# Relationship between CFM vs PSI

xr3tf
I have been "googling" to find the relationship between the above, and the answer has mostly been a resounding "NO".

Maybe my question is not "CFM" vs "PSI", but that is the only way I know how to descibe my question. I will explain the scenario:

I have air flowing at a constant speed (over the body of a car) which is being "collected" with a scoop and then channelled into a cavity with an open end, so the air flows through it. At the back of the cavity, I measure the "pressure" of the air by means of a Magnehelic meter which gives pressure in "inches of water" as the air passes over / through my measuring point back to the athmosphere.

If I increase the collection area of the scoop, I get more air being collected still at the same constant speed and the pressure measured increases as more air flows through the collection point.

My question is this: If the pressure as measured at the back of the cavity increases by 20%, what relationship does the "volume of air" collected by the two different scoops have to this 20% increase in velocity pressure? Is this a 1:1 relationship, or is it something different?

I am trying to equate that a 20% pressure increase as measured by my meter is as a result of a x% increased in air flow.

Gold Member
Your meter is screwed up if this is happening. A pressure probe should not depend on the volume of air it is sampling.

You can go from pressure to volumetric flow rate in certain situations, but not without working probes.

xr3tf

My meter is unfortunately not faulty.

Maybe I did not explain my scenario enough. With the amount of air being "scooped in" and being directed through an area that is smaller than the area if the scoop, ie a restriction, surely there must be some kind of pressure build-up at the exit of this restriction? Air speed through the smaller area probably increases which then leads to a lower pressure along the inside of the restriction etc, but that is not what the meter tells me at the exit.

I am not a mechanical engineer by any means - I simply have a problem that that should have a logical solution (I hope)

Gold Member
The ratio of that intake area to outlet area definitely makes a difference assuming there are no processes going on in between them such as combustion. Generally, as inlet area/outlet area gets larger, the pressure you measure should go up.

If you measure the pressure at the inlet and the outlet, you will have a pressure differential that can be used with Bernoulli's equation which relates pressure and velocity. You then have two unknowns, inlet and outlet velocity. You can relate those two using conservation of mass which says Velocity*Area is constant from one point to another in the flow (in certain cases, this being one of them).

There you have two equations and two unknowns. It is fairly simple to solve. You can then easily get volumetric flow rate from that outlet velocity and the area, which you know.

Mentor
You're looking for Bernoulli's principle (and your logic is pretty close to correct): http://en.wikipedia.org/wiki/Bernoulli's_principle

In short, the total pressure of your airstream doesn't depend on the geometry of the duct, but a restriction will cause the velocity pressure to rise and the static pressure to fall.

static pressure + velocity pressure = total pressure

So which are you measuring? What does the pressure gauge installation look like?

xr3tf
Having thought about this, I probably need a very sensitive flow meter to determine whether I have increased airflow with the bigger scoop.

I have fabricated a "matrix" of collection points that cover the largest portion of the cross section of the duct, in order to reduce eratic readings on the pressure gauge due to turbulance as the air enters the duct. The matrix allows prssures to be measured at the collection points, whilst air also flow through open areas in the matrix.

I use a Dwyer Magnehelic gauge for my measurement.

I tried using a brushless DC motor that was mounted in the airstream before as a means of measurement - I then measured the output voltage generated by the motor to compare, but it use to overspin very quickly once the scoop area was increased.