Better probes?


 
But since nobody has this new board yet I was hoping for a design that could be employed in an add-on style to the standard HM board we have now?
 
Brian Hilgert said:
It took me a while to dig them up, but here is what I used for the CDN probes, straight out of my hmcore.cpp:

{5.7079876e-4,2.1810467e-4,5.0643855e-8,2.17e+4}, // CDN Probes

Software side for themocouple is done? great! I just finished a rework (a minor one at that)of the HM 4.0 board to add the AD8495 circuit to the board. I opted to go with 603 size caps and resistors; I figure if I have to solder a MSOP8, 603s would be easy. Then I got to thinking it would be cool to make a 'mod chip' so that users could easily modify an existing HM4 to support K type thermocouples. I'm about to send that off to dorkbot today. I think cleaning up the silkscreen is/was harder than placing the components and routing the signals...
Click to expand...


thank you sir!


And again, anyone needs thermocouple to borrow... give me a shout.
 
RalphTrimble said:
But since nobody has this new board yet I was hoping for a design that could be employed in an add-on style to the standard HM board we have now?
Click to expand...
Which is what led me to this:



This would have you add a jack to your thermocouple, and make the board modification i mentioned earlier. I am going to send these off to Dorkbot today or tomorrow. I don't know if it will work as I intend, but at .34 sq inches, I can certainly test my ideas with some unpopulated boards.

The idea is you would cut the current traces leading to the pit prob jack, and solder this mod chip onto your HM4 board at the pit probe junction. I've looked into using this kind of jack (i hate calling them phone jacks, i've always called them stereo jacks. Phone jacks are rj11's or rj12's, like on a phone cord :p) and the general consensus is it won't distort the cold junction calibration more that a couple of degrees. At 600c, i can handle a few degrees of error. Anyway it's just an idea, and it will be cheap to test.

Ralph, I though you were considering/ had already added a RJ45 female to your heatermeter, then running an Ethernet cable to an external fan/servo/probe box. If that is the case, you would be better off placing the ad8495 circuit in that extension box. You'll need ground, 3.3v (or 5v if you are already running 5v to the box)and the pit adc. Otherwise your cold junction calibration could be way off.
 
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Awesome! You seem to know what you're doing so disregard if you've already thought it out, but that blue dotted line is a ground plane. So you'll have some teeny little connections on all four sides of that "pit" ground connection that are going to try to connect it to the plane. You can ripup that blue line then add a keepout rectangle on the bottom that should stop the copper plane from filling that area and it might save you a little work once you get the pcbs.
 
ThermoWork wrote me back about the probe coefficients in a nice verbose email that explained that because they don't integrate with third parties, they don't have any specs for the probe. It was nice of them to not just say "Nope, sorry!" and be done with it.

The good news is that the guess I made by searching high and low for a low cost thermistor with the right dimensions and temperature range may have paid off. Plugging the Beta 4300 and temperature range (100/200) into some online calculators with various standard curves gave me a single set of coefficients to try. I am tentatively saying that they're pretty close because I've got it strapped to a Maverick probe and they're both within 3 degrees sitting in my egg at 186F. I'll have to check them at some higher temperatures once this butt finishes cooking tomorrow. It also is touching the grounded braid of the Maverick and I haven't gotten a wonky reading yet.
 
Thanks Bryan. As you guessed, they didn't work.

I have an old analogue meter. I'm going to see if I can measure the resistance at some known temps, use this online calculator and see if I can obtain some coefficients. I'm really out of my element here. But, I'm going to give it a shot anyway. I have nothing to lose but a little time.

If it works, I'll post back.
 
Darren,
the key to doing this is to get readings when the temperature is stable. Due to the lag of the thermistor trying to get the reading "real time" will lead to frustration.

You can easily do freezing (ice cubes in water), boiling water, and room temp. That can get you something that should work OK. If you want to heat the oven, that is another way to add a couple more points.

dave

Darren C. said:
Thanks Bryan. As you guessed, they didn't work.

I have an old analogue meter. I'm going to see if I can measure the resistance at some known temps, use this online calculator and see if I can obtain some coefficients. I'm really out of my element here. But, I'm going to give it a shot anyway. I have nothing to lose but a little time.

If it works, I'll post back.
Click to expand...
 
EDIT: I'm still working at it. I just realized the temps were in C. I was entering F. Maybe I can get closer....

I got it closer, then lost it. I'll keep working on it. If I ever get it, I'll post my coefficients. I'm pretty sure about my temps. I'm using my Thermapen to compare temps. But, my old analogue Radio Shack multimeter may not be accurate enough.
 
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I hope this is something that pans out. I am getting tired of the problems with the Mavericks. Have some that if you touch them they stop working, some that just don't work anymore after 1-2 uses. Gets expensive
 
While likely exceedingly expensive for most, has anyone looked out there at any of the shops that design thermistor/thermocouple/RTD probes for NSF-certified food, industrial, or scientific work? Some of them have limits on the duration at certain temperatures, but I'm sure there has to be something on the market more reliable than the Maverick line.

http://www.omega.com/subsection/general-purpose-thermistor-probes.html
http://www.cooper-atkins.com/Products/Thermocouple_Probes/
http://www.cooper-atkins.com/Products/Thermistor_Probes/
http://www.us.endress.com/eh/sc/america/us/en/home.nsf/#product/TH13
http://www.madisonco.com/temperature/RTD.php
http://www.global-sensors.com/ProbeGuide.htm

It's a real shame that Thermoworks isn't more forthcoming with their specifications, because I would easily purchase their Model 113-178. It seems ideal for what we're trying to do: http://www.thermoworks.com/products/probe/tc_penetration.html I would rather pay $65 once over the potential of having to buy multiple Maverick probes. Bryan, is there anything that can be done to reliably determine the specs so we can use it?
 
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I'm not any sort of industrial engineer or whatever would have some sort of knowledge over how someone would calibrate this. You'd need carefully controlled conditions where you basically use a thermcouple to set the temperature of something, wait for the unknown probe to stabilize at that temp, then record the resistance. Do it every 10F for 0F to 600F, then feed that into a curve fitting program to get the coefficients back out. This would be outside the realm of my capability.

There's also the "Beta" equation. R = R0 * e ^ (B * (1/T - 1/T0)). Plug a bunch of numbers into that, get a table, use the curve fitting application. This relies on knowing the Beta value and wouldn't be 100% accurate, but may be close enough. The part might be 104GT-2 which has a Beta(100/200) of 4390.
 
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With that in mind I created a spreadsheet with that formula, and tried to find the R0 that would give me 100k ohms at 25C.

Beta 4390, 5185 ohms, 0.000731476,0.00022779,-5.52701E-020, off by 1F at 90F, off by 10F at 200F
Beta 4300, 5509 ohms, 0.000676599,0.000232558,-5.17306E-020, off by 1F at 90F, off by 6F at 210F
Just messing with numbers,0.000658,0.000232558,-5.17306E-020, off by 4F at 90F, off by 0F at 210F

I can't make the points match at both ends so that means that the coefficients will be wrong all up and down the scale. Not sure where else to go with it.
 
We could all try social engineering the information out of Thermoworks with varying stories of necessity. I wonder if I could get my buddy with a NASA.gov account to send them an email... :D
 
The thermoworks probes you linked(113-178) are thermocouples. While thost looked like well designed probes, it wouldn't be as simple as knowing the coefficients to get them to work.
 
Could you put a known probe(maverick) and a new probe together. Wrap some foil around it and put in your smoker. Then ramp the temp up and down a few times while logging both probes. Wouldn't this give you a large data set with temp from the maverick and resistance from the new probe?
 
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