Thermocouple problems during first cook


 
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Paul Nederveen

TVWBB Member
Hi folks,

I tried my first HM controlled cook this weekend. It worked and it didn't work. My setup is as follows:

Primo Oval XL cooker.
HM 4.2.4
2x iGrill Thermistor probes (from a still working iGrill bluetooth temp monitor)
1x Perfect-Prime TL0500 K-Type Sensor alligator clip seen here.
Micro Damper by Steve into a homemade damper plate.

I got the smoker going with the HM attached, set point at 215 (set for 225 later). Everything seemed stable, and I put the shoulder on to start smoking. So far so good. The HM held the temp perfectly around the set point and I ramped it after the first 6 hours.

After 12.5 hours the meat hit 170.4 and started falling. I figured it was a stall. Meat temp started dropping and kept dropping for the next 2 hours! I raised the set point a bit and then I got worried and started doing some investigation. My built in grill thermometer was reading only 200 degrees. I expect it to be a bit lower but 25 was a big gap.

I put the spare iGrill thermistor probe in next to the TC probe as shown in pic:


The grill thermometer and the thermistor registered the same 200F. The difference ranged from ~25 degree (TC=225, TM=200) up to over 60 degree difference (TC=~315, TM=~250). For sanity I plugged the same thermistor probe into the iGrill and the HM and they were exactly the same temp.

I had already calibrated my TC in freezing and boiling water. Stock settings for voltage and 0 offset. I checked it again today in boiling water and was reading 212-213 and .502V on the output of TC circuit. (as per this thread)

I need help on understanding why the TC seemed to have an increasing error and what I should be doing different.

Thank you all!

P.S. The pork was safe to eat and tasty but not as tender as I would have liked...beer helped me calm down eventually. :cool:
 
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I guess my first question would be, are you positive that your thermocouple is a type "K"? Is it a trusted name brand or is it an Ebay/Amazon special of unknown origin? I would first try using a known good type K from Thermoworks or Omega or... any number of other known good sources of probes. Just because it was sold as a type K doesn't mean it really is... If you still have problems after testing with a known good probe. Then we could start to trouble shoot the TC amp circuit. But no reason to go there just yet if you don't know you have a good probe.

You don't have a Fluke multimeter do you? a lot of them have a built in thermocouple input that you could also test the probe with... (Set your oven to 350 put the probe inside and see what it reads... Should be 350 +- 20 degrees or so.....) You know it's right at 212, this would allow you to check it at a different temp point..

Also check the TC socket and wire connections inside the plug, any corrosion/loose wire etc. will cause issues with a correct reading. If your probe has terminal forks crimped onto the wire then screwed into the plug, try removing the forks and wrap the wire directly around the screws and tightening them up. One less transition point and one less connection to worry about.

Also one more thing... Looking at your picture, I also have a Primo XL, because of where you have the probes located it could easily be 25 degrees between them. You have the TC close to the gap between the "D" plates and the outside of the grill, and the TC sitting over the D plates and not near the gap. If you really want to read the diff. you would really need to put the two in exactly the same spot... (Use the alligator clip and clip it right onto the other probe...)
 
Thanks for the reply Marc.

I have ordered another K type probe from Auber instruments. More $ than the Amazon one I got, but a bit less than a Thermoworks.

I do not have a Fluke meter unfortunately. I have a Wavetec, and it does not have a TC circuit built in. I'll ask around to my friends (if anyone on the bb is in Raleigh, NC area and wants to help let me know!).

I checked the connection on my TC and the wires go straight to the screw terminals, so nothing I can do there. Did I damage it by inserting directly into boiling water?

I see what you are saying about the location in the grill. Maybe that was a factor since the TC is more sensitive than a TM and the heat is coming around the D plates. I will try a few more experiments while I wait for my new TC to arrive.

Thanks!

P.S. Marc, since you have a Primo XL, where exactly do you locate your pit probe during a cook?
 
I try to keep my pit probe at least 1.5 to 2 in. form the meat and the same from the edge of the D plates. If I'm running two pieces of meat I'll put it in the center of the D plates as long as there's enough room away from the meat. Like you I drilled a hole through a clothes pin so I could run it in one direction and if I want to run it the 90 degrees different it fits in the spring hole of some clothes pins.

I doubt you damaged the TC by putting it in boiling water unless you also submerged the cable end. In that case you can probably save it by placing it in the oven at 300 ish for a couple of hours.. You know, the next time you make cookies or something... lol. It should drive/cook out any water. TC probes are pretty durable. You never want to get the cable end of any probe wet

I happen to be running a cook right now. http://hmgrill.dnsdynamic.net:9080/ Both meat probes are in the same piece of meat.. (Single 13ish lb brisket)
 
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OK, I tried this again on a simpler cook with my new Auber instruments K-type TC.

First I put the probes as close together as possible and away from the edge of the D plate. As seen here:




Lit everything up and let it stabilize. The TC ran consistently about 10deg above the iGrill TM probe. I eventually ran the setpoint up to 360 to get my desired 350. At this point the built in grill probe and thermistor are showing the same temp (pre-food). As shown here (where "Meat" really means "TM-pit"):


Partway through I flipped my spatchcocked chicken over. The probe setup is ~1.5-2" away from food per the suggestion from Marc. For some reason the grill struggles a bit staying at temp, maybe some older charcoal or something. Anyway, the TC and TM start to approach the same temp for some reason.



Maybe some of the problem was a damaged or cheap TC probe. However, I still don't really know how to "trust" the temp and what I should do to feel confident that the temp I set is going to be consistently close enough to the temp I need in my grill. Suggestions are appreciated!!!

Thanks,
Paul
 
My thoughts are don't over think it!... LOL
If I'm seeing things properly in the pic you have a long standard probe and an alligator clip TC probe clipped to it. So the standard probe is spanning several inches of cooking surface while the TC is just over one small spot, perhaps the TC was directly over a hot spot in the fire while the standard probe spanned over that hot spot and some colder spots and averaged out a little cooler. The different size, shape and mass of a probe will make a difference in how it reads temps and how quickly it will react to changes. If you want to track one against the other the best way to do that is with them both submerged in the same fluid, but most probes are water sensitive so be careful not to get the cable end in the water, that's usually where it leaks in.... or put them both in a "thermal well", perhaps put both probes inside a piece of iron pipe or something on the grill (for testing to satisfy your anxiety about the TC accuracy)
I've found the HM TC to be very reliable and accurate, however, if you do the SMD soldering yourself and leave a bunch of flux on the board.... that is probably the most common cause of inaccurate TC readings. I have seen this myself, a good cleaning with a brush and isopropyl alcohol over the SMD area will clear that up very easily (I actually use flux remover spray, but alcohol will work pretty much the same, just make sure to let the board dry out before you power it up)
A good TC is worth the investment IMHO. I went WAY over the top and bought a water/steam proof Thermoworks TC that can handle over 2,000F. It cost something in the mid $30's I think, not too bad really, but there's no way I'm gonna melt this one! I've left it on the grill for about a year and a half now and still kicking strong, even though don't take care of it, actually I abuse it pretty bad TBHWY. I've also got an amazon cheapie that seems to work well too, but I just use that on the test bench mostly because I prefer the TW probe.
One thing to check on any TC probe that is acting up is the wiring at the plug. Most of the TC plugs come apart, and sometimes on the inside the wires can be loose (the amazon cheapie shipped to me loose and worked like crap until I tightened it), or the wires can get pulled and twisted in there.... so pull that apart and check the wiring in the plug, particularly if you are running a cheapie TC from China....
 
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Ralph, do you have the part number for that 2000* TC? Wasn't able to find it on the thermoworks website. TIA
 
It's under their "high heat" TC section.... take that back, I guess they've redone their website since I visited last (another testament to the durability of their probes I guess).
If you go to Handhelds/Probes\TypeKTC\HighTemp you will see a list of their high temp TC's. The one I use is in the K-420 series, they have several varieties there. This series PROBE can handle over 2,000F but the cable handles a lot less, what I do is shoot the probe through in at grill level under the lid and leave the cable and threaded part outside the grill. The nice thing about the yellow cable is it is completely waterproof, as long as you keep it away from the heat it will serve you a real long time. They had two different thicknesses, I got the thinner one which doesn't cause an air leak when set between the nomex gasket of the lid. It's bendable so you can kinda point it wherever you want once its in the grill..
...but look over all the high heat options, they may have one with a cable that has higher heat tolerance or something that suits your rig better than the one I am using..
 
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I have the Auber clip probe you have and its pretty accurate. I have left mine in the smoker for over a year and it still works fine, although I never used it, for a cook, I just had it as a back up, jist in case.

Auber although cheaper still make ok probes, you just have to be careful with them, they tend to fall apart easier then thermoworks.
 
Thanks for all of the replies. I guess I just have to learn to place and trust the TC. I will occasionally experiment and see if there is any further tuning that needs to be done. In response to some earlier comments:

1) The TC and thermistor were over diffusion plates and not likely affected by hot spots.
2) I bought a pre-soldered TC circuit since I'm not quite up to SMD soldering. Doubt flux residue since I cleaned the board before starting my build.
3) I went with TC because they were supposed to be cheaper and sturdier and now I find they are as much or more than TMs. However, they ARE more options and likely last longer once done correctly.
4) I'll check the leads in the connector again just in case.

Thanks.
 
You wouldn't think it would be so, but if you take 4 probes and place them around your grill at different points you will see them register different temps. It is because of air currents carrying cold/hot air in certain areas and where the fire below is hottest etc. Your alligator clip doesn't have much area or mass so it samples the temp in a smaller area than a long probe like the food probe. TC's are generally cheaper and definitely sturdier, but the biggest feature IMHO is that you can get TC's that will handle high temps with ease, while standard probes will die between 500-700F, and I mean die, go buy yourself a new probe... been there done that TOO MANY times, over a year later my TC lives on....
 
Yep, Tc will last much longer then any probe. You can permanently attach them to your smoker and they will continue to work, even if you leave them outside year around.

Thermocouples are two different types of metals welded together, as long as the weld doesn't break, or the wire going to the plug, they will work fine. I have alot of probes that die for no apparent reason, sitting in a box, especially maverick probes, from water. None of my tc have died, even though I have one that the wires came out of the metal tip, I use it to test new heatermeters and its still accurate, but very sensitive.
 
Don't dismiss the idea of making your own thermocouples. It's cheap and you have the option of customizing them to any layout/design you wish.

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All good feedback. I've just got to get used to where and how I place the TC so I get the HM to control the grill to the temp I need it. This thread started because I didn't get my pork to the temp I needed because I didn't understand how it would work. A key point that Ralph pointed out,
Your alligator clip doesn't have much area or mass so it samples the temp in a smaller area than a long probe like the food probe.
Click to expand...

I'm super excited by how accurate and awesome the HM is at keeping whatever setpoint, actually set. :)
 
What I did when I was new to the HM (and if/when I use it on a new grill) is plug in all four probes and place them in various parts of the grill, grate middle, grate edge, hood etc and do a test burn. Watching how the temps track in the various regions will teach you a bit about how your grill actually cooks and will also help you pick the spot to set your pit probe. Once you get to know the grill you will know "this area is hotter", "this area is cooler" than where I have the pit probe, and use that to make your cooks go better....

Doing this I have learned that using a blower to stoke the fire creates a unique circumstance... If you force too much air you will create currents of cool air in your grill. If you have your pit probe set in the area where this cool air is being forced through the HM will stoke the fire to compensate and then cause overshoot, because when the target temp is achieved the HM turns down the blower and the rush of cool air stops so the temperature blooms. To minimize this you want to try not to place the pit probe in areas where the cooler currents flow in your grill, but IMHO the best thing to do is limit the amount of air the blower pushes by setting the blower Max speed lower and/or using a servo damper.

When doing low and slow I set my HM so it runs on the server damper only for the most of the cook, the blower just kicks in to stoke the fire when you first light or if have a dip and need to recover the target temp for some reason. I like this setup best because you can run the top vent wider open when you run by natural convection rather than forced air, so the smoke moves through the grill more steadily rather than getting trapped by a tight top vent and rolling around a few times before it exits.... and you are moving less air through the grill overall, so that means you are drying out your food less and using less fuel too, so it's a win/win IMHO....
 
Paul Nederveen said:
All good feedback. I've just got to get used to where and how I place the TC so I get the HM to control the grill to the temp I need it. This thread started because I didn't get my pork to the temp I needed because I didn't understand how it would work. A key point that Ralph pointed out,

I'm super excited by how accurate and awesome the HM is at keeping whatever setpoint, actually set. :)
Click to expand...

I completely disagree with the alligator clip comment. I don't think you realize (Ralph) that the alligator clip is just a mounting solution, and not part of the TC construction. The Auber build has a sleeve that encloses the weld of the TC. The alligator clip is just a means of attaching to the grill.

AND in terms of "thermal mass", There's really no difference between 1" sleeve or 6" sleeve. The wires are still welded on the end, surrounded by a steel braid, shoved into a small, tight, thin, protective sleeve with the same exposed measuring point at the end.

As far as TCs go? buy cheap. $60 for a TC? That BBQ must kick A$$!

I really think people get a bit too anal about getting that "perfect" graph on the flames page. The biggest reward is the piece of meat that comes out of it. Quit jonesing over that perfect "line", and be happy having a device that can keep temp within 5 degrees of set.
 
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You're not getting the concept at all and I don't care to argue with you, just to say to everyone else, use your own judgement here....
 
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