Atmospheric conditions - Do they affect flow on my orifice bench?

[Mouse]

Still having trouble understanding the difference between an orifice and a Pitot tube in a ratiometric setup. When I look at a flow rate calcultor for an orifice or a Pitot tube, they both ask for the air density. I also notice that the air density correction factor is roughly the same for both devices.

I just don't understand why a Pitot tube would not see the same ratiometric differences as an orifice.

[Tom]

Hello again,

I am glad that someone understood what I try to say.

Is here anyone who has a superflow and can tell me what kind of info you got with the certificated plate ?

@Mouse
Who says there is a difference ?


Thanks
Tom

[gofaster]

Tom,
When my turn comes I can PM you with the readings from my SF-110. That will give you a measure for a motor center bench as corrected through my FlowCom and PT software. One of the other members had mentioned having access to a SF-600 that he was comparing to his homebuilt bench results. I don't remember who that was, but hopefully he'll see this and post up. My new bench may not be done before the plates get here.

[Tony]

Ah ! The penny has dropped. I think I now finally understand what is really being discussed here. Sorry, I am a bit slow sometimes guys !!

As I understand it, a ratiometric orifice flow bench will always come up the same CFM figure for the same test object no matter what the atmospheric conditions. That is an excellent way to test, but it may not really reflect the true real CFM airflow on the day.

It is exactly like measuring the CFM through a running engine on a dyno. Colder air will flow more mass, and produce more power. And it may also do something similar on a flow bench. It will flow different masses of real air under different ambient conditions.

So what exactly does that tell us ?

I would much rather know that a certain port configuration measures as a very repeatable exact 100 CFM on my bench every time I tested it.

It may actually flow 98 CFM one day, and 102 CFM the next because the weather has changed, but how does knowing that actually help us ?

Surely the whole aim of a flow bench is to come up with a figure of merit for a test object that is repeatable and easy to measure with a minimum of introduced errors.

[Thomas Vaught]

Most people are not on the board to race flow benches, IMO. They are there to learn about flow benches, what type of bench best fits their needs and budgets, and can they use it to go faster.

I personally have built laminar, pitot, orifice, and mass air benches, and attempted to calibrate a SF turbine flow meter one time. Each has its strong and weak points.

The fun is learning about each one from this forum.

Tom V.

[Mouse]

Pitot tubes and orifices both fall into the class of devices called "pressure differential producers". They just measure pressure in a slightly different way. But none the less, they are both affected by the density of the gas pressure they are measuring.

Pitot tubes are associated with measuring air velocity on air craft and race cars. That is why you will always be told that the air density must be used with a Pitot tube. These are not ratiometric uses. In a ratiometric flow bench, Pitot tubes are no different than an orifice....you can calculate the actual air flow volume if you like, or simply rely on standard density calibration as usual.

Pitot tube calculation for standard density is not too complex:

Velocity FPM = 1096 * SQRT (Differential pressure/.075)

CFM = Velocity * Area of tube that the Pitot tube is located

For actual cfm replace the .075 with the correct density.

Actual flow rate is great for plotting fuel curves on carbs and EFI units, but is not any good for flow rate comparisons.

[Tom]

[color=#000000]Good morning @all,

@ gofaster
Thank you very much for your offer but I am not located in the US.
You stated that you have a SF 110 what kind of information / certificate you have with your calibration plate (orifice) ? Could you tell me that or scan it and bring it up here ?

We all have a good flow bench and we all know how to work with them and get the most potential out of our components.
As long as we are comparing, flowing

[maxracesoftware]