Single wall temp measuring - probe vs surface

[begreen]

There's a large discrepancy on what internal single wall flue temps are. Recently we've seen postings with a a multiplier for surface temps going from a third higher to 100% higher. I've usually struck in between at 50% higher. And then we have the variations of thermostats that can be 100F off. Which is correct?

Does anyone have instrumentation on single wall where they have measured with a thermocouple probe the difference between a Condar or Rutland pipe thermometer on single wall pipe and the actual flue gas temp?

 

[Jags]

We had a member do this in past years. I wish I could remember who....
If I remember correctly his readings were 33-50% higher than the surface temp.

I think there is some confusion in the math that some report. Just to clarify - 50% of a 400F surface reading is 200F for an internal temp of 600F. (and I believe that came from Condar).

 

[begreen]

Correct. 50% higher than surface is what I recall and have used, but I was hoping to get more data points from folks that have since installed digital probes. We have double-wall connector so I can't verify on our stove.

 

[quotejso]

I have the screw going into the pipe. You think that makes it more accurate?

 

[quotejso]

I guess i could check the surface with the infared and see if the screw make a difference

 

[mfetcho]

So if surface reads 200F then flue gas is 400F?

 

[Jags]

So if surface reads 200F then flue gas is 400F?

No. 50% of your 200F surface temp is 100 degrees. This would equate to a 300F internal flue temp (200 + 100).

 

[spirilis]

Hm, I have a thermocouple rig that I need to put together (probe inside pipe) and an IR thermo to compare... then I'll have some data to add...

 

[begreen]

Thanks. Temps will vary with the setup I think. My expectation is that there will be a different flue gas temp reading with systems that are straight up, than those that go up, take a 90 turn to an outside tee. Could be wrong though, but it would be interesting to know.

 

[mfetcho]

No. 50% of your 200F surface temp is 100 degrees. This would equate to a 300F internal flue temp (200 + 100).

Gotcha. Thanks!

 

[Highbeam]

I also suspect that it will not be a linear relationship. Like 50% higher at 300 but 100% higher at 500. It surely has to do with velocity in the flue as well.

I was always told 50% the other way. So a 500 degree flue gas makes a 250 degree flue surface temp.