Measuring water pressure differential in a pvc pipe

mancherje

I'm having problems with our pressure measurements on a pretty basic PVC water flow system. I drew up the general design in paint and attached it so you can have a visual of what i'm talking about. The second attachment is a picture of the actual pipe design.

We're controlling the flow rate into our pipe with variable height of a tall water reservoir at the inlet of the system. The outlet just pours into a bucket. The system is 1/2'' diameter pvc pipe all the way through, but in the middle is a 1/4'' elastic / collapsible tube. with the right flow rate we can induce partial collapse or flow oscillations in the tube which should create a pressure differential across the system. The pressure gauges are in PSI and measure 0-15PSI with 1/4 PSI increments

The problem we have is we cannot get the pressure gauges to show any measurement at all, they just sit on 0 the whole time. We had 100PSI gauges with 5psi increments and we figured those weren't sensitive enough. The lowest ones we could find in a store nearby are the 15PSI gauges we have now.

Do you think the pressure measurement part of our design is flawed? or do you think we should order more sensitive pressure gauges, maybe in mmHg?

Another thing that may be causing the problem is the pressure gauges don't sit 100% vertical, they are maybe ~50degrees up from horizontal due to steric issues, we can adjust the pipe set up if that's the problem.

thank you for the help!

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AlephZero

For measuring a static water pressure, 1/4 psi corresponds to about 6 inches of water gauge. So if you block the outlet of your pipe, you should be able to see a small pressure reading on the gauges depending on the height of water in your "tall water reservoir".

But when there is a flow through the system, that static pressure is being "used up" accelerating the water from rest in the reservoir to some velocity in the pipes (review Bernouilli's equation!) and the only dynamic pressure change between your gauges wlll be flow resistance of the pipes, which will be tiny for a low velocity flow in such a short pipe.

The dynamic pressure in the central narrow pipe will be reduced (Bermouilli's equation again!) which is probably why the pipe can collapse - the atmospheric pressure is higher than the water pressure inside.

You might be better putting some strain gauges on the central pipes to measure directly what the pipe is doing. With a data logger (or a computer interface) that will also give you time dependent measurements of oscillations in the flow with much better response than dial gauges would give, even if they were measuring something.