Some students of mine made a methane sensor with the mq 4. THis is how they tried to convert the analog values to ppm:
Since the closest thing to a known methane value that that they had available was straight natural gas coming through the line to a valve in the chemistry lab at the school, they used that as a sample to represent a saturation value (in other words, a million ppm). They read that the average commercial natural gas supplied is about 96 percent pure, so they let the saturation count as 960000 ppm. The analog read value with the sensor next to the valve (as saturated as they could get it) was 690. The lowest analog read value they got was 38.
Since the data sheet for the mq4 says that it's base 10 logarithmic, this is how they came up with their conversion:
960000=10k(690-38) or max val ppm= 10 k (max analog read-min analogread)
Log 960000 = k(652)
Log960000/652 = k = .009
so the final formula that converts the analog read to ppm is based on the formula:
Y= 10 .009(x-38) x being the analog read value
They know full well that even if the formula is more is correct that using a gas valve in the lab is at best a crude calibration standard.
has anyone out there tried to do such a value conversion with gas sensors? I'm not really a math guy, does this make sense.
The math looks right.
They might be able to get a better estimate of the high end of the range using a balloon. Put the sensor in the balloon and squeeze the balloon down around the sensor to get all the air out. Attach it to the gas outlet and partially fill the balloon with gas then take a reading. Needless to say - NO SMOKING ]
That ought to be very close to the top end. It'll probably be quite a bit higher than 690 but they can just plug the new value into the formula to derive k and away they go.
!!. The sensor you use isnt suited for (exact) school experiments..
It needs constant heatup ( 48 hrs contant power to heater)
Sensor is sensitive to many gases in addition to methane! Even breath will change output
Its extremly difficult to calculate correct ppm levels (advanved math)
................
bottom line
students should be able to explain:
the physics of sensor
maybee also the chemistry that takes place when gas is present ?
the principals for detecting theese changes with a computer
how electronics is suitable for this task
factors to be aware of -temperature, humidity, other gases, tolerance of componets and so on.
I , nor my students, have ever got reliable readings from a variete of MQ... sensors (many)
The important part is, to me, the understanding of the principal.
Knut N, Norwy
