So I was recently asked “how do I calculate the dilution factor on a canister?” Now ya’ll get this blog as a direct result. So here we go with the following example…
First we start with a clean and fully evacuated canister. Regardless of size a fully evacuated canister will have a pressure of -14.696 psig (note the g for gauge). Also note that -14.696 psig = -29.92” Hg.
Next we ship the canister out to the field and a sample is collected. Typically a canister is sampled until -2.5 psig (-5.09” Hg). Yes, not all of the canister is consumed. This has to do with the capabilities of maintaining “constant” flow with a passive flow controller, but that is for another blog.
From there we receive the canister in the lab and then very often the canister is pressured to 5 psig before analysis (this makes for faster loading of the sample onto the preconcentrator).
So how much has my original sample been diluted by going from -2.5 psig to 5.0 psig. To calculate the dilution factor for this canister we use the following equation:
Dilution Factor = [Pressure (after dilution) + Pressure (atmospheric)] / [Pressure (atmospheric) + Pressure (before dilution)]
Dilution Factor = (5 psig + 14.696 psig) / (14.696 psig + (-2.5 psig) = 1.61
So the concentration we get out of this canister we multiply it by 1.61 to get the dilution corrected concentration.
Now very often during the same conversation I get asked “what is my sample volume?”
Keeping with the same example, we calculate the volume of sample in the canister by using the following equation:
Sample Volume = Pressure Change / Initial Pressure x Canister Volume
Sample Volume = -14.696 psig – (-2.5 psig) / -14.696 x 6000 mL
Sample Volume = 4979 mL
For those of you who do not want to reinvent the wheel… I now offer you the
Which includes dilution factors and sample volumes (before and post dilution).*
*Model not included with calculator.
