As in this example above lets look at the flow of air the restrictions and pressure points along the way
The air from the vacuum source creates a depression on the D side of the orifice plate air begins to be drawn through the orifice and begins to create another depression on the C side of the orifice plate as the air move further it creates a depression in the B chamber of the test & settling chamber.
Now if the test point is completely closed (Blocked off) the depression measured at A-B would be whatever the vacuum source is capable of pulling at the current settings and the pressure drop across C-D would be 0 thus the incline would read 0 flow (This is because both C & D are reading the same pressure on both ports at both sides of the orifice plate).
Now this is where it gets a little confusing as when we open the valve on the test point 1 we begin to leak air into the settling chamber and air begins to move through the system (picture this opening of your test point valve as leak.
The air leak at the test point causes the pressure to drop in chambers B & C, as air flows through the orifice plate. Lets say we are trying to test are part at 28” of depression the now open test point has cause the A-B scale to drop (the leak) so we must add more pull from the vacuum source thus creating a bigger pressure drop along the way “the air begins to flow through the system”.
Since now the pressure at point D is lower (More vacuum) than B-C (test side where air is leaking in through the valve) the incline (Delta-P) manometer begins to move showing this “Pressure Differential”. The math tells us that for a given orifice size a pressure difference across it the orifice would be flowing a specific amount of air.
Confusing but this is the basics of the orifice bench. Now as you open your test point more and more you have to apply more vacuum to maintain your test pressure and the greater the vacuum needed to pull the air through the orifice thus creating a grater pressure differential “Delta-P”. This causing the incline manometer to rise farther up the scale and again the math will show you are flowing more air.
The first key is the orifice plate is designed to work at a given Delta-P for the PTS DM bench 16” thus a 200cfm orifice plate will have a pressure differential across it of 16 inches when it is flowing 100% or 200cfm. If we were to do some simple math then if my test pressure is 28” and my Delta-P is 16” then at point D to atmosphere we should be able to measure very close to 28 + 16 or 44” of vacuum. This explains how the orifice Delta-P effect the usable power of the bench, if the orifice and instrumentation were set to read say a 6” Delta-P your vacuum needs would only be 34” so from a design perspective you might be able to use less motors.
Now you may ask why is the PTS DM set to use a 16” Delta-P this is for two specific reasons. At the time of design the smallest sensor available was a 16” sensor. And two combined with the electronics inside this sensor provides the highest level of accuracy across the digital range. Our sensor manufacturer has since come out with smaller sensors and we are looking at building prototypes of the PTS DM that would use a Delta-P of say 6” for smaller benches but again this may not be as sensitive as the 16” sensor application.
Ratio metric! What does this mean and how does it apply to the PTS Bench? This means that the basic operation of this particular style of flow bench uses a confined pressure differential of same air to determine flow. In other words the air Density is the same on both sides of the orifice eliminating the need for adjustments due to temperature, barometric pressure and humidity.
Though the PTS DM does use these figures in the configuration settings they would only be changed if you were making drastic changes in any specific area for test purposes. Example you wanted to see if your bench flowed the same in Denver as it does at home in Ocean City Maryland. The discussion of the effects of temperature and barometric pressure came about because of the setup and configuration of the SuperFlow SF 1XX series of benches. These benches do not use a “Confined Orifice” there orifice has one side facing the atmosphere so flow would follow a different path “Test piece > Vacuum Motor > chamber > orifice > atmosphere” thus the air flowing through the motor is heated versus atmosphere. In these benches the ambient temp & humidity along with the bench internal temp all play a role in the formula to determine Flow.
(Thanks Rick (1960FL) for taking the time to put this together!)