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### Topic: Breath Pressure Sensor (Read 5861 times)previous topic - next topic

#### baum

##### Jun 22, 2011, 08:07 pm
I am looking for a sensor that can sense how hard the using is blowing into a pipe. I have seen some, but their max. pressure is ~16psi. But atmospheric pressure is about 14psi, leaving only 2psi to increase the pressure! Any product suggestions, or alternate routes that will accomplish the same things.

#### PaulS

#1
##### Jun 22, 2011, 09:22 pm
Quote
I have seen some, but their max. pressure is ~16psi. But atmospheric pressure is about 14psi, leaving only 2psi to increase the pressure!

Are you sure that isn't ~16 psi above atmospheric pressure?

#### baum

#2
##### Jun 22, 2011, 09:25 pm
It is above. but atmospheric pressure @ sea level = 14.7 psi. So if the max. measurable pressure is 16psi, I only have 1.3 psi to increase the pressure. That's not much.

#### baum

#3
##### Jun 22, 2011, 09:39 pm
Paul:

I am very sorry to have wasted your time with this silly question.

When reviewing the sensor I had questions about, I realized that it is not 16psi, but 75psi max.

Thanks!
baum

#### jackrae

#4
##### Jun 22, 2011, 09:43 pmLast Edit: Jun 22, 2011, 09:47 pm by jackrae Reason: 1
Commonly available pressure sensors are generally referenced to atmospheric pressure, which is 0psig or 14.7psia.  "g" stands for gauge (referenced to atmospheric pressure) and "a" stands for absolute (referenced to a total vacuum).   You can of course get ones referenced to absolute zero but these are generally limited to specialised industrial applications and are therefore quite expensive.

Therefore if yours is capable of going to 16 PSI then that'll be 16 above atmospheric  16psig or 30.7psia

Putting it into context, blowing to 16psi above atmospheric is about, or above, the limit of the average human.

Edit:  Whilst writing the above your note re the actual sensor came in.  This is a differential sensor so with the LP port left open to atmosphere the measured range is 0-75psig.   If you connected the LP port to an absolute vacuum, the range would still be 75psi but the measured range would be 0-75psia or -14.7 to +60.3psig

#### baum

#5
##### Jun 22, 2011, 09:48 pm
ok...
so the one i linked above is the mpx5500 (72.5psi max, this one is differential (what's that?))
the one i looked at earlier is the mpx4115 (16.2psi max, this one is guage)

which one should i get? '4115 looks like it means 16.2psia, right?

#### jackrae

#6
##### Jun 22, 2011, 09:55 pm
The 4115 is an absolute type sensor so is unsuitable for your application

Go for the 5500.  The term differential means that the sense element looks at the difference between the HP and LP ports (there are two to see in the photo)  As stated above, leave the LP one disconnected and connect your blow-tube to the HP port.

#### baum

#7
##### Jun 22, 2011, 10:03 pm
Or...

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MPX5700GP-ND

it is gauge, same price, package, pinout, etc.

#### jackrae

#8
##### Jun 22, 2011, 11:37 pm
The human lung can only produce about 1 to 2 psi so if you use a 100psi sensor you'll only be using around 2% of its range.  This means you'll only get around 2% of the 5 volt output range (0.1 volts).  This will severely limit the resolution of your intended application.

May I suggest you conduct an experiment by making a u-tube out of 1/4" bore clear plastic tube with each side around 4 feet long.  Fill with water so that each side contains around 2 feet.  Now blow down one side as hard as you can and get a colleague to note how high the water moves up one side.  The pressure you blow is the difference between the water levels in each side of the tubes.  If you're lucky you might get a difference of around 1.5 feet.   To convert this to psi,  2.3 feet of water equates to 1 psi.  You will be surprised at high feeble your lungs are.

Select a device that has a lower range, say 0-5psig or 0-5psi differential (often referred to as DP)

#### baum

#9
##### Jun 23, 2011, 12:00 am
Good point. The lowest range MXP series sensor was 1.5 psig, but that is too low.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MPX5100GP-ND
?

It is 0-14.5psig.

Thanks!
baum

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