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Speed With Pressure Sensor Question

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Speed With Pressure Sensor Question

Postby Ramius-II » Wed Sep 11, 2013 12:02 pm

Hi All!
Okay so I have a small pressure sensor (MS-5541) and I wish to make it a little more sensitive when measuring water pressure when the sensor is moving through the water. Currently the tube in front of the sensor is about 1/8" in diameter. One thought I had was to use a larger tube (3/8") that narrows down to the size of the sensor (1/8"). The delima is, is it better to do a small gradual tapered down angle of say 15 degrees or a larger taper of say 30 degrees?

Thanks, Ed
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Re: Speed With Pressure Sensor Question

Postby KevinMc » Wed Sep 11, 2013 6:34 pm

Since there's no flow in the pitot tube (or at least there shouldn't be) a step or taper will not make a difference to the pressure reading at the narrow end. Unfortunately, I don't believe increasing the tube diameter will make your system any more sensitive. Remember that what you're measuring is force per unit area (PSI, or some other measure) so what you measure at the narrow end of the tube will not be affected by the larger diameter at the pickup end either.
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Re: Speed With Pressure Sensor Question

Postby Ramius-II » Wed Sep 11, 2013 9:14 pm

Hi Kevin,
Thanks for the input. I was thinking of this more like a venturi than a pitot or it does not make any difference?

Ed
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Re: Speed With Pressure Sensor Question

Postby KevinMc » Thu Sep 12, 2013 7:55 pm

Ramius-II wrote:I was thinking of this more like a venturi than a pitot or it does not make any difference?

It's all about the geometry of your pick-up.

If your pressure sensor connects to a tube that sticks straight out of the front of your boat and has negligible flow within that tube you have a pitot and my original answer stands. (This arrangement will see a pressure increase as speed builds.)

If on the other hand you've got a device that the water flows through and you're measuring pressure off the sidewall of that flow-through tube, then you have a venturi and smooth tapers are what you need. (This arrangement will see a pressure decrease as speed builds.) That said, the only place I can think of that I've seen that kind of structure is the vacuum source on an aircraft. (For running vacuum driven instruments, not measuring speed.) See below.

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Re: Speed With Pressure Sensor Question

Postby Ramius-II » Thu Sep 12, 2013 8:59 pm

Got it!
It just "seemed" that if the pickup tube was 3x the sensor's diameter then the pressure would increase.

Thanks, Ed!
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