Calibration Help - Having problems

[jakesportingservice]

Hi everyone I am having some issues with calibrating my bench. Here is the description of my bench it is a msd style bench with the top added section and baffle. It also has the rotating orifice with a board of equal thickness on both sides of the disk there are pics of this in the readers flowbench area.
My inclined specs are tube id .1875 reservoir id .83 scale length 27.00" scale vertical height 13.5" fluid density .876 marvel mystery oil scale angle 30
My problem is that i have verified the inclined and it matches that of the vertical. Now when i flow my calibration plate the numbers are way off on my largest range.
The orifice disk is .040 thick and has six range sizes that are like this range 1 .885 = 37.1 range 2 = 71.3 range 3 = 148.2 range 4 = 297.8 range 5 = 443 range 6 = 576.6 all were calculated from the spread sheet with a .6 cd and the orifice is square edge.

When the plate i flowed at work which flowed 272 cfm on a blue bench 600.
On my bench i leaked checked and came up with 2 cfm of leak which i am fine with now the problem is that when i flow this plate on my bench it shows 37% on range 6 which is 213.34 cfm another test on range 5 showed 55.5% which comes to 245.86 cfm and on yet another test on range 4 it showed 89% which equaled 265.04 cfm .
I have tried a hose on the pickup tube and have even moved the tubes close to the walls so they didn't sense velocity but i still get all the same readings. I seam to be lost at the moment and cant figure this thing out. Also i have done a exhaust test on the largest range and that showed a 39% figure which i think is pretty normal for the exhaust to read a little higher.
When the verified inclined checked out i did the leak check and that seems to be very minimal as is 2 cfm does everybody agree with that? I have read the past posts but really couldnt figure mine out .
Does any body think the board above the disk is influencing the intake test?
Any help would be greatly appreciated Thanks Jake

[49-1183904562]

[thomasvaught-1]

Agree. I have seen flow changes where the downstream piping was affecting the orifice reading vs discharging into a plenum. same deal with the "entry" to the orifice acting as a "orifice".

I personally am a fan of the one direction benches unless you can move the orifice plate to the leading edge side of the plenum. I have tested square plates with the calibration orifice in the center and round plates and both repeated well IF the plate was on the leading side. Hard to do that with the Blue Bench/ MSD
type "trapped rotating orifice plate" without some flow work vs a series of calibration plates.

Tom Vaught

[jakesportingservice]

Hi rick the plate is on range five in the pic and it is about 3/4"on all sides on that range and about a half inch on the largest range. How does anyone suggest to fix the problem.

I know everyone will probably say scrap the rotating disk but it seems not to leak . should i open the entry hole up to help it not to act as an orifice in a orifice? or try to work with the cd. I think if i put the cd to .77 the calculation came out to what my plate flows.

Tom sorry i didnt realize until now that i didnt thank you for your input i know that you are very knowledgable when it comes to this kind of stuff and am very glad that you are trying to help me with this design.

[bruce]

I'd work with the Cd but, make sure it is linear across the whole range of the plate ie from 100% to 40%.

[jakesportingservice]

Thanks bruce that saves a bunch of work i not sure what you meen by making sure its linear across the range from 40% to 100% ?
I've made another plate and flowed it 2.560 hole and it flowed 64.5% and that calculates to 371.907 for my bench but the spread sheet says it should flow 445 .

So if i take it correctly i should adjust the cd to .77 for the largest range and use that number to calculate my flow from
on the 3.491 inch hole. That is not really a size that i will be using very often but if i ever do any big stuff i might need it so that is why i put it in. The ranges that i will be using most often will be range 4 and 5

[bruce]

What you want to do is test a plate at 100% and one at 40 or 50% of the range and see if they match using the range Cd. If not then your plate is not linear using that Cd and presents a calibration problem.

Might want to check your other ranges this way also. You might have to adjust the Cd some so the top of your range and bottom of your range falls within an acceptable limit for your use.

[jakesportingservice]

Thanks Bruce i think i understand and will have to check some of those things and post the results . Thanks for the help, Jake

[jfholm]

Jake,
I don't know if I missed it somewhere else, but what is your test depression? Also what are your diameters of your orifices. Also did you test the calibration orifice on the SF600 at the same depression?

I also like Bruce's idea of checking the cd on each orifice.

John

[jakesportingservice]

Hi John the test depression is at 28" on the vertical and yes i did test the plate at 28" on the blue bench. The orifice sizes of the disk are in the first post.
Here's some data that i just collected and dont know if this is the correct way that bruce explained to check but here goes these test were done using range five orifice and a plate with a 2.560 hole square edge this plate has not been tested on the blue bench yet but i will test it to see the results
39% @ 5" water converted to 453.83
55% @ 10" water converted to 450.93
78.75% @ 20 water converted to 456.26
94% @ 28" water 461.54
97.75% @ 30" water converted to 463.63
Let me know if this data helps or do i need to make a bunch of orifice plates and test them to get the data needed to help? Thanks to everyone helping Jake

[bruce]

My thoughts would be you need plates for each size you need to test. One plate is not going to work for all your ranges.

Different plates will work for various ranges though. I would work with one static depression and not mix and match depressions.

If you plan on testing at 28" then make all your calibration plates at 28".

Other forum members might have other ideas, so don't take my thoughts as "gospel" . . .

[jakesportingservice]

Thanks bruce i will have to come up with some measurements and make a bunch of plates to test @ 28" water and get that data and post it. might take a day or two to get the results.

Thanks for all the help Jake

[slracer]

Jake, As the orifice gets closer in size to the hole in the plate holder, you are approximating an "orifice in a pipe" design which has been discussed at length on the forum. The link below says the limit is a ratio of 30% of orifice diameter to "pipe" diameter. At the bottom of the discussion, they supply "modifiers" to correct for larger ratioes. If this starts to match your results, it would suggest this is your problem. You can solve it mathematically or by changing the orifice holder. Ed (rocco86) will love this one, pure math! -- Doug

[jakesportingservice]

Thanks for the info Doug but i have no idea how to do all that math although if i did it probably would be very useful. I'm way better at building things than doing formulas so i think i'll just rely on using the spread sheet here on the forum and change the cd. Once i have my other orifices finished i will collect some data and see what i come up with.

Thanks for your input Jake

[86rocco1]