Differental pressure range

[blown928]

I've been reading everything I can here, and starting to get a handle on things (I think). I still am not sure which style bench I'm going to build, but while thinking about it I remembered my workbench in the garage is built on this hefty old Probot frame:




This is going to be the base of my bench. I like Tony's KISS bench, 2 holes, one blowing and one sucking, with motor speed control for depression and digital pressure readouts. I just have to figure out how to get that into this frame

As for the digital pressure readout, I'm going to roll my own. I'm building the signal condition/amplifiers and thinking about feeding the analog voltages into a that I have.

I'm going to use a range of 0" to 40" for the static pressure reading, but what will I see for a range on the differential pressure? 0 to 1"? 4"? 0.25"? Does it depend on the type of bench? What is a 'normal' range here?

Bill
(why be normal?)

[86rocco]

That depends on the construction of your bench. I'll speak only about orifice type flowbenches since that's what I'm most familar with. The differential pressure changes for a given CFM air flow varies with orifice size. So, the differential pressure you'll see is completely under your control by selecting orifice size, and because of that, the choice of incline for your manometer is somewhat arbitrary. 6" and 12" inclines are common mainly because there are good quality commercial manometers available in these ranges.

Download the spreadsheet in and play around with the numbers a bit, pick an arbitrary pressure difference for you manometer height and change the orifice size see how the CFM's are effected, then keep the orifice size fixed and see how varying the pressure difference effects things. This way you'll get a feeling for what orifice sizes will work best for the type of testing you'll be doing.

[wing64k]

Hey guys,I didn't want to start a new post,so i"m asking this question in here,and I know this has been discussed alot but it's still not clear to me,so don't laugh to hard,can someone explain coefficent discharge(Cd)why we use it and the formula for it and if it is needed for a pitot style bench?This subject for some reason goes right over my head.

Thanks Dean

[86rocco]

The co-effiecient of discharge (Cd)is strictly related to orifices. It's measurement (or estimation) of how efficient a particular orifice is at allowing are to pass through it. Knowing the Cd allows us to calculate the flow based on the area of the orifice and pressure different across the orifice. Most commonly a sharp edge round orifice is used because they're relatively easy to construct and because the Cd of these orifices are know to fall within a fairly narrow range around 0.62. There are other shapes which are considerably more efficient but the sharp edge ones are the easiest to accurately describe and construct.