Diff btw Stoker and Guru probes and possibilities for modification


 
M

Matt A 3

TVWBB Member
I am looking at the Stoker and Guru. There are many differences, and thanks to the contributions of everyone here I think I have a handle on most of the differences that are important to me.

I would like to understand the probes better. Essentially I’m trying to figure out if I will somehow be able to use different probes with either of these units, and if so, how difficult that would be.

My current understanding is:

- Both are type K thermocouples

- Neither use standard industrial interface plugs and thus the probes are proprietary to the Stoker or Guru hardware

- Stoker – thermocouple lead wire attached to a “smart plug”. The plug performs the temperature calculation and relays this to the Stoker. The plug also has a unique identification that allows the Stoker to identify it (thus facilitating multiple probes on a single lead with a splitter)

- Guru – thermocouple lead wire attached to a plug, but with the plug adding no value other than to relay a signal to the Guru, which measures the temperature. Identification of the probe is based on the Guru jack that the probe is plugged into.


A few questions –

(1) Is my above understanding correct?

(2) Would it be possible to make the Stoker plugs compatible with standard industrial thermocouple probes? For example,
(a) take a Stoker probe/plug and cut the lead wire pretty close to where it goes into the plug
(b) attach a standard thermocouple connecter, for example Grainger item #3FXN6 or 3HL36 – this gives you a “dongle” of a few inches in length
(c) then you plug in a standard thermocouple probe – beaded, lance, immersion, air, surface, etc.

(3) Could you do the same thing as described above with the Guru plug?

(4) Since the Guru plug is not “smart”, could you go even further and fabricate your own “dongle” using some thermocouple lead wire (e.g. Grainger item #5ZY35 or 5ZY36) and whatever the plug type is? And any ideas on what plug you would use?

(5) If any of the above worked, would swapping the probe require recalibration of the temperature?

You probably think I am out of my tree for asking these questions. But I am curious and trying to figure out if I can make the units a little more flexible (and in the case of the Guru, if I can completely free myself from proprietary probes).

Thanks,
Matt
 
1) Yes, you have it pretty well surrounded.
2) Yes. John at Rock's BBQ (Stoker land) has talked about offering the standard plug instead of a TC on the end of the wire.
3) Yes.
4) Quite probably.
5) Generally not. Thermocouples are 'law of physics' devices. As long as the right alloys are used, they all behave pretty much the same. What you are really recalibrating is the virtual junction temperature used in electronic thermocouple interfaces. That isn't really changing when you change a Guru probe.

This is a difference between the Stoker implementation and the Guru.

Inside the Stoker plug, the junction with standard wire occurs within about 1/4" of the point at which the temperature is measured, an arrangement that offers high theoretical accuracy.

In the Guru, the actual second junction occurs where the thermocouple wire meets the jack/plug metal outside the box while the temperature reference is inside the box. The difference in temperature between inside the box and outside becomes error in the result (of roughly the same amount).

Most of the time it doesn't matter that much, but under difficult conditions it could be 20 degrees. You can calibrate (compensate) for only one set of in the box -- out of the box temperatures at a time. If you want maximum accuracy, you need to do the calibration at the grill with the air temperature about where it will be in use.

This does not apply to the Stoker because both the second junction and the temperature reference are close together and inside an small enclosed space, so the calibration tends to track even as conditions vary.

Good questions, Matt.
 
Thanks much for the detailed reply.

The "inside/outside vs. outside/outside" the box measurement error porential is interesting but I wonder how relevant it would be.

Perhaps if you kept the Guru probes in a very cold place and the main unit in a warm place, then when you fired up the unit you'd have a big differential in temp between the cold junction and where the temp of the cold junction was menasured.

But after a few minutes of use I have to believe that the metal of the plug would heat up to be nearly the same temp as the inside of the box where the cold junction measurement occurs. I say this because the plug is recessed into the box, so the plug is pretty well "in the box" when plugged in. I suppose if the actual junction is in the non-metal part of the plug that sticks out of the box that would be relevant.

Perhaps this whole cold junction issue is at the source of the comments made on an on-line review of the Guru (here) which observed probe accuracy problems and got a response from the Guru people that the unit has to "warm up" for 20 mins to be fully accurate. Perhaps the same scenario as really cold probe plugs going into a warm unit that I described above??

Cheers,
Matt
 
As you suggest, it may not be that relevant. Most of the time it certainly is not.

The second junction (nominally the cold one) is inside the plastic cover where the wires are crimped to the two contacts of the plug.

That plastic cover probably insulates reasonably well and would allow the cold junction temperature to more closely approach the internal temperature better than a metal covered jack. If you built your own, you would want to do it the same way.

You could further improve the consistency of results by putting both the Guru and the plug of the thermocouple into a clear/semi-transparent box where they would tend to approach the same temperature.

If the circuitry of the Guru generates much heat, it will always be significantly hotter than the air that surrounds it. I assume that it does not run hot or closing in in a box might cook it.

Of course, for the older models without a temperature display none of this matters a bit.
 
This has absolutely nothing to do with saving money.

Perhaps over-thinking is part of the fun for someone like me.

Don't worry, I'm not under any illusion that using a different probe will somehow make me a better brisket cook!!

Cheers,
Matt
 
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