Lid probe temp issue?


 
R

Rick Kramer

TVWBB All-Star
I was using a WSM I bought off of CL the other day. The previous owner had mounted a decent quality therm in the lid opposite the lid vent. When using my "tested" ET-73 in the vent hole I was reading an average of 15-20* higher than the mounted therm. The only thing I can come up with (other than the mounted therm is faulty) is that because the ET probe was in the top vent, and the vent hole has sorta a flue effect to it. Maybe it actually is hotter on the upper vent side because of the chimney effect? Whatcha think?

Rick @
 
Absolutely right!
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Lotza guys have been questioning the accuracy of their dome thermometers, even the new weber ones... Usually because they're measuring temps in different locations and comparing them.... then they'll test em both in boiling water and find the thermometers are pretty close
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Ron is correct. Temperatures will vary somewhat depending where you read them in the smoker. The lid temp will be different than what is read at the top grate, which is different than what is read at the bottom grate. It will be different when read on the edge of the grate versus in the center of the grate due to the heat flowing up the sides of the barrel.

You can drive yourself crazy trying to figure out which is the 'correct' reading. I've found that they are ALL correct readings just simply different. As they are all within what I consider allowable tolerances (10-15%), I don't worry about it.

The best way is to learn how your smoker works for cook times versus temperature versus food type. Don't get all wrapped up in what thermometer is better or worse. It's a waste of time.

Russ
 
Thanks guys ... but I need more! Curiousity has gotten the best of me.
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Being the proud owner of three WSMs and having cooked on one for over five years I know pretty well how they work. I know about the temp diffs between the top vent and the bottom grate and from side to side. What I didn't know was there was such a temp diff between the top vent side of the lid and the polar opposite side of the lid. As I said in my first comment one might think it is due to the "flue effect" from the smoke/heat being forced out the vent holes. Fair enough, sounds reasonable. IF our temp probes measured air temp. (more commonly known as wind chill) I don't think most of our probes do that. If that is the case then why are the temps hotter on the vent side of the lid? I have a theory but I'd like to hear what others think.

Is this a make or break issue? Nah. But it would make a difference in my goal lid temp if I'm using the mounted therm alone.

Rick @
 
I would think the one reading lower is reading a lower temp rather than some increase in temp from a flue affect. Seems like the heat would be hottest right at the coals and then move down from there. I don't see it getting hotter as it moves it's way out the top vent.

More likely seems to me that there is some airflow causing the reading where the mounted thermometer is to be lower.
 
Far be it from me to argue with a Wizard and with all due respect ... but ... I always thought that once the WSM was set up with the water pan and grates in place that the heat rose as it went up the cooker? My formula using the lid vent as my temp measuring point is/was, 265* lid temp = 250* top grate temp = 235* bottom grate temp. So unless something has changed, the WSMs temps do get hotter as they rise in the cooker. Naturally the hottest point in the cooker is the coals, but once the water pan is in place that changes the dynamic of heat flow and distribution.

Or so I thought ...
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Rick @
 
I think the amount of meat and how cold the meat is has an effect on a lid mounted therm. Seems a vent mounted therm is more tempered sitting in the exhaust, and the amount of cold mass doesn't effect it as much as the static one.
At least that's the way I see it..
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Tim
 
Yeah Tim, that's exactly the way it seemed when I did a multi-stage cook on it the other day. (multi-stage cook = finishing one meat then starting another) In this case it was a ham replacing a couple of finished turkey breasts. The mounted therm seemed more sensitive to the actions that affect temp .. lid lifting, fresh/cold meat being placed on the cooker, lid lifting
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Whereas the vent therm always came back to it's original (higher) temp much quicker than the mounted.

I like your explanation Tim. It's more/less what my theory was. Thanks for at least confirming I'm not the only who thinks this way. Of course it may just be an Illinois thing.
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Rick @
 
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rick Kramer:
Far be it from me to argue with a Wizard and with all due respect ... but ... I always thought that once the WSM was set up with the water pan and grates in place that the heat rose as it went up the cooker? My formula using the lid vent as my temp measuring point is/was, 265* lid temp = 250* top grate temp = 235* bottom grate temp. So unless something has changed, the WSMs temps do get hotter as they rise in the cooker. Naturally the hottest point in the cooker is the coals, but once the water pan is in place that changes the dynamic of heat flow and distribution.

Or so I thought ...
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Rick @ </div></BLOCKQUOTE>

Rick,
You bring up a good point. I don't use water therefore, my frame of reference is different. I still doubt that the temp gets hotter as it moves from just around the water pan up through the vent. That said, I would tend to agree with Tim - that meat was causing the one thermometer to read low - not "hotter" air coming from the vent causing a flue affect and causing the other thermometer to read hot.

One thing I can say with confidence, when you get into the cook and things stabilize, there won't be a significant difference in temp no matter where you take your reading from. I say this from a little experiment and this was confirmed in a test someone on this board did (Chris) that I read quite some time ago.

p.s. Wizard means I posted a lot on this board. That's no guarantee any of my posts were correct.
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jerry N.:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rick Kramer:
Far be it from me to argue with a Wizard and with all due respect ... but ... I always thought that once the WSM was set up with the water pan and grates in place that the heat rose as it went up the cooker? My formula using the lid vent as my temp measuring point is/was, 265* lid temp = 250* top grate temp = 235* bottom grate temp. So unless something has changed, the WSMs temps do get hotter as they rise in the cooker. Naturally the hottest point in the cooker is the coals, but once the water pan is in place that changes the dynamic of heat flow and distribution.

Or so I thought ...
icon_confused.gif


Rick @ </div></BLOCKQUOTE>

Rick,
You bring up a good point. I don't use water therefore, my frame of reference is different. I still doubt that the temp gets hotter as it moves from just around the water pan up through the vent. That said, I would tend to agree with Tim - that meat was causing the one thermometer to read low - not "hotter" air coming from the vent causing a flue affect and causing the other thermometer to read hot.

One thing I can say with confidence, when you get into the cook and things stabilize, there won't be a significant difference in temp no matter where you take your reading from. I say this from a little experiment I did and this was confirmed in a test someone on this board did (Chris?) that I read quite some time ago.

p.s. Wizard means I posted a lot on this board. That's no guarantee any of my posts were correct.
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</div></BLOCKQUOTE>
 
Well, well, look who's a Guru now.
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I know what you're saying about the rankings Jerry. My Karma rank is "Pro" ... yeah right! Pro BS'er maybe.
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Beating this "dead horse" a little more... another way to look at this might be: A) The bottom grate is near boiling H2O which won't get hotter than 212* and the bottom grate receives very little radiant heat because of blockage by the water pan.. soooo.. temp @ bottom grate tends to be lower. B) The top grate is affected less by the 212* water because it's farther from it and the top grate is exposed to more of the heated air streaming upwards... sooo it's somewhat warmer than bottom grate. C) The dome thermo reads less partly because the rising air is cooled slightly by the meat as it passes and because this air is cooled slightly by exposure to the sides and lid of of the cooker. D) The temp at the lid vent is higher because the rising air is moving more quickly; it isn't exposed to the allready mentioned cooling effects as long as the other locations... and .. therefore retains more of its original heat...
 
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