PTS Flowbench Forum

If you have not downloaded this spreadsheet yet you really need to. It does all the math for you

http://www.flowbenchtech.com/forum/view ... ?f=14&t=73

As Tony said we all didn't just one day wake up and understand all of this, it's been years and probably 4-5 forums now till we got to this point where we now have all the good "flowbench toys"

I by no means am the forum Guru, there is a wealth of flowbench knowledge to be found among the forum membership!

See thing is if was just messing around with this for novice reason then would be different.. In my case its actually something i want to work with and use in conjunction with my engine builds.. Maybe a set of plans from Bruce will give me a broader understanding.. aswell.. This thread is not over.. still more questions to ask.. Will do so tomorrow..

I wanted to keep it all as simple as possible.

Basic concepts and understanding are IMHO more important than precision at an early stage of understanding.
My rather simplistic formula may not offer the ultimate in precision, but it probably gives more of a realistic hands on feel than plugging numbers blindly into a spread sheet.

What you already have can very likely do what you require after a few modifications.
The first step would be to upgrade the air blower system and then work out what test pressure and orifice pressure drop and flow it will support, consistent with the type of testing you plan to do.

A suitable orifice and sloping manometer will then provide the measurement range you require.
Like most measurement systems, the tool needs to suit the job.

FWIW, that spreadsheet contains a detailed description of the math behind the the orifice calculations.

Tony wrote:I wanted to keep it all as simple as possible.

Basic concepts and understanding are IMHO more important than precision at an early stage of understanding.
My rather simplistic formula may not offer the ultimate in precision, but it probably gives more of a realistic hands on feel than plugging numbers blindly into a spread sheet.

What you already have can very likely do what you require after a few modifications.
The first step would be to upgrade the air blower system and then work out what test pressure and orifice pressure drop and flow it will support, consistent with the type of testing you plan to do.

A suitable orifice and sloping manometer will then provide the measurement range you require.
Like most measurement systems, the tool needs to suit the job.

Yup i appreciate learning it the old fashion way first. does give a better understanding. i will look at the spreed-sheet bruce mentioned..

It has 2 Vac motors what there capable of i am not sure do not have the spec for them.. .. how do i test this? range i require would be atleast 200cfm at the
most.. and from our calcs was a 6inch orffice @ 6inch but this was nor practical as we mentioned? how do i go about testing? i have a few of those cd type orifice.. 1.5inch to around 3inch or so...

Testing an unknown vacuum motor is fairly straightforward, all you need to do is fire it up with a series of orifice plates that will readily stick to the air intake by suction pressure alone.

You then measure the negative pressure at the eye of the spinning rotor (just down stream of the orifice plate) with a very small bore probe and a water manometer.
If the probe is fairly long and skinny, you can probe straight through the orifice hole, being very careful not to strike the spinning rotor.
If you measure the orifice pressure drop, and measure the orifice diameter, airflow can be calculated.

This does not need to be extremely accurate, but with a series of odd assorted hole sizes, you can plot some points on a graph of inches pressure versus CFM flow.
All you need are a few points, and you can then sketch in a fairly good flow curve.

The working pressure the vacuum motor has to work against in an actual working flow bench will be test pressure plus the pressure drop across the measurement orifice.
For a PTS bench, that would be 28 inches test pressure plus sixteen inches orifice pressure.
So we would be interested to discover the CFM the unknown blower under test can deliver at 44 inches of working pressure differential.

It is unlikely that any of your odd assortment of experimental test orifices produce an exact 44 inch pressure drop, but if you can plot a rough curve, CFM at 44 inches can be estimated fairly closely from nearby closest points on the curve.

The next thing to test would be the efficiency, or how many CFM per amp the vacuum motor can produce. Efficiency varies a lot between different vacuum motors, and under different points on the flow curve with a given motor.

Just load up the vacuum motor with a sliding plate across the air intake to 44 inches depression, and measure the motor amps.

Not sure where in the World you are located, but at 230/240 volts, most blowers would fall into a range of 8 CFM per amp to 15 CFM per amp.
At 110/120 volts it would be more like 4 CFM per amp to maybe 8 CFM per amp.

There seems to be no relationship between vacuum motor size and efficiency.
Some of the really small ones can end up being quite good, but not necessarily so.......
Efficiency is very important because in the end the maximum airflow your flow bench can reach will be limited by how much electrical power you have available.

This testing not difficult to do once you get set up and organised to test a whole bunch of different vacuum motors.
And it is fun, quite instructive, and well worth the effort.

I am from South Africa.. voltage is 240volts at my place.. its quite funny every night i go to bed with a fried brain... But every night i come back for more info...

Okay lets get started.. first of what is a bore probe? and how would i use it with a water manometer? abit more detail if possible on exactly what to do.. down stream of orifice would be threw the hole were the test orifice sits bellow that would be the vac motor.. try to get it as close to there as i can with this probe..get that bit, but not what a probe is or how to use it in conjunction with manometer..I do understand this method will allow me to calculate airflow .. if i know the pressure drop. and orifice diameter.. will this formula be the same we used before.. to calculate airflow threw and orifice? only difference is i would not use 6inches.. but whatever i test the drop to be via that probe method...

So i take all those cds i have with different size holes and use them? in this test id assume..

And in doing all this i need to test under combined 44inches... but how do i do this if my incline rise is only 6 inches.. and i dout i can get to 28-inches my vertical manometer reads up to 12scale.. =24inches wc... am i making sense here or am i going ofcourse..

With the vacuum motor running, and a suitable orifice plate stuck to the air intake by vacuum, you get a very thin hollow metal tube and connect that to a vertical manometer, and poke the end of the metal tube through the orifice hole into the vacuum motor air intake.

The air pressure there will be much lower, whatever depression the vacuum motor can pull through that particular sized orifice.

For a vertical manometer, all you need is a bucket full of water and a few metres of clear plastic hose. The water will be drawn up the hose by an amount equal to the suction depression across the orifice.

The water rise could be several feet, so the hose needs to come out of the bucket, up to maybe full ceiling height, then back down again. You just measure between the water level in the bucket and the height the water column rises in the hose.
Only the two relative heights matter, the hose does not need to be straight.

Hello everybody,

New memeber from Finland looking info using square edge orifice calibrating. I'm building PTS bench now and liked an idea that using square edge orifice I don't need to flip it around when change intake/ exhaust. Working also with round slider plate with lever or electric motor from old battery hand drill running threaded rod.

How thin material can be used to make square/ any edge orifice without risk bending/ vibrating? Other observations if using square edge? I'm going to use DM.

When orifice plate is top of the MDF plate does it have different CD in exhaust mode when orifice is " bottom" of the 5 inch opening?

How about thin both side beveled sharp edge?

I'll post some pictures when about to be ready

brgrds, Juhani

I think the best way for calibration is check weight method because you can easily adjust you scale by using that weight. Or you can get more information on manufacturer website because they also define the calibration process that is suitable for their scales.