Difference in dome and vent temp

[Chuck Bench]

So, the other day I was wondering how far off my Weber factory WSM thermo was off so I stuck a Tru Tel thermometer in the vent and it showed 30 degrees higher than my Weber thermo....I was like WTH so I've been looking for alternative to the factory thermo in the same spot/shroud...

Well, fast forward to yesterday...my wife wanted to fry some french fries because I just bought her this huge commercial french fry cutter and the frying thermometer is a little crappy...you can physically move the thermometer inside of the housing where the degree marks are at...I wanted to make sure it was reading right so I put it in a boiling pot of water to check and calibrate it, which it was off a few degrees...

While I was at it, I put the Weber WSM thermo in the pot and it read dead on 212...put the Tel Tru in the pot and it read dead on 212...

so my conclusion is that the temperature at the dome thermometer and the temperature 10" away at the same level in the dome is 30 degrees off...

How is this possible...I don't understand it, unless the hot air is going out of the vent essentially making the vent temp hotter...

On a side note, I'm going to be ordering a Stoker this week with the works so this really doesn't matter, but I'd still like to figure out the characteristics of my WSM

 

[Bob Sample]

Yes. Your probe through the vent is in the exhaust stream and will read higher than the ambient temperature in the smoker

 

[Darren Wilson]

I have just done a similar experiment.

I had been taking temps through the lid exhaust and found almost 50F difference between the dial and probe. Thinking that the dial is out of calibration as you keep reading about them online, I went on the probe temp. Just fired up the WSM and got it to temp on the dial. Removed the dial from the lid and placed it in a pan of boiling water. 212F/100C reading!!! Placed the probe in the water - 212F/100C!!! Done the same with the handheld probes, and all read 212F/100C!!!!

For the 2nd part of the experiment I used a piece of thin wire to attach the remote probe to the dial probe in the lid and monitored the temps. Dial was saying 235F but the remote probe was saying 260F!!! Both read identical temp in the boiling water not 10 minutes earlier !!! I made sure that the end of the probes were equally aligned in the lid.

The only explanation I can think of is that the cable on the remote probe is picking up a higher temp from inside the lid as obviously the wire wasn't placed in the water.

Very strange indeed but at least I know that the dial thermometer is a lot more accurate than I thought it was.

 

[Brian Johnson]

Yes. Your probe through the vent is in the exhaust stream and will read higher than the ambient temperature in the smoker

So if the stock dome therm has one number, a probe placed at the cooking grate gives another number (which further depends of it is centered over or outside of the water bowl or if its too close to a big hunk of meat), and a probe through the top vent hole gives yet another number, the question becomes what's the best indicator of pit temp?

 

[Darren Wilson]

So if the stock dome therm has one number, a probe placed at the cooking grate gives another number (which further depends of it is centered over or outside of the water bowl or if its too close to a big hunk of meat), and a probe through the top vent hole gives yet another number, the question becomes what's the best indicator of pit temp?

In that situation, given that all the probes are calibrated, I would go with the one that you feel is the correct one!! lol

 

[Brian Johnson]

In that situation, given that all the probes are calibrated, I would go with the one that you feel is the correct one!! lol

Ha! Pretty much.

For what it's worth I think I remember hearing someone say not to get too hung up on the actual temperature but to use the thermometer more as a guide/indication of what's going on inside the WSM. Basically that you're watching the therm mostly to make sure that your temp isn't steadily climbing/falling or making any changes too rapidly as any of those can be indicative of a problem (no more fuel, not enough/too much air flow, etc.).