I replied in the discussion on 4 gas analysers that I had received an interesting email re HCs as % rather than ppm.This was the reply I received:
"Sure. However, it is not normal practice, and doesn't make that much sense.
Let's recap:
% is actually (the Europeans remind us), '%V/V' , or volume of gas of
interest as a fraction of total volume of all the gas. 15% CO2, as an
example, is actually 0.15 CO2. The '%' identifier is only to indicate
that the true ratio has been multiplied by 100 for convenience of
expression - to make concentrations range between 0 and 100. You can
see why - 0.50% CO is actually 0.005 when expressed as a decimal
fraction. Lots of zeros - and too much room for confusion.
HC fractions are even smaller than CO. The HC gas fraction in typical
engine-out gas is maybe 0.0002. This could be expressed as 0.02% HC
alright, or as 200 ppm. Back to the CO argument above, too small of a
number if expressed as %. Further, if typical engine-out HC is 0.02%,
and then the post CAT level would be maybe 1/10th of that, or 0.002% -
which is 20 ppm. (There are 10,000 ppm in 1.0% gas fraction.) At the
end of the day, the units of measure track the typical concentration of
gas - and the gas fractions for HC have been typically so low that the
preferred unit of measurement has been ppm.
Most modern cars are now producing post CAT HC levels in the sub-10 ppm
range, by the way. 10 ppm is 0.001%. Pretty small, and thus too far
below 1 for us to easily handle.
So, the reason that HC concentrations are diplayed in ppm is to keep the
numbers above 1. Decimals seem to confuse us. (Remember that before
CATs, cars would produce maybe 12% CO2, 2.5% CO, and 500 ppm HC. Back
then, the units of measurement seemed fine. Today, we should really be
measuring CO as ppm - as post CAT numbers are maybe 0.02%, or 200 ppm.
By reducing the CO level by a factor of 100, we are falling out of the
% argument for that gas too.)
Getting back to your original question, our analyzers now measure HC as
methane, and there we do indeed display HC as % in that case - with
0.001 % the smallest number - equivalent to 10 ppm resolution. (You
have to have almost 6 methane molecules to get the same HC content as 1
hexane molecule, though, so this kind of makes sense, as the typical
'HC' you see as methane will be 5 or 6 times higher than HC as hexane.
Got it?
Let me know if you have questions or comments.
Thanks,
Bob Schrader
Bridge Analyzers, Inc"
Hope you find it interesting.
Cheers
Peter Wright