How do peak flow meters measure flow rate?

[areamer02]

TL;DR Summary

Peak flow meters are simple mechanical devices that measure flow rate without an electronic gauge. How do these meters get a L/min measurement without directly measuring volume or time?

I am in pharmacy school and my professor recently went over lung diseases with us. An important (and cheap) tool that many patients can use to detect whether their lung function is declining is a peak flow meter, which tells the patient how much air they are moving when they exhale with as much force as possible. My professor mentioned that he has no idea how this device measures L/min without having a timer or anything to measure volume on it. The meters are often entirely mechanical, with no electronic component. I assumed that it is calculated by using a spring to detect the amount of force or pressure being applied within the meter and then using some formula to calculate flow from force/pressure. However, I have looked up what I can and have been unable to find a formula that I think would work in this scenario. So, my question is how can these meters calculate air flow without any electronics? Is there a simple (or maybe complicated) formula to derive air flow (L/min) from force (kg*m/s^2) in this circumstance?

 

[Tom.G]

The one test I had decades ago was a tube about 1.5in. (4cm.) diameter and perhaps 4in. (10cm.) long with a slit along part of its length. There was free-floating piston in it that was initially set at the mouthpiece.

When you blow into it, the piston is moved along the tube, exposing more of the slit for air to escape. The greater the air flow rate, the higher the pressure, causing the piston to move further and uncovering more of the slit.

Ahh! wikipedia to the rescue:
More info and photos here:
https://en.wikipedia.org/wiki/Peak_expiratory_flow
Cheers,
Tom

 

[anorlunda]

Summary: Peak flow meters are simple mechanical devices that measure flow rate without an electronic gauge. How do these meters get a L/min measurement without directly measuring volume or time?

and then using some formula to calculate flow from force/pressure.

You don't always need a formula. You can just calibrate it with a series of measurements.