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: 385

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?

Ted

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?

RSS

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.

Latest Activity

Ben Jacobs replied to Isaiah Borel's discussion Blown Cellulose VS Blown Fiberglass in the Attic
"Hi Isaiah, I have experience with both fiberglass and cellulose. I think that cellulose is…"
2 minutes ago
Dolores Bevilacqua posted events
1 hour ago
Bob Blanchette replied to Dave Robinson's discussion Can We PLEASE Stop Calling Them “Ductless” in the group 1000 Home Challenge
"Concealed units in hallway ceilings aren't new. Apartments have been using them for decades."
1 hour ago
Graham Irwin replied to George Kopf's discussion How does efficiency factor into a grid parity world?
"George, You raise a good question. The concept of "free and unlimited" energy is not…"
1 hour ago
David Byrnes posted an event
Thumbnail

Best Lead Source For Energy Auditors at Your computer

November 1, 2014 at 12am to January 3, 2015 at 7pm
In 2009 I started my energy auditing company, Green ID, with over $3,000 in credit card debt to…See More
1 hour ago
Empower Efficiency added a discussion to the group Job Board
Thumbnail

Empower Efficiency is hiring!!!

Empower Efficiency is a woman-owned firm dedicated to making energy efficiency and renewable…See More
2 hours ago
Empower Efficiency joined Diane Chojnowski's group
Thumbnail

Job Board

This group is for posting jobs related to all aspects of the home performance industry including…See More
2 hours ago
Profile IconEmpower Efficiency, Ron Revia, Matt Austin and 3 more joined Home Energy Pros
2 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service