The L-shaped probe grew legs, so this was improvised with a straight metal tube. There seems to be a persistent notion that angling the probe toward the appliance (into the exhaust stream) is the correct technique, but wouldn't this result in interference from ram air pressure of the rising flue gases?

Tags: CAS, CAZ, analysis, combustion, draft, pitot, safety, spillage, tube

Views: 461

Reply to This

Replies to This Discussion

I'm guessing that is the short metal tube that comes with the DG.  If that is the case, go with perpendicular to the pipe.  Might want to trim some of the pipe insulation off as well before the job is completed.  The insulation looks a little close to the vent, based just on the pictures above (clearance to combustables).

Thanks Dan. Yes, it's the straight tube from the DG bag. Interestingly, Minneapolis sells what they call a "Static Pressure Probe" and distinguish it from a "Pitot Tube", which they don't sell. Both are right-angled metal probes, the difference being that pitot tubes measure ram air (or velocity) pressure through an opening in the tip that static pressure probes don't have.

The principle is familiar to the aviation geek in me (aircraft use a pitot tube to measure airspeed and a static pressure port with an aneroid barometer to measure altitude). After further discussion in our office and examination of one the aforementioned static pressure probes, angling the tip toward the appliance would, in fact, align the holes in the probe perpendicular to the exhaust stream for a true static pressure reading. In that context, the technique I hear people cite about pointing the probe into the stream makes sense, as does your recommendation to hold the straight metal tube perpendicular to the flue.

I think the tight clearance is a result of the photo angle here.

A pitot tube on an airplane is pointed forward because you want to measure air speed and if it points into the wind it measures, well, the speed of the air.

Draft in a heater is different - we aren't looking for MPH but Delta P.  These are 2 different things.  Both the pitot tube and the bent static probe have holes perpendicular to the air stream so are measuring the same thing.  The straight pitot tube in a heater flue should point straight in to measure the same thing that the bent static probe measures when either pointed toward or away from the air stream.  

Interesting that, in your pictures, the probe pointed into the airstream measures zero.

Yep - and the static pressure port is usually on the side of the fuselage, perpendicular to the airstream.

The reading pointed toward the airstream fluctuated between -0.2 and 0.2 or so. If we interpolate the "true" static pressure as about -2.0 (halfway between zero and -4.1), then the interference of the velocity air pressure directly entering the tube, which the manometer would interpret as positive, explains the zero reading.

Isn't there a measurement missing here?...

Yeah, it should have been taken perpendicular to the flue, as Dan pointed out.

Too cryptic, sorry.  

Shouldn't there be another tube and another pressure being measured?...

Hmm. I've understood the intent to be measuring the delta P of flue gases and the ambient air, but it's been quite a few moons since I've done combustion testing on a regular basis. What am I missing here?


Still too cryptic

Where is the 2nd hose measuring caz depressurization?

I see what you mean - but the title of the thread is:

Which is the valid draft test, noir which is the valid CAZ test

Is it a valid draft test without both numbers?


Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Grzegorz is now a member of Home Energy Pros
2 hours ago
Rod Fox replied to Rod Fox's discussion Utility Meter Accuracy
"Dennis, that's a ton of useful info and it will take me a bit to fully assimilate those…"
3 hours ago
Ben Haas replied to Rod Fox's discussion Utility Meter Accuracy
"Yup..... 1200 amp service to the compound.....  3 -400 amp panels, all single phase since 3…"
15 hours ago
Walter Ahlgrim replied to Rod Fox's discussion Utility Meter Accuracy
"The more I think about it if the problem cannot be the current transformers reading small loads,…"
20 hours ago
Dennis Heidner replied to Rod Fox's discussion Utility Meter Accuracy
"Note to readers 0.5A instead of 5A for reading with A/D.... the edit to correct that didn't…"
21 hours ago
Dennis Heidner replied to Rod Fox's discussion Utility Meter Accuracy
"Rod, I have some of the CT's that you referenced in the openenergymonitor system.  They…"
21 hours ago
Tauran Ivall replied to Tauran Ivall's discussion Energy Efficient Entry Door options out there?
"Graham, How did this Plasproinc door cost?  Do you have any photos you can share a link to?"
21 hours ago
Dennis Heidner replied to Rod Fox's discussion Utility Meter Accuracy
"1200A service... really?  or did you mean 200A.  That size service would likely result in…"
22 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service