Thermal Couple Vs Thermal Probes (thermistors)

Nathan Stock

New member
I have heater meter 4.3 with thermalcouple for the pit probe. I have noticed that when I put one of my extra thermal probes (thermistors) in a similar location to the pit probe it will read about 15 degrees cooler.

As a test I put the 3 - Thremalprobes ( thermalworks pro series) and the two thermalcouples into the oven and set it to 225 with the heatermeter & multi meter for the other themalcouple and all the temperatures tracked with a few degrees. The is also true when they are at ambient temperatures.


I am wondering if the difference in temperature I observe in my smoker are do to the air current and the responsive difference between a thermistor and a thermal couple?
 

Bryan Mayland

TVWBB Hall of Fame
I would try wrapping the two in aluminum foil together (with paper between them if the thermocouple is exposed or the metal shield is connected to one of the two pins) so they experience closer temperatures. If they track together closely in the oven though, I'm not sure why they would read differently anywhere else. If the thermocouple was more sensitive, you'd see on BOTH sides of the thermistor probe, bouncing back and forth where the thermistor is averaging the temperatures due to its mass.
 

Nathan Stock

New member
I collected a little bit more data and have attached the log below. I put the thermalcouple & two thermal probes in a cold oven and set it to 225. They started within a couple of degrees of each other but during the warm up they diverged (thermalcouple and thermal probes by ~14 degrees). From the data you can see that any time there is a temperature change they diverge. Late in the log I turned on the convection and then I wrapped the thermal couple and probe #2 together in a tin foil packet.

I have surmised that the thermal couple is slower to respond then the probes. This can be noted both during the warm up and when I opened the oven to wrap 1 thermal probe and the thermal couple together.

I suspect that the temperature would diverge more in my UDS as that is less table then my electric oven.

Oven.png
 
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Bryan Mayland

TVWBB Hall of Fame
I would reach the same conclusion from that graph, that the thermocouple is reacting more slowly and that it reads slightly higher than the thermistor. You can adjust the gain of the thermocouple by changing the mv/C value in the webui. Standard is 5, higher values will lower your temperature, lower values will raise it, and the effect is amplified the higher the temperature is.

I'd be curious to know what your 3.3V voltage is though if you have a chance to measure it (just measure on the probe expansion header on the right side where it says 3V3 and GND). HeaterMeter assumes that the 3.3V power is exactly 3.300V but it has no way to measure it. The voltage regulator has a pretty wide tolerance though so it could be as low as 3.135V or as high as 3.465V, and these could generate much higher or much lower thermocouple temperatures. This is somewhat mitigated by the fact that the bandgap measurement is lower when the 3.3V power is higher, but the bandgap has an even larger tolerance for its voltage, so it isn't reliable.

If you could measure the 3.3V power voltage, it would provide me another data point for trying to figure out a better correlation. I'd also need the bandgap measurement. You can find this by using the device's LCD menu and going to the "Manual Mode? Yes/No" menu, then hold the LEFT button until the display switches to the diagnostic screen, displaying the Pit temperature dump. Short press left again to get to the next diagnostic display and the bandgap is displayed on the bottom left like "BG332".

The easiest way to get a temperature reading that you can have more confidence in from the thermocouple is to put it in boiling water and then adjust the mV/C up or down slightly until the thermocouple reads the correct temperature, somewhere between 4.75 and 5.25 most likely.
 

Nathan Stock

New member
I measured the 3.3V from the pin labeled 3.3 to the ground connection on the board and got ~2.99volts

I wasn't able to figure out how to get to "manual Mode"... I scrolled thought all the option and can not find it.

I did calibrate the thermacouple with boil water. I spot checked the boil temperature vs altitude and it should 208.5 - 209.5 ( based on my google earth altitude) The thermistor read 209, but the thermal couple was higher so I tweaked the value to 5.09 and now everything lines up at boiling temp.
 

Bryan Mayland

TVWBB Hall of Fame
Thanks for the info on the voltages. "Manual Mode? Yes/No" is on the device itself, using the buttons press RIGHT twice. It goes Setpoint -> Manual Mode. On that screen hold the LEFT button until it changes to a screen filled with numbers, then (short) press LEFT again to see the BGXXX display. This is on the firmware from within a year or so ago, so if you're on "v14 Release" it won't be there.

Wow 2.99V on the 3.3V seems a little low, it should be 3.125V or higher but I'm not sure what would cause it to be lower.
 

Nathan Stock

New member
Thanks for the info on the voltages. "Manual Mode? Yes/No" is on the device itself, using the buttons press RIGHT twice. It goes Setpoint -> Manual Mode. On that screen hold the LEFT button until it changes to a screen filled with numbers, then (short) press LEFT again to see the BGXXX display. This is on the firmware from within a year or so ago, so if you're on "v14 Release" it won't be there.

Wow 2.99V on the 3.3V seems a little low, it should be 3.125V or higher but I'm not sure what would cause it to be lower.
Bryan- That was a typo on my part... it should have said 3.299 so it is right on the money
 

Nathan Stock

New member
Is manual mode the same as "manual fan mode"
- this is what I get when i press the right button twice. If I hold the left button for a long press it just takes me back to home screen with Pit temp & fan %

20201006_224842.jpg
 

Bryan Mayland

TVWBB Hall of Fame
Oh yes, "Manual FAN Mode?" my bad. If it doesn't go to the diagnostic screen then it is an older firmware since that was added in 20180718 firmware (haha wow *two* years ago? How old am I?)

Are you not on the Snapshot firmware? Everyone should be on Snapshot, it's the best!
 

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