Converting adc into ppm - gas sensor

[Denzel]

My project the VIPER uses a hydrocarbon gas sensor to detect LPG.
http://www.jaycar.co.nz/products_uploaded/RS5610.pdf

The problem is, when reading the analog data from a Picaxe the data is between 0 and 255 naturally. Do you know how I could convert this reading into ppm? So I can make a useful graph.
When the gas level is high I get an analogue reading of about 218 and the PDF (link above) says the sensor is sensitive to 200 - 10000 ppm LPG. So I guess the question is, what would 218 be in ppm?
Is it just a simple ratio equation? 255:10000? Where 218 would equal about 8549 ppm
Any help on this would be much appreciated.

Thanks

 

[Andrew Cowan]

The datasheet should tell you the mV per ppm.

Supply voltage * 1000 / 255 = mV per unit.

A

 

[BeanieBots]

Unfortunately, the "key" information on the datasheet is missing.
Therefore it is not possible to say for sure. Most of the graphs are reasonably straight so the relationship is fairly linear under constant conditions. However, it does warn about temperature and humidity influences so if you want to accurately display the value, you should consider also measuring those parameters to compensate.
It should then be possible to convert your ADC reading into ppm numbers.

Do you have anything you can calibrate against?
I'd start with a simple linear relation like you suggest and see if the numbers produced are of acceptable accuracy. Test at different tempertures and humidities. Don't forget that the PICAXE can only cope with numbers up to 65535. So to show a large number, keep it smaller than that and just add the required number of "0"s at the end as part of the displaying routine.

 

[Denzel]

...

I do not actually want the picaxe to convert the data into ppm. I have the analogue reading from the picaxe sent to the computer via XBees and I want Visual basic to convert it. so I dont have a size restriction on my resultant ppm.

I considered getting a gas detector which gives a readout in ppm and compare it with my analogue reading when measuring LPG but I dont have one

for this particular test I am using a simple cigarette lighter without the flame, does anyone know know how many ppm roughly that would be if I held the sensor right up to it?
would about 900 ppm be close?

 

[goom]

I would think that 900 ppm would be well below the flamability limit of butane, so would not sustain a flame at that level. The syoichiometric fuel/air ratio is more like 65000 ppm (by mass) or 31000 ppm by volume.
You really need a sample of known concentration to calibrate the sensor. What about bubbling some butane into an inverted water filled test tube, then adding air to fill it. Not very precise, but it would give an idea.