Help with orifice calibration

[cspeier]

Hello Guys

I've been reading for a few months, now I have a problem of my own. I have a custon built orifice style bench. It a Mercdog design with my tweaks. My problem.

I'm having a problem with my exhaust flow being accurate. I decided to flow the exhaust side with my flow plate, like I do my intake. The exhaust numbers seem to be low compared to a 601 in town. When I get to the 6,7,8 range on my plate, the manometer goes backwards. It basically will go to the 5 range, then start backing up. I recheck the intake, and it's dead on. Is this normal? I replaced the lines thinking it was sucking air, but still the same. To be honest, I've never had to use the 6,7,8 range when flowing a exhaust port.

ANY HELP would be appreciated! Thanks a million for a killer forum for such a problem!

Chad Speier

[larrycavan]

Hi Chad

A fair bit of information on your bench would be helpful as this particular problem is considered. Only one thing will cause a manometer to reverse and that's a pressure reversal between the manometer connections. Something in particular with your plenum design or flow disk seal is the likely culprit. Also consider that when flowing intake [suction] through the cabinet, it's actually aiding in sealing the joints and the opposite is true when flowing exhaust [blowing through it].

How well your flowdisk seals and the sizes of the orifice holes could play heavily into this.

Without getting into all the specifics of the plenum design and flow disk hole sizes, a simple test to rule out some variables is the most expeditious approach.

Here's what I would do to rule out a leaking flow disk. It may or may not be easy to perform. That all depends on your ability to get to the flow disk.

Select the range you experience this pressure reversal in and move the lever to that position. Then, take duct tape or whatever else you have and seal off ALL THE OTHER holes in your flowdisk. Repeat the test and see if the problem is still there. You should be able to flow at 3 - 6" of test pressure and get a result while not blowing the temporary seal off the flowdisk holes.

Larry

[cspeier]

Thanks Larry..

I'll try that in the morning..

The bench is basically a MSD design, with eight orifice holes. The largest being 3 inches. I can get the exact sizes of the others, all info is at work. The bench has seven motors. It'll flow 480 @ 36inches. I've had the bench working for 6 months or so. The intake side is within 1% of a SF 601. The exhaust, well it's not even close. I was thinking some of the problem is the exhaust has to push thru a 3 inch 3/4 piece of wood. I called the designer of the bench and software and he said the calibration shouldn't be different from the intake side. What got me thinking is that a SF has a different range for the exhaust side. I'll check it out. Is there anything else in particular that would be useful for you? Thanks.. I appreciate it.

Chad

[larrycavan]

Chad,

Keep one thing in mind. The exhaust flow and the intake flow are not likely to match because the Cd on the flow disk can be different from side to side and likely is on the one you've got. It it's not, I'd be totally impressed.

Larry

[cspeier]

Larry

Before I even got started, I found something strange. I have my calibration plate on my bench. I made sure I had it good and sealed. I put clay around the edge. I put a piece of tape over the hole. It still went backwards! Do I have a air leak?


I taped the other holes, yes it still goes backwards. HELP!!!


Chad

[larrycavan]

Chad,

I have to think about this one a bit.

You say it's the MSD design. Do you have any baffels installed anywhere?

Is there an added top chamber like the SF has? or Is it just as the photos of the Mercdog bench would indicate?

And, when you sealed the calibration plate off, were you performing Intake or Exhaust test?

Larry

[cspeier]

No baffels, no top chamber, I was on the exhaust side. The intake side is sealed tight.

[larrycavan]

Chad,

I'm thinkin on it....NICE JOB on the bench!...

Possibile places I can think of for internal leak would be:

center divider where it meets the rear door of the cabinet

possible motor back flow problem...use just one. If exhausting, you already have the right side door off or open so you could reach in and seal the unused motors with more magical duct tape.

Did you buy plans from Mercdog?
If so, what do they think?

[cspeier]

I think the plans are simple, on the money and worth every penny. The software alone is worth the $30.00. I've done the math like SF have and the software is dead accurate. I changed a few things. I also made a panel with needle valves so I wouldn't have to keep changing over the manometers.

I think I found the problem. I got a candle and started to check for leaks. I found the problem at the calibration plate. I got some two sided 3m tape and used it. NO LEAKS.. The clay was just blowing out. I have the new figures, I'll post them as soon as I do the math. Thanks Larry..

[cspeier]

Nope.. don't think this is correct..

Orfice hole dia. cfm
#1 .922 139.2
#2 1.300 180.8
#3 1.530 217.5
#4 1.830 253.1
#5 2.043 535.7
#6 2.295 632.7
#7 2.555 870.3
#8 3.098 1160.6

Calibration plate has a .985 hole.

[larrycavan]

Glad to hear it. You're a good craftsman by the looks of the bench photos.

[cspeier]

Think I jumped the gun! I don't think my numbers are correct. Can it possibly flow 1160 thru a 3.098 hole?

Chad

[larrycavan]

Assuming 28" of test pressure is what you're using, I get a Cd of over 1.0 when I run your numbers. That's very high.

Was the leaking calibration plate the problem causing the manometer to reverse in the range you had problems with?

[cspeier]

No not the problem.. I have a dumb question. On the intake side on #8 hole at 28 inches it will flow 480 cfm. I have 535 cfm on the #5 hole on the exhaust side. Do you think the reverse on the manometer is telling me that I don't have enough motors to flow at that orifice range? Basically it's reading negitive pressure(or positive), such a thing? I checked everything you've mentioned and always the same end result. Thanks Again..

Chad

[larrycavan]

Chad,

That thought crossed my mind. I won't rule it out. What happens if you reduce the hole size by placing duct tape over it? Can't hurt to try that. Reduce it by 25% see what happens. If it's still the reversal problem, reduce it more.

If the problem is still there, then there has to be an internal leak past the flowdisk.

One thing I don't like about our benches is that on exhaust tests, at higher test pressures, depending on how tightly the disk fits, how flexible it is, etc...it can be possible for air to be leaking through other, unselected range holes. That's why I suggested sealing them off.

I would give the hole reduction a whirl and see what it does.

Larry