I'm trying to figure up some orifices.Gonna be sharp edged from 2mm aluminum.
Not sure what to enter into the "Pressure Difference WC inches" input.
Manometer looks like this.
A foot note on the spreadsheet states, "Pressure Difference will be your intended 100% or vertical rise on the Inclined manometer".My intended 100% is 24" or 60cm,while my vertical rise is 12". Depending on which value I enter, 24" or 12",I get different CFM data.
I'm the author of that spreadsheet. First go to the inclined scale sheet, plug in the numbers for your inclined manometer, the tube inside diameter, well I.D. etc. make note of the value of Δp at 100%, use that value for the pressure difference in the orifice calculation sheet.
Ed
A side note to Linux users, and others that don't have MS Excel: I've tested the spreadsheet on Openoffice, and everything works properly so the spreadsheet can be available to you as well.
That also answers my question, somewhat,on what the Delta p column was for.
I corrected some variables for my manometer setup on the scale sheet.@100% = 24" (delta s) = 23,79" (delta p).
I have some digital manometers on the way.When I try them out, and measure across the orifice, will it be showing 23,79" @ 100%?
What test pressure is this all calculated at?I've been told that it doesn't matter,but the charts I've looked at on "CFM vacuum flow through an orifice all show the test pressure indeed make a difference.Granted some charts are in psi and others in Hg.
I somehow think inches water will also be pressure dependent.
Orifices are rated as xxx cfm @ yy" so let's say you bought an orifice from Bruce that's listed as flowing 250 cfm at 16" but if your inclined manometer goes up to something other than 16" so you need a way of calculating what air flow rate 100% on your manometer correlates to, in order to do that, you need to know what the delta p at 100% is. The delta p is also there for the benefit of those that have digital manometers as a point of comparison.
The airflow through an orifice (or any other restriction) is a function two basic things, the actual physical properties of the orifice and the pressure difference across the orifice so if you know the properties of the orifice, you can calculate the flow.
In an orifice type flowbench, there are always two separate pressure differences we're concerned with, the one is the pressure difference across the part we are testing, this is usually called the test pressure and in most cases we deliberately control the test pressure. The other pressure difference is the difference across the orifice. It seems to be very common among people just getting started with a flowbench to get these two pressures confused, they are separate but related entities and how they are related is at the heart of how a flowbench works. An orifice type flowbench works is by comparing the airflow through an unknown item (the part we are testing) to the flow through a well known item i.e. the orifice. By applying a known pressure difference (test pressure) across the part you are testing (usually 28") then, allowing the same parcel of air to also flow through the orifice and measure how much of a pressure difference that amount of airflow creates across the orifice and because we know the properties of the orifice we can calculate the airflow and because it's same air that flowed through our part, we then also know how much air flowed through our part.
