Another calibration question

[smooth blend]

[color=#000000]I

[84-1074663779]

All you need to do is mark off your sloping manometer in increments of flow. You might make 30" of manometer scale length equal to 100% flow for example.

If you decide to calibrate both your orifice and manometer to work at 13" of rise, then 100% flow will correspond to what the orifice flows in CFM at 13".

Now pressure developed across the orifice is flow in CFM squared. If flow doubles, pressure goes up four times. If flow triples, pressure goes up nine times.

From this you can see that the 50% flow calibration mark will be one quarter the way along the scale (7.5"). Remember when flow doubles (50% to 100%) pressure goes up four time (25% to 100%)

So you start marking off your scale:

50% flow ( 0.5 x 0.5 = 0.25) one quarter the way up or (0.25 x 30") = 7.50 inches
51% flow (0.51 x 0.51 = 0.2601) (0.2601 x 30") = 7.803"
52% flow (0.52 x 0.52 = 0.2704) (0.2704 x 30") = 8.112"
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100% flow (1.0 x 1.0 = 1.000) (1.000 x 30") = 30.00"

[smooth blend]

Ok that explains why the marks are non-liner.
What about getting the initial calibration mark? I need a known cfm orifice at a know depression to calibrate right?
thanks

[84-1074663779]

After you have made a suitably calibrated flow scale, of whatever length is convenient, the next step is to decide on a suitable measurement depression at which to set the 100% flow point.

The biggest factor here is probably available blower power. Thirteen inches rise is a nice convenient number, but it could easily be made something else. That will mean that the 100% flow mark will be exactly 13" above the manometer zero fluid level, and that fixes the manometer slope.

The actual CFM flow reading at 100% will depend on the diameter of the measurement orifice. For a thin sharp edged orifice:

CFM = 13.55 x square root of pressure (in inches of water) x diameter squared (in inches)

For example, thirteen inches of water, and a two inch diameter orifice:

CFM = 13.55 x square root of 13, multiplied by 2 squared
CFM = 13.55 x 3.6055 x 4
CFM = 195.4

This assumes still undisturbed up stream air. Turbulent, or high velocity up stream air can make that figure higher or lower. But that formula has been generally accepted by most people here as being a fairly accurate starting point.

[smooth blend]

[color=#000000]That is exactly what I needed
Thank you very very much!
However I do have a question

[84-1074663779]

Hi Julian,

The above formula assumes the orifice is located flush in a large flat expanse with stationary (completely dead) air upstream of the orifice. Pretty much like laying a calibration orifice flat on your bench top, with entering still room air.

If the air entering the orifice is highly turbulent, measured flow will most likely be a bit less, and the pressure drop may fluctuate and be unstable.

If a directed jet of high velocity air is headed directly towards the orifice, as if you pointed the nozzle of a high pressure air hose directly at the orifice, a lot more air is going to get through !

The big problem with the MSD bench design is that the orifice is directly below, and in direct line with the test hole. The air is highly disturbed and unstable at that location. The flow coefficient of the measuring orifice being highly dependant on what you are testing. Not a very good way to test.

The flow calibration figures are all over the place compared to the theoretical step up in orifice diameters. It is a truly horrible design in that respect.

Nearly as bad would be placing an orifice in a pipe. While any orifice can be calibrated, there will be no direct simple relationship between orifice diameter and calibrated flow. The closer the orifice size and pipe diameter, the worse it gets.

Placing a one inch orifice in a two foot diameter pipe would probably come pretty close to the above formula. Placing a two inch diameter orifice in a three inch pipe most certainly would not. Pipe mounted orifices are convenient for industrial process flow measurement provided you never need to change ranges quickly. For a multi measurement range home flow bench, pipe mounted orifices are a disaster.

Your best bet is to have a very large settling volume immediately upstream of the orifice, where the air can slow down. My bench has a settling volume of around fifteen cubic feet. It simply cannot be too large.

If very calm air enters the measurement orifice the flow will very closely follow expected measured diameter changes, and if that can be achieved you can be pretty confident in your measurements.

But if the flow steps are all over the place with different diameters, your measurements will never be stable and repeatable.

Hint, the largest orifice is always the most difficult to get right, because flow velocities are going to be at maximum throughout the whole bench.

If you can measure identical pressure drops with identical orifices, that is one orifice in the measurement location, and an identical orifice placed over the test hole, you are doing o/k. Say exactly thirteen inches of water drop across both, that is the ideal. If you can get within one or two percent with your largest orifice size, it is an achievement to be really proud of.

Do that, and all the smaller sizes will be even closer, and will fall exactly into line with theoretical expectations.

[larrycavan]

[zc1.8crx]

tony
so does that mean if your test pressure is 28'' then the inclined manometer should be set 28'' above the manometer zero fluid level.

[cspeier]

I'm not doubting all the talk about the design of the MSD bench. BUT, I've never seen it. My design has been accurate and steady. I've done alot of back to back with a SF600 and I don't see much difference. Maybe I got lucky! However, I must add that when I had the piece of steel cut for the top, the guy was either drunk or didn't know how to read a tape. The hole was way off from center and more towards the front. At first I was worried. I honestly think this was a good thing. Maybe some of the issues you mention on the design were fixed by the mistake. The more I think about it, it seems possible..

[larrycavan]

Chad,

If it's working that well. Count yourself among the fortunate. Generally, that design is problematic. Yours is the first one I've heard of that the builder didn't struggle with to calibrate it. It's nice to hear that something went smoothly for someone every now and again.

Larry

[cspeier]

Honestly, it's been a struggle to calibrate it. I had some issues. After my findings with the FP1, I know now how well it was calibrated. Look at the thread, it's amazing..

Chad