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Author Topic: Your thoughts on calibrating mq-3 alcohol sensor  (Read 1295 times)
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Haverhill, Ma
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I just ordered an mq-3 alcohol sensor from sparkfun. Here's the sensor https://www.sparkfun.com/products/8880 and here's it's datasheet http://www.sparkfun.com/datasheets/Sensors/MQ-3.pdf. I would like to make a breathalyzer that is somewhat accurate. Btw, I won't be using this to judge whether or not I can drive safely. I'm just looking for more accuracy than "buzzed", "drunk", and "dead".

To calibrate this sensor, they recommend using .4mg/L (200ppm) alcohol concentration, 21% Oxygen concentration, 65% relative humidity, at 20 Celsius.

I'm pretty sure I can create the above environment by evaporating .5ml of alcohol in a 1 liter bottle of ambient air.

Since I'm using this sensor as a breathalyzer, should I use ambient air or air from my lungs (15% Oxygen, 90% humidity at 36 Celsius) in the 1 liter bottle for calibration?

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my thoughts:

Quote
they recommend using .4mg/L (200ppm) alcohol concentration, 21% Oxygen concentration, 65% relative humidity, at 20 Celsius.
so use that for calibration.

The datasheet has some graphs to see how to compensate for temperature differences :

36C => 10% diff
85% RH => another 10% off

so exhaled air has a "read error" to compensate of ~20%.

If you are a large person you might also need to compensate for the litres blood. 70KG person has 5.5 litre blood.
from - http://wiki.answers.com/Q/How_much_blood_does_each_person_have - As a general rule blood actually constitutes about 7-8 % of body weight.

cheers!
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Rob Tillaart

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This is an introduction to the MQ gas sensors : http://playground.arduino.cc/Main/MQGasSensors

I think you need a calibrated device to compare your readings.
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Haverhill, Ma
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Thanks for the replies. Still not sure what I'm going to do though. 20% error margin seems a bit too much to be concidered accurate, and I don't have a calibrated unit for comparasion.
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Your calibration proposal in the first post is not that bad.
Alternative. Buy a bottle of whiskey, go to the police station and ask their device to help you calibrate yours. First test both sober, then one glass , measure both, etc.
Do not forget to take notes :-)
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Rob Tillaart

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Haverhill, Ma
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I was thinking of doing that. Unfortunately, the local law enforcement around here aren't too friendly. I contacted them before to get some info for my police scanner and they made it clear that they had better things to do. I may start asking around and find a friend who knows someone in law enforcement. That may be my way to compare the results.
 
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