on page 19 "Never immerse Probes in water while cleaning." In the real world most people would expect something that gets covered in cooking meat needs to be cleaned in the sink so it is going to happen. When it does the fix is to just leave the probe in an elevated temperature setting for a few days. Not too hot, perhaps dangling in front of an air heater outlet in winter or in the sun in summer. The moisture will eventually dry out of the probe and the readings will come back to normal. Prevention is better than cure so the best cleaning is not to put the probe anywhere near the sink and clean it with a non-scratch sponge scrubber from the bend to the point.
How could the manufacturer have constructed this better?
Insulate the thermistor leads from bead to termination with the cable that goes up through the probe tube. One lead had a short section of fibreglass insulation on it to prevent shorts but it needs liquid protection too. Another way would be to effectively seal the top of the tube with a high-temperature silicone or epoxy, taking care not to produce a flex point that would promote the early failure of the cable.


Davo
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Re: Faulty Readings on Thermometer

Post by Davo » Tue Jul 24, 2018 9:55 pm

Yes I found that out very early when relying on electric temp readers and yep, guilty for dunking into water :oops:

I've still got the original Weber Style monitor when they first began to become available but not being a tech head, found the probes a real pain in the arse to clean...so easy to get any kind of moisture dripping down inside that probe but isolating or insulating wouldn't be easy due to the long exposure to heat they have to endure plus then, it has to be pliable so it can bend, twist etc so anything that would have to be used for insulating would have to withstand heat plus be flexible so that it won't crack and be compromised. The only substance I can think of is that silicone that the new age non scratching cooking utensils are made of. Silicone seems to be heat resistant and pliable enough to be used as some kind of insulate and it doesn't seem to onsorb moisture either...well at least my silicone basting brush doesn't.


Cheers


Davo
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