calculating thermistor coefficients

I'd like to use an inkbird probe as the pit probe.

I can see the latest snapshot firmware includes them, but Byrans comment suggests they aren't accurate over 200F

[www] Adds support for Inkbird thermistor probes used in models IBT-2X, IBT-4XS, IBT-6X. I calibrated one of these from 45F to 200F and it should be within about 1F across that range. I wouldn't recommend using one for a pit probe because I can not do the correlation up to temps above boiling.

Is there a reason the correlation cant be done for higher temps?
I have multiple probes, so can run one in the inkbird and heatermeter side by side and measure at small temperature increments?
Or is it much more complicated than that?
 
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Bryan Mayland

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I can't remember why I didn't do them in the toaster oven like I did with the Thermoworks. I think I got some weird results that threw off the temperatures at food probe temps when I calibrated them from 400F down to room temp.

You can do your own correlation but the math is a bit complicated, you have to measure the resistance at dozens of points across the curve. The curve is non-linear so you can't just pick a few points and hope to approximate it. See: Inkbird probe curve vs Thermoworks

Once you have your table of resistances and temperatures (in Kelvin), just use something to curve fit it to the equation (where A, B and C are the values you put into the config):
T = 1 / (A + B * ln R + C * (ln R)^3)

If you're handy with SSH, HeaterMeter can help you out. Put the known probe in PIT, the unknown probe in FOOD1, tie them together so be certain they are reading the exact same temperature, and use the command `lmclient LMUP,start`. Then slowly traverse through the entire temperature range you want to correlate and execute `lmclient LMUP,fit` and it will spit out the values of A, B, and C. Finally, return everything to normal with `lmclient LMUP,stop`. So easy, right?!
:coolkettle:
 
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Thanks @Bryan Mayland. So simple... or not :oops:

I've sent a message to one of the inkbird reps asking if she is able to help. She has asked me to clarify exactly what I need to know, and said she will do her best.

Do you have an example of what the coefficient data should look like, maybe for one of the Thermoworks probes?
 

Bryan Mayland

TVWBB Hall of Fame
The only probes we ever received data for was the original Maverick ET-72/73 probes and it came in the form of a temperature to resistance chart (that was a PDF scan which I can't seem to google any more). I guess technically what you're looking for is the datasheet for the thermistor used, which would contain all of the characteristics needed to calculate the coefficients, but even just a table of resistance to temperature that contains enough datapoints to cover the range is good (30-50?).
 

IvanTrail

New member
Couple questions.
First, I am not good at SSH, but I tend to dig in and learn until I get it or I just totally screw something up. So, is there a preferred terminal program for the PC that is noob friendly?

Second, how slow is slowly? Is this in the magnitude of annealing titanium (0.6°F/hr), or letting your car warm up (50°F/min)?

Thanks.
 

Bryan Mayland

TVWBB Hall of Fame
Second, how slow is slowly? Is this in the magnitude of annealing titanium (0.6°F/hr), or letting your car warm up (50°F/min)?
haha "Slowly" means "as slowly as needed so that you're reasonably sure both probes are reading the same temperature".

Like, you don't want to just hit them both with a blowtorch for 5 seconds and then dunk them in ice water. You want them the be at the same temperature because the software measures the temperature of one, then instantly assigns that temperature to whatever the resistance of the other is. If you're going too fast, then one might be reacting to temperature change more quickly or more slowly and assigning that temperature to the other's resistance would make it correlate incorrectly. I usually wrap them up tightly together, put them in the toaster oven set to 400F and when the temperature has stabilized for 5 minutes, I turn off the toaster oven, start the correlation, and let them return to room temperature for as long as it takes.

I also use PuTTY for all my SSHing. I see it is also available for $1 on the Windows Store but don't pay for it because that's people just trying to sell you free open source software to make a buck off someone else's work.
 

JKalchik

TVWBB All-Star
I also use PuTTY for all my SSHing. I see it is also available for $1 on the Windows Store but don't pay for it because that's people just trying to sell you free open source software to make a buck off someone else's work.

If that's going to the developer, I wouldn't have a problem with that..... but I'd be willing to bet a cup of coffee that Simon Tatham doesn't see a cent of it.
 

IvanTrail

New member
Thanks for the reply. I appreciate the insight. It makes sense.

If I do something like this, would you be interested in the results? Is there an online repository for such things? Or is that just too much of a hassle for the returns?
 

Bryan Mayland

TVWBB Hall of Fame
Are you doing them for Inkbird probes? I probably would not take any updates to those coefficients because I think they're good where they are. If you're doing a new set of probes and have an exact name and model number I'd add them to the dropdown list in the webui.
 

IvanTrail

New member
They are probes I bought a while back to replace ones on an old no name thermometer. Chinese stuff off Amazon. Probably not worth it because of all the turn over of those vendors I would guess...
 

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