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: 367

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

Kurt Shafer replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
"Reading the Invisco description you will find that it is designed to be surrounded by cellulose to…"
14 hours ago
Glen Gallo replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
"Kurt, I think cost of operation is always a factor but I am not sure I would leave it just cfm.…"
14 hours ago

Paul DelVaglio just added their location.
(via Member Map)

16 hours ago
Paul DelVaglio posted a photo
16 hours ago
reflintorm replied to Jamie Kaye's discussion Flickering LED lights
"I think, Better to replace the dimmer switch if it is not recovered then once go through the…"
20 hours ago
Kurt Shafer replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
"Glen,  You are the first person who has ever told me they prefer lower CFM. What you left out…"
yesterday
Glen Gallo replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
"Tamarack has lower CFM than typical fans and has a insulated lid that automatically closes when fan…"
yesterday
Tom Conlon's discussion was featured

Whole House Fans - Love 'Em or Not?

I just searched this forum for "Whole House Fan", but I didn't find much about them (except …See More
yesterday
Tom White shared Brandon Walton's blog post on Twitter
yesterday
Brandon Walton's blog post was featured

12 Things Every Home Performance Contractor Should Have on Their Work Truck

Every home is unique and differs from the last. It would be a perfect world (from a project…See More
yesterday
Griffin Hagle replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
yesterday
Griffin Hagle replied to Tom Conlon's discussion Whole House Fans - Love 'Em or Not?
"Kurt, Whoops, looks like I got my links mixed up. Thanks for pointing that out. Here's the…"
yesterday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service