Initial Test Results

[Dom G]

On Monday I was finally able to run some initial tests with my MSD based bench. Before I give the results, here's a quick profile of my bench.

* six 16-1234 motors
* 1 meter inclined manometer scale
* 12" rise
* orifice disc with 8 measurement ranges (from 29 cfm-
495cfm)
* cabinet leakage at 28" using the 29cfm orifice was .5 cfm
for the intake. haven't done leakage testing yet for exhaust

Using a 160cfm calibration plate that Bruce made for me, my bench at 28" measured 155cfm. That was using an orifice on my disc that should flow 189cfm at 100%. I ran several tests which were all within 1cfm.

I'm pretty happy with this result. However, during the testing I made a few discoveries.

1. Unless I have something really out of wack in the bench, six of these motors will not get me close to my goal of being able to measure 400cfm. As I recall Mousehouse1 had trouble getting to 400cfm with 10 of these motors. I 'm going to address this in a separate post.

2. I do not believe just using the control valves will give me enough precise control without adding some type of electronic means to throttle back at least one motor.

3. When I ran a test using the calibration plate on the next largest orifice on my disc, the results were no where close. I used an orifice on my disc that should flow 300cfm at 100% at 28". Using the same calibration plate the bench measured about 43cfm. By the way, I had no trouble getting the depression to 28" which sort of surprised me.

There has been much published and stated that you need to always select an orifice size that will put the inclined manometer in the upper portion of the scale, 60% or higher.

Has anyone else had similar results when the likely result would fall below 60% of the inclined scale?

So at this point, I'm not sure if I simply just need more horsepower so I can measure flow to 400cfm, or if I've got other issues also.

As always, I would appreciate any comments, suggestions, observations.

Thanks.

Dom

ps...this is fun but frustrating

[larrycavan]

[Mousehouse1]

Dom I see you are starting to go through the same thing I was getting on my bench.

I had a couple of problems with leakage. The main one was the orifice disk. I am replacing it with several orifice plates that I will have to change to change ranges. Chad (another member on this board) used 7 motors of a different brand and was able to get over 450-500cfm if I remember correctly. Is the outside of your bench painted? I am going through my bench right now and painted all of the wood. Another member posted the amount of leakage he was getting through the wood of his bench. I was surprised to say the least.

Are you sure your orifice disk isn't leaking a little? If it is you won't be able to get your flow figures any higher. I had the same problem. I am hoping to have very little leakage when I have it back together.

I believe Mouse is using 6 of these motors in a small air box he uses on his pitot bench. I had to many leaks for my motors to pull enough vacuum. That was my problem.

I have gotten a lot of help from Larry, John, Bruce and others on this board. They have a lot of information and I haven't found any of it to be misleading.

I am also having fun building my bench. I just hate the problems I have had to work through. If it wasn't for this board I would be lost and would have made something else out of the wood by now.

[Dom G]

Larry

I meant I selected a range that is calculated to flow 300CFM max at 12" pressure differential and tested a known calibration plate that knew flows a specific value @ 28" of test pressure. Sorry I wasn't clear.

In any event, I'd like to find out from others whether the measurement results get totally and unreliably skewed when the percent on the inclined scale falls below a certain point.
And what that point is.

Dom

[Dom G]

Mousehouse1,

Currently my bench is unpainted. But as I mentioned, I ran a leakage test and it showed a leakage of .5 cfm (1/2 of one cfm) which is nil. Now if I'm right, that test means my upper and lower air plenums including the orifice disc are tightly sealed. If that's true, others are suggesting vacuum power or exhaust pressure is being lost through the cabinet walls as a result of porosity and not making its way to the plenum areas. Maybe its possible, but I'm having some trouble accepting it will make much of a difference. If I was a betting person I would say switching to the good motors would make a huge difference.

I'll have to go back and reread the posts relating to this subject to see if anyone has before and after results once they painted their cabinet.

How close are you to revising, painting, and retesting your bench? If you are close, since we have the same motors, I may wait until you have your results from retesting before I make any major changes.

Let me know.

Dom

[larrycavan]

Dom,

The technical term, I believe is "turn down ratio".

The scale is most accurate in the upper ranges BUT that doesn't mean it's completely useless at say...30%. There certainly is a plus/minus in the lower portions of the scale where accuracy is concerned but it won't be as far off as what your posting indicates for test results on that range.

I would say you should be able to trust your inclined within a few percent of accuracy down in the lower regions of the scale...If you couldn't then testing for cabinet leakage would be rendered an arbitrary number.....

Something else is wrong there....You should have seen close to the 155CFM value of the cal plate in your 300CFM range too. That would put it at 51% on the inclined scale.

47cfm is 15% of 300...if you're down that far...there's a leak..

It's not surprising that you could easily pull 28" of test pressure because you are only pulling against a relatively small restriction of the cal plate...then again..with a reading that low it is rather suprising after all...

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....

Make certain your exhaust control valve isn't open a little...

You WILL find it...have no doubt...

Larry C


Larry C

[Dom G]

Larry

I'll try the other upper ranges and see what I get as a next step.

Although performing the cabinet leakage test is suppose to be done using the smallest orifice opening in the bench, can any reasonable conclusions or suspected problems be drawn from trying to perform the leakage test using each of the orifice sizes? Just a thought. I don't know enough at this point to know whether it can physically be done.

Dom

[larrycavan]

[Nick]

I have found below the 30% line the numbers drop off.

also remember your motors see test pressure plus pressure difference. So if your test pressure is 28" and your incline scale is 12in high, at 100% on the incline, the motors see 40".

I made this mistake thinking that 4-2 stage motors would pull 400cfm, but they do not, they only pull about 300cfm at 28" and 100% on the incline. There is a motor chart on here somewhere.

Just my 2 cents

Nick

[Mouse]

One thing that would be very helpfull for anyone trying to help would be to present some actual test data. For instance, the following would be very helpfull:

Calibration orifice size to the .001"
Measuring orifice size in question to the .001".
Test pressure
Differential pressure in inches of water (% of incline means nothing).

Without these data, there are too many assumptions and all anyone can do is take shots in the dark trying to troubleshoot difficulties in these systems. With that data, a person can sit down with a calculator and see the problem in a matter of minutes. Spreadsheet users, a matter of seconds.

John

[Dom G]

Mouse

Here's the information you suggested would be helpful.

Calibration orifice size to the .001" = 1.498" (sharp edge)
Measuring orifice size in question to the .001". = 2.531"
Test pressure = 28"
Differential pressure in inches of water = 12"

In addition, the measuring orifice size that yielded 155 cfm at 28" of pressure using the same calibration plate (1.498") is 2.009."

Thanks.

Dom

[Mouse]

Dom,

Your measuring orifice (2.531") should have a dif pressure of 3.45 to flow the same as your cal orifice, which should flow 160scfm @ 28".

John

[Dom G]

John,

Can you translate what having "a dif pressure of 3.45 to flow the same as your cal orifice" means? At this point in my learning curve, I can't relate to it.

Thanks.

Dom

[86rocco]

By that he means that 160 cfm through the 1.498" orifice would generate a 28" pressure difference and the same flow rate through the 2.531" orifice generates a pressure difference of 3.45"

[larrycavan]

That's 53% on your inclined Dom....in the 300CFM range...

Larry