PTS Flowbench Forum

Lol no problems man just being double sure.. or i would confuse myself further..

So these orifice plates in this examples.
2.0 inches 66.38 CFM
2.828 inches 93.8 CFM
4.0 inches 132.76 CFM
would be the ones that go into the bench to depict max testable flow. of what ever i am going to test..

what test pressure do i use for them to flow those figures listed above eg:2.0" =66.38cfm, what will the vertical manometer need to read? 6-inches of wc?

next question :
a 6-inch orifice would flow 199.14cfm

So that's the orifice size i would need to use to test my cylinder head? which from the net is rumoured to flows around 170cfm am i correct?

another question.. what is the math involved from the incline manometer reading while testing, to the vertical manometer test pressure.. what is the math equation there to determine cfm flowed of the test object... if u can break it down for me would help understand abit better... use my bench as a example if so i can understand it better including the scale i have on my incline manometer..

As you already know, flow is an inverse square function of the water rise in the sloping manometer.
If you define the flow range as being 100% flow = 100% full scale manometer scale reading
Then 50% flow = 25% sloping manometer scale reading.
This will always be true on any flow range.

It is a convention to greatly simplify the whole flow bench reading process by fitting an inverse square law scale to the sloping manometer instead of having a linear pressure scale.
The same manometer inverse square scale works on any flow range.
So you run your test item, and the water rises up to the 73 % mark on your inverse square law scale.
If you know that 100 % is maybe 390 CFM (or whatever) then 73% of that is easily worked out as being 284.7 CFM.

You can work out the required scale as follows, where 100 % = 1.00 = 18.00 inches scale length
100% flow = 1.0 x 1.0 or 1.00 times 18" up from zero is full scale.
99% flow = 0.99 x 0.99 or .9801 times 18" or 17.6418" up from zero along the scale
98% flow = 0.98 x 0.98 or .9604 times 18" or 17.2872" up from zero
*
*
53% flow = 0.53 x 0.53 or .2809 times 18" or 5.0562" up from zero
*
25% flow = 0.25 x 0.25 or .0625 times 18" or 1.125" up from zero

Test pressure is completely independent of flow as far as the bench is concerned.
Generally you wind up your blower speed to provide some suitable test pressure across what you are testing. Twenty eight inches of test pressure is the industry standard for most things, but a small bench may have insufficient blower horsepower to reach that at very high flow rates.

Ten inches test pressure is another fairly frequently used standard.
For carb testing 1.5 and 3.0 inches of mercury are industry test pressures for four and two barrel quoted carb CFM.
You can actually use any test pressure you and your bench are comfortable with, and then convert the measured CFM to some other "theoretical " test pressure.
Again the inverse square law applies.
Twice the flow would require four times the test pressure.
You can convert the flow at one test pressure to what it would be at a different test pressure by using the pressure ratio between the two test pressures, and applying the inverse square law.

lovendhra wrote:Lol no problems man just being double sure.. or i would confuse myself further..

So these orifice plates in this examples.
2.0 inches 66.38 CFM
2.828 inches 93.8 CFM
4.0 inches 132.76 CFM
would be the ones that go into the bench to depict max testable flow. of what ever i am going to test..

what test pressure do i use for them to flow those figures listed above eg:2.0" =66.38cfm, what will the vertical manometer need to read? 6-inches of wc?

next question :
a 6-inch orifice would flow 199.14cfm

So that's the orifice size i would need to use to test my cylinder head? which from the net is rumoured to flows around 170cfm am i correct?

Could you answer the last bit there. about orifice ... while grasp rest of your last post..

Tony wrote:As you already know, flow is an inverse square function of the water rise in the sloping manometer.
If you define the flow range as being 100% flow = 100% full scale manometer scale reading
Then 50% flow = 25% sloping manometer scale reading.
This will always be true on any flow range.

It is a convention to greatly simplify the whole flow bench reading process by fitting an inverse square law scale to the sloping manometer instead of having a linear pressure scale.
The same manometer inverse square scale works on any flow range.
So you run your test item, and the water rises up to the 73 % mark on your inverse square law scale.
If you know that 100 % is maybe 390 CFM (or whatever) then 73% of that is easily worked out as being 284.7 CFM.

You can work out the required scale as follows, where 100 % = 1.00 = 18.00 inches scale length
100% flow = 1.0 x 1.0 or 1.00 times 18" up from zero is full scale.
99% flow = 0.99 x 0.99 or .9801 times 18" or 17.6418" up from zero along the scale
98% flow = 0.98 x 0.98 or .9604 times 18" or 17.2872" up from zero
*
*
53% flow = 0.53 x 0.53 or .2809 times 18" or 5.0562" up from zero
*
25% flow = 0.25 x 0.25 or .0625 times 18" or 1.125" up from zero

Test pressure is completely independent of flow as far as the bench is concerned.
Generally you wind up your blower speed to provide some suitable test pressure across what you are testing. Twenty eight inches of test pressure is the industry standard for most things, but a small bench may have insufficient blower horsepower to reach that at very high flow rates.

Ten inches test pressure is another fairly frequently used standard.
For carb testing 1.5 and 3.0 inches of mercury are industry test pressures for four and two barrel quoted carb CFM.
You can actually use any test pressure you and your bench are comfortable with, and then convert the measured CFM to some other "theoretical " test pressure.
Again the inverse square law applies.
Twice the flow would require four times the test pressure.
You can convert the flow at one test pressure to what it would be at a different test pressure by using the pressure ratio between the two test pressures, and applying the inverse square law.

Since my scale is 18inches long starts at 1 from the bottom to 6 on the top.. 100 % = 6.00 = 18.00 inches scale length?

Trying to absorb to much info at 1 go is a brain fry

Yes in "theory" a six inch orifice plate would be required to flow 199 CFM at only a six inch water rise.
But this is just not practical, for reasons I will now go into.

I wanted to try to keep all this as simple as possible for as long as possible to explain the concept of an orifice flow bench, using the bench you already have there that you can relate to.

One step at a time to digest....

A major decision to be made right at the start of building any orifice flow bench would be the design water pressure drop within the bench.
Your bench uses six inches, but usually a much higher pressure than that is chosen.
This effects the diameter of the orifice, and the rise of the sloping manometer, and also most importantly of all, the velocity of the air through the orifice, and the amount of blower power required.

Taking the last point first.
As you decide to operate at higher test pressures and higher orifice pressure drops, the air blower has to fight against a rising total pressure it has to pump against. This requires a LOT of electrical power which is almost always what limits each of us to what we can realistically build and run.
If your available mains supply is 20 Amps, 30 Amps, 80 Amps, that is all you have to work with.

The next important consideration is the air velocity through the measurement orifice.
If a large diameter orifice is running at a very low pressure drop, the air velocity through the orifice will be quite low. The problem with that, is that any slight up stream turbulence or instability in the moving air stream entering the orifice will effect the flow through the orifice in a major way.

A very small diameter orifice running at a much higher pressure drop, the air will be screaming through that hole with much violence and fury, and it will be much more immune to any slight up stream air turbulence created by the flow path within the bench.
A higher design water pressure drop is ALWAYS an advantage, particularly on the highest flow range, if you can manage to do it.
The PTS Forum flow bench has a design pressure drop of sixteen inches which has been found in practice to be a very good compromise.
This will require much smaller and much more realistic orifice diameters to run up to ranges reaching several hundred CFM with excellent accuracy.

The flow bench you have there would seem to have been designed to test very small engines probably with air from a single vacuum cleaner. The pressures are low, and the max possible airflow is also pretty low, but probably perfect for what the guy that originally built the bench was using it for.

A reasonable guide to thinking all this through might be to consider the entire air path from the test hole, through the entire flow bench including through the internals of the air blower, and the orifice plate.
If you decided that a 100mm diameter flow path was appropriate, its no good trying to pass all the air through a 25mm diameter hose at some point, everything along the flow path must be scaled appropriately.
The measurement orifice MUST be the greatest single constriction in the entire air flow path (apart from what is being flow tested) if everything is going to work well together.

So thinking in terms of a six inch measurement orifice is just not realistic unless the entire flow bench is being built to an enormous scale.

lovendhra wrote: Since my scale is 18inches long starts at 1 from the bottom to 6 on the top.. 100 % = 6.00 = 18.00 inches scale length?

Trying to absorb to much info at 1 go is a brain fry

Its definitely a fair mental leap to absorb all of this, we have all of us here been at the stage of mental confusion you are now at.
I am trying to feed it to you slowly, a bit at a time.
Stick with it.
A light will suddenly turn on in your head, and you will have an AHH!!! moment, got it !!

The water travels along the sloping manometer tube according to the differential pressure drop across the measurement orifice.
But airflow in CFM is not a linear function of pressure drop, or water travel, it is a sharply rising curve.

You can mark off your manometer scale in linear inches of water rise, as your manometer is marked, and calculate the inverse square law curve function with a calculator.

Or you can fit a non linear scale to the sloping manometer that already incorporates this non linear curve function to read directly in percentage flow, where 100% on this scale equals the flow rating of the orifice plate.

It just simplifies the whole process of reading a flow figure, by eliminating a lot of calculation.
You can do all the calculations once, and create for yourself a non linear flow percentage scale for your sloping manometer.
Or you do the calculation for every single reading you take for the rest of your life.

I just read this from the beginning a few times over.. honestly speaking the more i read it time after time the more the whole thing makes sense.. This bench would not be practical for my needs.. or the type of engines i work with... as i understand.. what confused me was i was comparing this to a pts bench lol.. not understanding the main factor is the pressure drop of the bench mine been 6inches and pts been 16inches.. why it confused me how come u guys 2inch plate flows 200cfm for exmaple.. were mine was flowing only 66cfm for example.. then if i got to the forumla we used CFM = 13.55 x square root of.. and in my case used 6inches of rise.. but pts style plate would be 16inches. think i am correct in saying that.. This bench as 2 motors on it.. but still i dout the blower power will be sufficient for my needs.. rather disappointing in a way since i do not test small engines at all.. It has however been a good learning curve.. i had 3 diffrent cylinder heads lined up for testing... most of them flow in the 150-250cfm regions.. and this bench will not suffice.. so what do i do from here Build a PTS Style Bench ??

Or am i jumping the gun

lovendhra wrote:I just read this from the beginning a few times over.. honestly speaking the more i read it time after time the more the whole thing makes sense.. This bench would not be practical for my needs.. or the type of engines i work with... as i understand.. what confused me was i was comparing this to a pts bench lol.. not understanding the main factor is the pressure drop of the bench mine been 6inches and pts been 16inches.. why it confused me how come u guys 2inch plate flows 200cfm for exmaple.. were mine was flowing only 66cfm for example.. then if i got to the forumla we used CFM = 13.55 x square root of.. and in my case used 6inches of rise.. but pts style plate would be 16inches. think i am correct in saying that.. This bench as 2 motors on it.. but still i dout the blower power will be sufficient for my needs.. rather disappointing in a way since i do not test small engines at all.. It has however been a good learning curve.. i had 3 diffrent cylinder heads lined up for testing... most of them flow in the 150-250cfm regions.. and this bench will not suffice.. so what do i do from here Build a PTS Style Bench ??

I think all of us here started out our air flow testing with something less than wonderful and no real knowledge of what we were trying to do.
For me it was a leaf blower... which promptly burned out its motor in protest.
I then built three more flow benches, each much better than the previous, this was all many years ago, long before this Forum even existed.

There was not a lot of readily available information around back then, and Bruce started this Forum because he too wanted to learn and share information with other people interested in low cost affordable do it yourself at home flow bench testing.

Over the years Forum members have helped each other, contributing ideas and knowledge, and the final result are the PTS Forum flow bench plans, which have slowly evolved, and is now at least as good, and probably a lot better than a commercial bench that ANYONE anywhere in the world can build themselves. No single person could have achieved what Forum members have achieved by working together and helping each other without any profit motive or any ideas of personal gain.

I honestly believe that you could not do any better than to obtain a set of Forum flow bench plans from Bruce, along with any of the parts you have difficulty in sourcing.