How do I measure gas concentrations?
I have built a gas detector using a conventional gas sensor (similar to Figaro sensors).
I want to calibrate it. I’d use a container of a known size and place a drop of petrol into it. I would heat it in warm water, letting the droplet slowly evaporate. Voila – an atmosphere of patrol vapour.
My question is, how do I calculate the strength of the vapour, in say parts per million (ppm)?
Calibrating the container volume is easy, using various kitchen measures. Measure the volume you want in water and pour it into a larger container. Mark the level, and empty the container. We have a known volume. A calbrated syringe also gives odd but accurate volumes.
The petrol can also be measured using a small syringe.
I am aware the atmosphere in the container would need to be stirred, as the vapour is heavier than air and would settle.
What however is the formula (ae) required to calculate the concentration? Alternatively, what do I do to measure a concentration of 10 PPM?
If you are measuring the petrol by volume, you will need the density. From the density and volume, you can get the mass (m = rho * V). From the mass and the molecular weight, you can get the number of moles of petrol (n = m / MW).
(EDIT: The volume and density as mentioned above would be for the LIQUID state, everything below here is gas state.)
Once you have the number of moles, you’ll need to add that to what’s already in the box as air. Use the ideal gas law to calculate the number of moles of air that is already present.
n_air = P*V/R*T, where P is atmospheric pressure [you should be as exact as possible if you're calibrating the instrument], V = volume of the box, R is ideal gas constant, and T is temperature.
After the petrol has evaporated, the volume of the box has not changed, clearly the ideal gas constant has not changed, and presumably, the temperature has not changed either. Therefore, you can determine the new pressure by the ratio: P0/n_air = P1/(n_air+n_petrol) or,
P1 = P0 * (n_air + n_petrol)/n_air
With the new pressure, you can determine the volume of the petrol vapor.
V_petrol = n_petrol * R * T / P1
Next, get the volume% of petrol: V_petrol / V_box.
ppm is just vol% * 10^6
(EDIT #2: If you’re going to do a “drop” of petrol, that makes it hard to measure the volume. Also, because of the volatility of petrol, it would be difficult to try to weigh a drop as well.)
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