how it works

[Jason]

Ok, I just watched a video on YouTube, and I have a question about the "inches of water". They were testing a BBC head at 28" of water. The manometer showed about 70% of flow. Where does the water in the manometer come from? Is it basically the air that can't flow through the port that makes that rise?

[86rocco1]

The manometer is just a very simple pressure measuring device.A U-tube manometer is the simplest type, it's just a U shaped piece of tube which has been half filled with water, one end of the tube is open to the atmosphere and the other end of the tube is connected to your pressure source it cause the level of the water in the tube to change and 28" inches of water means that the difference in the water level between the two legs of the U tube is 28".

[larrycavan]

In an orifice bench arrangement, [SF300, 600 BUT NOT SF110] generally you'll have 2 manometers and two plenums.

The orifice sits in between the two plenums.

When the bench is running, you are measuring two distinct pressure differentials.

The test pressure differential & the plenum pressure differentials.

The test pressure manometer is connected to the top plenum and to atmosphere. It's generally scaled in inches. It reads and functions exactly like Ed explained above.

The inclined or % FLOW manometer is measuring the pressure differentials between the top plenum and the lower plenum. It's reading is also, behind the scenes, in inches of water. The scale has is has been redone to show a percentage rather than inches of water column. This is done to allow for much easier calculation of the flow to CFM.

It reads % of a flow range. For example 70% of 300CFM.

[Otto]

the 28" is the test pressure the question would be 70% of what range? maybe these would help you understand how the sytem works
and then this

boy these guys are fast responders

[Jason]

Thanks guys for your responses, and thank you all for your help in getting me up to speed, I really appreciate it.