Caibration Problems

[Flathead]

Hi Rick
Since installing baffles bench has gone into over read mode.
"Fitted circular baffles plates with rounded edges 4 3/8 " under top deck and the other same distance under metering plate. Tested at this stage and found bench over flowing, 100 against 150 off the scale , 100 against 200 reads 95%"

This is where I got scale from http://www.dtec.net.au/Tech%20Articles/ ... erator.xls Will check against others listed in the forum.

Thanks for your help
Grant

[Brucepts]

What are you using for a measuring orifice inside the flowbench?

[Flathead]

Hi Bruce
Inside measuring orifice is one of the plates you made for me. So either the 200cfm @28 or the 150cfm @28 then I run tests with the 150 and the 100 against the 200 and the 100 against the 150.

Grant

[coulterracn]

I was having some problems with calibrating my bench. I had ordered Bruce's orifice plates. I didn't specify whether I needed the internal orifice plate or external calibration orifice plate.

I was looking at your sketch, my question to everyone is; will relocating the upper baffle plate so it's the same distance off the internal orifice plate instead of the upper test port? Will this affect air flow across the orifice?

You will probably be happier making your percentage scale per the spreadsheet posted elsewhere on the forum. Then make the scale adjustable along the length of your tubing so you can set '0' with the liquid level.

Ray

[Brucepts]

Ok, makes sense now!

The plate that goes inside your bench has to be rated for the rise of your inclined manometer. So if the incline rises 14" you need a plate rated at 14".

A 200@28" plate at 14" only flows 141 hence the reason you are maxing out your manometer or it's not reading correctly.

[Flathead]

Hi
Well now I am confused. So with the plates I have 200 150 and 100 all @28" how do I go about calibrating my bench?

Grant

[Brucepts]

The plates you have are calibration plates and are used on top of the flowbench, you run the depression up to 28" and measure your cfm on those plates. Static pressure you test at has no effect on your internal plate other than a higher static pressure means you need more blower power.
You need an internal plate that is rated at your inclined manometer rise (which is your differential pressure) ie 6, 8, 10, 14 etc. The rise of the incline is not the length of the incline, the length could be any length ie 12, 16, 36, 50" etc. So to read 200cfm@28" at 100% you need an internal plate rated 200cfm @ 6, 8, 10 14" etc. For an example; the PTS Digital Manometer uses a 16" differential pressure so any internal plates are rated for XXX cfm @16".

The 200cfm@28" plate would will flow:

141 @ 14"
119 @ 10"
107 @ 8"

Hope this is makes some sense now? Go back and reference the Flowbench 101 post in this section might clear some of this up for you (I'm not the best at explaining this via the written word).

[jfholm]

Grant,
What Bruce has said is what you need to do. The reason is on your inclined manometer you are raising the fluid level to a height of 14" if that is the difference in height between 0% and 100% no matter how long the manometer is. So you need to know how much air does your orifice flow at 14" h2o because that is where it will be hitting 100%.

So in this case since you know that the one orifice you have from Bruce is 200 cfm at 28" h2o you can calculate the diameter of the orifice. I used .605 CD (coefficient of discharge) to calculate a diameter of approximately 1.69". Now if you download and use the spreadsheet that Ed has put on the forum for us to use you will find that an orifice of 1.69" with a CD of .605 will flow 141 cfm at 14". That is what Bruce put in his post just before me.

So to calibrate your bench for a 28" test pressure, which has nothing to do with your inclined it is just how hard you are sucking air through the test piece i.e. your head or calibration orifice, you need an orifice that will flow 141 cfm @ 28". That would be an orifice with a diameter of 1.42". If you have Bruce make you an orifice with that diameter you would put that on the outside of the bench where the head would mount and then on the inside of the bench you would put your orifice that flows 200 cfm at 28" because it will only be flowing 141 cfm at the inclined 14" h2o pressure. Then run your bench up to 28" h2o test with the 1.42" orifice mounted on the outside and your inclined manometer set at 14" rise should read 100%. If your inclined manometer does not read 100% then adjust it so it does. Now keep in mind it should not take much of an adjustment to do that. I would guess no more than an inch either way if even that.

Now to try to grasp this concept just remember: How do you increase the flow through an opening? You either increase the flow pressure or you increase the size of the hole. Since air is compressible it makes a big difference in the flow capacity of an opening just by increasing the pressure. And in this case you are using two different pressure readings between the inside orifice and the outside orifice for calibration. That is why you have two different size orifices flowing the same amount of air.

Also to note: if you have the 141 cfm @ 14" orifice inside the bench and you are testing a head and it is reading 50% on the inclined at 28% that means what you are testing is flowing 70.5 cfm @ 28" h2o.

John

[Flathead]

Hi Guys
Thanks for all the help and comments. I guess if was the shy kind I might think about stopping but not my way I just have to keep pushing on so here goes risking that I may still have some almighty blind spot!!!

So I got 3 plates from Bruce all at 28" 200 150 & 100 cfm I see the error of my ways with that now so maybe a bit of progress before I rush off and get plates @14" I figured I could us what I have by converting cfm ratings to 14" yes? no? maybe?
Assuming I can, I have 3 plates that should flow: Plate 1, 141cfm @14" , Plate 2, 106cfm @14" and Plate3, 71cfm @14" I then ran tests with these plates.

Plate 1 (141cfm @14') inside the bench tested:

Plate 2 @14" test pressure resulted 69.5% on inclined manometer = 98cfm should be 106cfm???
Plate 3 @ 14" test pressure resulted in 49% on inclined manometer = 69cfm should be 71cfm
Plate 3 @ 28" test pressure resulted in 71% on inclined manometer = 100cfm should be 100cfm

Plate 2 ( 106cfm @14" ) inside the bench tested:

Plate 3 @ 14" test pressure resulted in 65% on inclined manometer = 69cfm should be 71cfm (Same as above repeatable data yeah!!)
plate 3 @ 28" test pressure resulted in 94% on inclined manometer =99.6cfm should be 100cfm.

The thing with this is I don't know if I have made any progress at all as to get these results I had to tilt the inclined manometer to 48 degrees.
Given my previous misunderstandings will leave it at that and await further assistance.
Really appreciate people taking the time to help thanks.
Grant

[Brucepts]

Pretty good numbers!!

Don't worry about the angle, key is to set the incline so it matches your plates, well diameter and incline tube diameter will change your rise/angle. The spreadsheets get you close, testing gets it calibrated