# How to measure flowrate using a differential pressure sensor

In summary, when building a digital peak flow meter to measure asthma, it is important to understand the relationship between fluid pressure and speed. One way to measure this is by using a differential pressure sensor and inducing an obstacle with a known pressure drop. Alternatively, a commercial flow sensor or an electronic anemometer could also be used.

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.

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?

Simon Bridge said:
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 isn't useful.

Differential pressure sensor.

Last edited by a moderator:
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?

Simon Bridge said:
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

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.

Dimi33
In order to use a differential pressure sensor to measure flow, you have to induce an obstacle of a known pressure drop per flow range and measure the differential pressure across this obstacle. For example, If you induce an obstacle within a pipe that creates a ΔP of 5mbar when 14 l/min pass through it, by using a differential pressure sensor, you will know when measuring 5mbar, that 14 l/min pass through the tube.

[Questionable link deleted by the Mentors]

Bernoulli’s Equation is on there, too.

I hope it helps!

Last edited by a moderator:
If you are building just one or a few you are likely better off buying a commercial flow sensor and interfacing it to a small computer. Even a single-board computer would be more than needed.

Here is one commercial unit that showed up on a Google search:
https://www.festo.com/us/en/p/flow-sensor-id_SFAH/?q=~:festoSortOrderScored

(above, and many more, found with:

Another possibility is look at how an electronic anemometer for wind speed works to see what you can come up with. There are those that count how fast a windmill is spinning and the 'hot wire' anemometer that sense the temperature change of a heated wire as air flows past it.

Here is a link to a 'mass air flow' sensor as used in medical equipment for monitoring the air flow in patient ventilators.
https://www.seeedstudio.com/blog/20...00-200-for-gases-medical-ventilator-solution/

That should keep you busy for a while!

Cheers,
Tom

## 1. How does a differential pressure sensor measure flowrate?

A differential pressure sensor measures flowrate by using two pressure sensors, one located before and one located after the flow restriction (such as a pipe or valve). The difference in pressure between the two sensors is used to calculate the flowrate using the Bernoulli's principle.

## 2. What is the principle behind using a differential pressure sensor to measure flowrate?

The principle behind using a differential pressure sensor to measure flowrate is based on the Bernoulli's principle, which states that the pressure of a fluid decreases as its velocity increases. By measuring the difference in pressure before and after a flow restriction, the flowrate can be calculated.

## 3. How accurate is a differential pressure sensor in measuring flowrate?

The accuracy of a differential pressure sensor in measuring flowrate depends on several factors including the quality of the sensor, the calibration method used, and the conditions of the fluid being measured. Generally, a well-calibrated differential pressure sensor can provide accurate measurements within a range of 1-2%.

## 4. Can a differential pressure sensor be used for all types of fluids?

While differential pressure sensors can be used for a wide range of fluids, they may not be suitable for all types. For example, highly viscous fluids or fluids with high solid content may not provide accurate measurements. It is important to check the compatibility of the sensor with the specific fluid being measured.

## 5. What are some common applications of differential pressure sensors for measuring flowrate?

Differential pressure sensors are commonly used in applications where accurate measurement of flowrate is critical, such as in industrial processes, water treatment plants, and HVAC systems. They are also used in research and development settings for measuring flowrate in laboratory experiments.