Yes, I see the confusion. I would like to measure volumetric flow rate; volume (liters) over time (minutes). The end result is to measure a persons exhaled air flow rate.
I thought I could use a surrogate measurement and conversion. For example if someone blows a pinwheel, I could take the number of revolutions over time and convert. Measuring revolutions with a magnet?
Does this seem like a good idea? What equipment can help me accomplish this for a low cost? Are there any better ideas?
The person can use a SCUBA regulator as a starting apparatus. Then the exhaled air is going through a pipe. Otherwise it is difficult to get a person's air to be confined for measurement. Alternatively :
1 Balloon
2 Radioactive Xenon gas
3 Charged dust in the air
4 Flames containing ionized iodine
5 Vaporized iron gas exhaled, deposited on cryogenically cooled tungsten plate with seed hematite crystals.
Maybe you can clarify for me, what exactly you want to do, and is this a class project, hobby project, medical research, or what.
Do you plan to monitor a persons breathing while they sleep?
Will you want them to take a deep breath, then blow as hard/fast as they can for one second?
Those ideas seem overly complicated (or way over my head). I am thinking about a small tube (between straw and pvc size) with a spinning turbine. One alternative, I believe thermistors could be used to measure flow rate?
UPDATE: Jack hit it on the nose - medical hobby project. The person will take a deep breath, blow as hard as they can and measure the peak flow.
What you're trying to make is called a "spirometer". With enough known values & proper math...you should be able to design one using any small flow sensor, or even a pressure sensor.
Thanks for the clarification. That brings to mind the toys at a new years eve party. You blow on them, and they unroll (like a sleeping bag). If you don't blow hard they only unroll halfway.
You may be able to get your readings by measuring pressure. One pressure sensor that can be used with arduino is the BMP180 used to measure pressure. It may or may not be of help to your project.
Are you sure you need a flow rate, or just pressure.
Since you are taking the sample over just about one second, through a resistive pipe/straw, flow rate may be skewed.
filo47:
UPDATE: Jack hit it on the nose - medical hobby project. The person will take a deep breath, blow as hard as they can and measure the peak flow.
Probably the easiest solution to make reliable would be to measure the static pressure drop in a venturi. You could size the venturi to suit your accuracy requirements and the resolution of your pressure sensor.
Jack. I just need flow rate. Pressure would only be a means to an end. The tubes that are used for spirometry typically have a diameter of only a few inches.
I had not considered (remembered) the venturi effect. Would that mean multiple pressure sensors?
What kind of reading, result is to be produced? Something like " 5 liters per minute " ? and you will calculate that from just one reading (about a second), or you plan to do the measurements for a full minute ?
I would like to measure flow rate from the beginning of the exhalation until the end (maybe 5 seconds). Producing a graph of liters per minute vs seconds. From that data I can make calculations. For example, the peak flow rate.
Ok, sounds like you want to sample many times during that (5 second period), maybe 100 times a second?
Then do math on those (500 samples), like min(), max(), avg(), and whatever.
You may want to sense, and start your counting/sampling when a minimal value is attained, and tell them not to stop till they hear the buzzer (5 seconds, or whatever you decide).
I am beginning to see how that could be useful.