Initial Test Results

[Dom G]

Whether its the correct way to make the calculation, I simply divided 160 by 300 to arrive at 53% knowing that the sqrt of 53% is the number that I need to get to on my meter long inclined scale.

I would be interested to know in layman terms what 3.45 differential pressure means and how you converted it to 53%?

Dom

[larrycavan]

[Mouse]

Dom,

To add to Larry's post, differential pressure is the difference in pressure from one side of your measuring orifice to the other side. This is how air flow is measured in your flow bench, by measuring the difference in pressure across the measuring orifice. By comparing this difference in pressure with the diameter of the orifice, the cfm can be calculated. Also known as Delta P (change in pressure).

If you apply a test pressure of 28"wc to your calibration orifice, your delta P is 28"wc, since the other side is atmospheric pressure, which is considered 0"wc.

I feel that when setting up and trouble shooting a flow bench, it is best to use inches od water column ("wc). It just leads to less confusion and fewer variables at this time. When all is found to be working correctly, then I would impliment my % of incline readings.

John

[larrycavan]

Dom

Did you find out what the problem was yet?

Larry

[Dom G]

Larry

I just got done performing the following steps you recommended a day or so ago.

"Move on up and test the other ranges with that same plate at 28" and see what you get.

If it won't add up...and you can get to your flowdisk....select the range that's giving you problems and seal the rest of the ranges off with duct tape...that will isolate the selected range as the only air path between the two chambers....tape around the sealing ring as well...."

First, I tested the two higher ranges. As I expected, both tests which used the same calibration plate yielded extremely low results.

Then, I pulled the front cover off the bench and cleaned the orifice disc and surrounding sealing ring real well. Next, I selected the orifice hole (2.531" ) that was originally giving me low readings and duct taped everything else so it was the only air path.

Much to my surprise, nothing changed! So now I'm wondering whether the problem lies with the placement of the pressure pickup locations in my plenums. And I simply got lucky when I did my first test with the calibration plate and the result was real close. Buy the way, I located the pressure pickup for the upper plenum about three inches in from the outside cabinet wall and about three inches down from the bottom of the top. The pressure pickup for the lower plenum is about 3"-4" from the outside edge of the hole in the center of the divider panel where the air flows through.

If it's not the pressure pickup locations, there are two other possibilities I can think of. One is the chalking around the outside edge of the sealing and spacer rings of the orifice disc which I doubt. The other is something I built into my bench that I didn't mention previously. Because of all the talk of leakage problems, especially doing exhaust testing, I added two thin , low profile (7/8") pressure fingers that extend from the outside edges of the divider panel to about 3" from each side of the orifice. My thought was to try it and see if it helped improve the seal of the orifice against the divider in both intake and exhaust mode.

So now I need to decide what change to make next as I continue to troubleshoot. My instinct tells me changing the pressure pickup location for the upper plenum is probably the logical next step. But I'm guessing.

Any suggestions.

Thanks.

Dom

[bruce]

Ok I've never built an orifice style bench but wouldn't you want your pickups to be as close as possible on either side of the orifice disk? Is your pickup getting any flow directed at it?

You want your pickup to be out of the flow of any air or you will see "velocity" (maybe not the correct term to use) instead of true differential pressure.

[larrycavan]

[Dom G]

Thanks Bruce and Larry for your input. Tomorrow, I'm going to try to use the plastic tubing approach to temporarily relocate the pressure probes and see if that makes any difference.

As far as having a massive air leak, is there any other way to pinpoint the likely location? Upper plenum? Lower plenum? Other?

One thing I've wondered about is whether during cabinet leakage testing (with a 29cfm orifice) it should take turning on 4 motors to get the depression to 28"? Maybe normal. I'm just looking for some other indicators that there's an air leak I haven't found.

Dom

[Thomas Vaught]

On the SF bench the orifice probes extend from the top of the orifice angled mounting plate to right next to the orifice hole BUT they are about 3/16 off the surface. They are mounted in plastic retainer type deals glued to the board surface.

The test pressure probe is located in the top chamber in a dead spot in the bench. I use a probe inside a second tube with several small holes in it to eliminate and velocity issues and to just get static pressures.

Tom V.

[larrycavan]

[larrycavan]

FPexcel shot...

[Dom G]

Larry

Very nice looking bench.

Dom

[larrycavan]

Dom,

It occurs to me that we haven't discussed any exhaust tests on your bench.....why not go ahead and perform some calibration tests on the exhaust and see what the results are? It may reveal something....

Larry C

PS thanks for the nice comment on the bench..

[Mouse]

Dom,

Are you leaking air through your unused motors when trying to get 28" from 4 motors?

John

[Dom G]

Mouse

I don't know whether I'm leaking air through the unused motors when trying to get 28" of depression. I suppose I could try blocking off 3 or 4 motors and see if that makes a noticeable difference in fewer motors getting depression to 28". I'll give it a try if there is nothing obvious to some of you regarding the following summary of my testing today which follows.


Larry

You were correct that we had not discussed any exhaust tests since until this afternoon I didn't do any.

Here are the changes, testing and results from the things I just finished.

1. By using vinyl tubing, I temporarily relocated both of the orifice probes and re-ran the intake test using the calibration plate (1.498" sharp edge) and the orifice (2.531") I originally had trouble with.

Nothing changed, the test results were almost identical.

2. Next, I decided to run an exhaust test using the same orifice and calibration plate as above since they were already in place. All other orifices on the disc as well as the sealing ring were still duct taped so air could only pass through one place the test orifice.

The test results were virtually the same as #1 above. 15%

3. My last test was to do a cabinet leakage test in exhaust mode using the smallest orifice (29 cfm).

At 28" pressure, the amount of leakage was about .5 cfm which is the same as the intake leakage test I performed originally.


I said to myself this doesn't seem to make sense. I had examined the inside of the cabinet as best I could and there weren't any obvious problems that I could see. I looked for cracks in caulk joints that might suggest panel movement or gaps of any type....nothing stood out.

In thinking about my test results it seems like there's a common denominator present. I'm wondering whether the way I built the reservoir for the inclined manometer could be the problem since it impacts all measurements. I took a digital picture of it and hopefully its attached. If not, someone will need to describe how to attach it. I remembered a previous post by Bruce I believe describing it, but I couldn't find it.

Would appreciate thoughts and suggestions because if I don't find the problem pretty soon I'm going to put this bench in the corner of my shop and forget about it for a while.

Thanks.

Dom