Faulty Readings on Thermometer
Posted: 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 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.
Problem: Our Weber Style 6741 Wireless thermometer 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.