PTS Flowbench Forum

How many plates would be needed for 100cfm to 330cfm?

I have been reading on here about calibrating benches, that is what I am trying to do now, but I get confused so easy, so tell me if I am thinking right . The spread sheet says that a 1.78dia hole should flow 173.8 cfm at 16.4"wc rise on the incline (if I read the spread sheet right) so if I put the same size hole on the top of the bench should the % reading be 100% ? Or if i place the 2.05 plate on the top it should be a 100% for sure right, because it should flow more than what the 1.78 hole would correct?

swnf,

Yes your 1.78" hole will flow 173.8 CFM with a 16.4" pressure drop.

If you also place an identical 1.78 inch hole on top of the bench, there "should" be a 16.4" pressure drop across that orifice as well at the same flow.

So basically, if you crank up the test pressure to 16.4" across the test piece on top of your bench, your sloping flow manometer should also read 16.4" if everything is perfect.
In fact the two manometers should follow each other exactly all the way up from zero.

Now the air flowing into your 1.78" test piece on top of your bench is undisturbed room air.
And the air flowing into your 1.78" measurement orifice is flowing through the settling chamber inside the bench.

The readings "should" be identical in a perfect world, with a perfect flow bench, but with a good bench the two pressure readings should be extremely close if not identical.
It is an excellent way to test your bench.

Yes, the 2.05" hole will flow more air with a much lower pressure drop.
You should find that the same 173.8 CFM requires a test pressure of around 9.315" to raise your sloping manometer up to 16.4" (=173.8 CFM).

Thanks for the info. I will tinker with it today some, will let you know if it works
I can not thank you folks enough, for all the help.

Ok, here is what was done. Redone the incline manometer, went to Lowe's purchased two steel yard sticks that have millimeters on there they are a whole lot easier to read. Placed the 1.78 orifice plate on top, same on the inside, Then turned on the bench, set the pressure to 236mm, if I understand right the incline height should be 472mm, when the u-tube is at 236mm. I set the incline to 472mm. Got the bench to do the same thing (repeatability i hope). Went to the pts forum found this formula L=(v*v)*T so in my case, T=899mm, V=% value, L=length from 0 mark. Figured from 50% to 100%, marked it off on the incline. Made a bore adapter, tried to flow my big block ford head (dove-c) on the exhaust side, pulling through the port not exhausting. Here is how it done, % scale only goes down to 50%, so started testing at .300 lift
.300-59% of 131 is 77.29
.400-66% of 131 is 86.46
.500-71% of 131 is 93.01
.600-73% of 131 is 95.63
.700-74% of 131 is 96.94

The 131 comes from the spread sheet for a 1.78 orifice at 236mm or 472mm on incline.
Now I know the ford exhaust port is bad but is it that bad? or am I missing something? Please tell me if I am.
stock valve some porting already done.
Do not think something is quite right.

I think we need to start from scratch here.
How long is the scale along your sloping manometer from zero to max ?

899mm

O/k.

Your sloping manometer directly measures the pressure difference across the measurement orifice by driving the water up the tube.

What you need is to add a scale to your sloping manometer that converts manometer water rise to percentage air flow.

You are not interested in reading pressure with your sloping manometer, but reading flow off the scale of your manometer.
This same scale will work on any flow bench, with any orifice size, and any length sloping manometer.

With any orifice, the pressure drop increases with the square of the airflow.
For example, if flow doubles, the water rises four times as far up your manometer.
If flow triples, the water rises nine times as far up the scale, and so on.
Because of this, the flow scale along your sloping manometer will be very non linear.

Working backwards, if the whole scale length is made to represent 100% air flow.
One quarter of the scale length will represent 50% air flow.
(when flow doubles, from 50% to 100%) the water rises four times as far (from 25% along the tube to 100% along the tube).

So the percentage flow markings along your 900mm long scale should look something like this:

0%................50%.........................................................100% Flow
0mm.............225mm.....................................................900mm

Are you with me so far ?

The next step is to fill in all the other flow values along the scale.
Starting at 100% and working down.

100%...... 1.00 x 1.00 x 900mm =900mm
99%.........0.99 x 0.99 x 900mm = 882mm
98%.........0.98 x 0.98 x 900mm = 864.4mm
97%.........0.97 x 0.97 x 900mm = 846.8mm
*
*
49%.........0.49 x 0.49 x 900mm = 216.1mm
*
30%.........0.30 x 0.30 x 900mm = 81.0mm

You will quickly reach a point where the scale %flow markings become so close together that an accurate flow reading is very difficult.

To read lower flow values you need to switch over to a smaller measurement orifice, that will shift you back up into the higher percentages.
Likewise if your flow manometer tries to go over 100% flow, you need to switch to a larger measurement orifice.

I think I have done what you said.
May have wrote it wrong.
On my scale I marked it off at 100% all
the way down to 50% in one mm increments
from the top down, rounding the numbers.

1*1*899= 100%
.99*.99*899= at 881mm I mark 99%
Did it just like that all the way to 50%
Is that the wrong way