PTS Flowbench Forum

What I mean is, if the pressure drop across the measurement orifice is one inch, the water will rise one inch.
But in rising one inch at the sloping manometer, it has to travel three inches along the sloping tube to get there.
That puts the inch divisions on the scale three inches apart and makes it easier to read.

Okay cool.. thanks for the more detailed explanation..

Can you give me any feedback on this cd orifice i have and what they flow. or how to figure out actually how to use this bench...

The next thing you need will be a suitable measurement orifice to suit the flow range of what you are going to test.

Don't worry about extreme accuracy at this stage, the important thing is to just get your head around the basic concepts.
The amount of air a given hole size will flow is:
CFM = 13.55 multiplied by the square root of ( inches of water x hole diameter in inches squared)

Assuming six inches water rise is the max full scale calibration pressure drop for your bench.
And let's assume a 1.5 inch diameter round hole in a thin sheet metal plate is the flow measurement orifice.

CFM = 13.55 x square root of (6" x 1.5" x 1.5")
CFM = 13.55 x square root of 13.5
CFM = 13.55 x 3.67
CFM = 49.7

So 6 on your sloping manometer corresponds to 49.7 CFM with a 1.5 inch measurement orifice.
A larger measurement orifice hole diameter will make your sloping manometer be able to read higher flows at six inches rise.
A smaller measurement orifice hole diameter will be able to read lower flows at 6 on your scale.

So you end up making a few different measurement orifice plates to give you different flow measurement ranges.

Tony wrote:The next thing you need will be a suitable measurement orifice to suit the flow range of what you are going to test.

Don't worry about extreme accuracy at this stage, the important thing is to just get your head around the basic concepts. The amount of air a given hole size will flow is:
CFM = 13.55 multiplied by the square root of ( inches of water x hole diameter in inches squared)

Assuming six inches water rise is the max full scale calibration pressure drop for your bench.
And let's assume a 1.5 inch diameter round hole in a thin sheet metal plate is the flow measurement orifice.

CFM = 13.55 x square root of (6" x 1.5" x 1.5")
CFM = 13.55 x square root of 13.5
CFM = 13.55 x 3.67
CFM = 49.7

So 6 on your sloping manometer corresponds to 49.7 CFM in this example only.
A larger hole will make your sloping manometer be able to read higher flows at six inches rise.
A smaller hole will be able to read lower flows at 6 on your scale.

Thanks for your response and assistance i myself really eager to grasp this concept and understand it properly..

Okay so understand the maths involved...

So from what you said say my measurement orifice i assume this is the 1 that fits in the bench that measures the pressure difference.. 2.0 inch diameter round hole in a thin sheet metal plate is the flow measurement orifice as u mentioned in your example..
CFM = 13.55 x square root of (6" x 2.0" x 2.0")
CFM = 13.55 x square root of 24
CFM = 13.55 x 4.89
CFM = 66.25

So number 6 on my bench scale would read 66.25 cfm is this correct? so the bigger the test plate diameter more cfm it can read?

Yes, 66.25 CFM is correct.
And a larger diameter hole would increase the flow measurement range even more.

You can scale your sloping manometer to read any flow range by simply fitting an appropriate measurement orifice to the bench for the range required.
It is usual to have several different orifice plates, and a quick and simple means of swapping them over.

The next thing to understand is that the water rise is non linear with flow.

If you have a certain orifice size, and you double the airflow, the pressure drop increases four times.
Three times the flow creates nine times the pressure drop. Exact square law relationship.

Now looking at this the other way around.
If 6 inches on your manometer scale corresponds to 66.25 CFM
Half the flow (33.13 CFM) will only raise the water one quarter of six inches, or 1.5 inches.
One quarter the flow (16.56 CFM) only raises the water one sixteenth of six inches, or 0.375 inches.

Because of this inverse square law effect it becomes increasingly difficult to read the low end of any flow range, and flow ranges greater than about 4:1 are not really practical.
And it is why a sloping manometer is so very helpful by spreading out the scale as much as possible.

So basically you might have a whole series of orifice plates, each doubling in diameter, and each plate providing a 4:1 flow range to cover as wide a total flow range as you require.

If the low end of the range is too compressed to read accurately you have two choices.
Either reduce each flow range to 2:1 or 3:1 instead of 4:1
Or fit a much longer sloping manometer tube and scale, still with a six inch rise.

A six inch rise bench with 2:1 flow ranges might have a series of orifice plates as follows:
1.414 inch orifice 33.12 CFM
2.0 inch orifice 66.25 CFM
2.828 orifice 132.5 CFM
4.0 inch orifice 265 CFM
5.656 inch orifice 530 CFM

Okay before i get my head around what you just typed.. another quick question ... The car engines i work with specially is the toyota 4age 20v engines.. at this stage just doing inlet port testing or going to as soon as i understand how the flow-bench works..

1.. From the info online these heads flow around 170ish cfm stock..

2. From my last calculation of orifice diameter 2.0" we got 66.25cfm... so max flow through that orffice would be 66.25cfm.. and from what i see you guys stress alot the orifice in the bench depicts what the max flow that can be tested.. so this would mean i need to increase me test orifice size to something much bigger to read those figures? not worried about accuracy just the thought of flow testing 170cfm.. am i correct or am i missing something here?

You beat me to the draw.
Just added more info to my previous post while you posted.

Tony wrote:Yes, 66.25 CFM is correct.
And a larger diameter hole would increase the flow measurement range even more.

You can scale your sloping manometer to read any flow range by simply fitting an appropriate measurement orifice to the bench for the range required.
It is usual to have several different orifice plates, and a quick and simple means of swapping them over.

The next thing to understand is that the water rise is non linear with flow.

If you have a certain orifice size, and you double the airflow, the pressure drop increases four times.
Three times the flow creates nine times the pressure drop. Exact square law relationship.

Now looking at this the other way around.
If 6 inches on your manometer scale corresponds to 66.25 CFM
Half the flow (33.13 CFM) will only raise the water one quarter of six inches, or 1.5 inches.
One quarter the flow (16.56 CFM) only raises the water one sixteenth of six inches, or 0.375 inches.

Because of this inverse square law effect it becomes increasingly difficult to read the low end of any flow range, and flow ranges greater than about 4:1 are not really practical.
And it is why a sloping manometer is so very helpful by spreading out the scale as much as possible.

So basically you might have a whole series of orifice plates, each doubling in diameter, and each plate providing a 4:1 flow range to cover as wide a total flow range as you require.

If the low end of the range is too compressed to read accurately you have two choices.
Either reduce each flow range to 2:1 or 3:1 instead of 4:1
Or fit a much longer sloping manometer tube and scale, still with a six inch rise.

A six inch rise bench with 2:1 flow ranges might have a series of orifice plates as follows:
1.414 inch orifice 33.12 CFM
2.0 inch orifice 66.25 CFM
2.828 orifice 132.5 CFM
4.0 inch orifice 265 CFM
5.656 inch orifice 530 CFM

If i use : what u quoted 4.0inch orifice size you got 265cfm

this is formula we used before...

CFM = 13.55 x square root of (6" x 4.0" x 4.0")
CFM = 13.55 x square root of 96
CFM = 13.55 x 9.79
CFM = 132.76

How come you got 265cfm..?? i guess it deals with the 2:1 scale? if i understand correctly my scale is 1-3.. every 1inch in rise.. water moves 3inches on the scale? But if possible explain how you get to figure 265cfm?

Tony wrote:You beat me to the draw.
Just added more info to my previous post while you posted.

Hope you don't mind so many questions really appreciate the advise...

Yes you are right, I had a brain fart.
Relying on my fallible memory instead of checking it first.
2.0 inches 66.38 CFM
2.828 inches 93.8 CFM
4.0 inches 132.76 CFM

Have to go out now for about an hour....
But I will be back.
Ask away....