# How to measure flowrate using a differential pressure sensor

## Main Question or Discussion Point

I have to build a digital peak flow meter. I ve read that you can measure the volume metric flow rate using a differential pressure sensor. H ow do i go about doing so. The peak flow meter will be used to measure asthma.

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Simon Bridge
Homework Helper
1st work out how the pressure sensor works.
This should let you know the relationship between what it says and the flow rate - which will also depend on the geometry of the device.
Do you know the relationship between fluid pressure and it's speed?

1st work out how the pressure sensor works.
This should let you know the relationship between what it says and the flow rate - which will also depend on the geometry of the device.
Do you know the relationship between fluid pressure and it's speed?
Initially i thought of just measuring the pressure at one of the inlet ( using it as a gauge pressure sensor). Hence, i would get the pressure readings and then just graph them. But it does not tell me the volume metric flow, and that data probably isnt useful.

Differential pressure sensor.

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Simon Bridge
Homework Helper
If you know the gauge-pressure at a position in some flow, that can be useful for telling you the speed of the flow... provided you know the relationship between pressure and air speed. You usually also need other information like the geometry of whatever the air is flowing through.

So:
Do you know how pressure relates to air speed?
Do you know how air-speed relates to volume flow?

If you know the gauge-pressure at a position in some flow, that can be useful for telling you the speed of the flow... provided you know the relationship between pressure and air speed. You usually also need other information like the geometry of whatever the air is flowing through.

So:
Do you know how pressure relates to air speed?
Do you know how air-speed relates to volume flow?
I believe flow rate = cross sectional area x fluid velocity

Im not sure about the pressure vs velocity

pressure = force/area
force= mass x acceleration
velocity = integral of acceleration

so pressure= ( mass x velocity x time) / area

Simon Bridge
Homework Helper
Flow rate: well done.

Pressure/velocity: nice reasoning, but that would be (loosely) the pressure exerted by a jet of air directed at a surface on that surface
You need the pressure of the air going past a surface. fast air has low pressure.

... look up "bournoulli's equation".

You could probably work out the pressure-speed relationship for your design empirically.

Have you thought of using a Pitot tube or an orifice? How critical is the pressure drop caused my the measurement device? I would imagine if you are measuring pulmonary air flow, the lower delta p the very better.