Convert ppm to percent or percent to ppm:
This article describes the simple math used to convert between parts per million (PPM) and percentage, such as how we convert a measurement of 800 ppm of CO2 in air (indicated on our colorimetric gas measurement tube shown at page top) and a percentage concentration of that same gas in air.
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Suppose we are given a parts per million measurement (ppm of X in Y) and we want to convert that to a percent concentration of X in Y.
% Concentration = ppm / 10,000
Percent concentration of substance X within another substance Y = ppm concentration divided by 10,000
( Parts per million (ppm) of X ) divided by ( 10,000 ) = Percentage concentration of X in Y
We are calculating the percentage concentration of X in Y even if "Y" is not being stated. People often talk about concentrations without stating the "in what" substance (Y) because they think we know what the heck they're talking about. Or We don't. Or they don't.
If I make a Carbon monoxide (CO) measurement and my instrument tells me I've got 800 ppm of CO, and my hygienist asks me to write the report in percent concentrations what now?
Just watch the decimal point and we can avoid embarrassment.
To convert 800 ppm of gas X to percentage of gas X in air divide the ppm by 10,000
800 / 10,000 = 0.08%
Notice where we place the decimal point if you want to stay out of trouble.
To convert an 8 percent concentration of substance X to parts per million of X (in whatever, air, water, gas)
8.0 % (concentration) x 10,000 = 80,000 ppm
Thanks to Leroy Jenkins for technical editing to clarify this point.
When converting percent concentrations of anything to parts per million remember that a 1 % (concentration) = 10,000 ppm or parts per million.
This procedure is the opposite of what we just did above. Now we're given a % concentration and we want to express that in parts per million or ppm.
Formula to convert percent concentration into parts per million:
ppm = ( 10,000 ) x ( % concentration )
Where the heck is that 10,000 coming from in these conversion formulas or in the ppm <-> % conversion table we give below?
Remember that 1% means one part in 100 so we can write
1% = 1 / 100
or writing that in decimal form (without the percent) we'd write 1% = 0.01
Remember next that parts one part per million means 1 in a million, so we can write
1 ppm = 1 / 1,000,000
Finally, remember that one percent concentration expressed in parts per million is therefore written as
1% of a million = 0.01 x 1,000,000 = 10,000 ppm
You can always check your math by using 100% - as Shelly Weinberg (statistics & queuing theory instructor at IBM Systems Research Institute) said, "Are you scared of the math, pick a number, any number. Still scared, OK let's try 1. -- or 100%. 
Let's try converting 100 percent of something to parts per million.
If we have 100% or "all" of something expressed in parts per million, we should end up with exactly a million parts per million, right?
100% can also be written as 1. But when converting percent concentrations to parts per million we write:
100 x 10,000 = 1,000,000. which is a million (when speaking English).
1 part in a million = 1 / 1,000,000 = 0.000001
But to convert that result to a percent number we always multiply by 100. Forgetting this is what makes trouble.
So when we want to convert a parts-per-million number into a percent we divide the number of parts by a million and then multiply that number by 100 to express the same number as a percent "concentration".
If we have just one ppm that can be written as:
1 / 1,000,000 = 0.000001 or one part per million
To convert this raw decimal fraction into a percent we multiply it by 100:
0.000001 x 100 = 0.0001% percent ppm concentration
Or if you like, perform the division and then simply move the decimal point two places over to the right to convert the decimal fraction to a percent concentration.
Is it possible to have more than a 100% concentration of one substance A in another substance B?
No. Since if we get to 100% concentration of substance A, it's taking up 100% of the space and there's no room for the second substance B.
For people who hate multiplying two numbers or who like me are always scared that they put the decimal in the wrong place, here is a table converting between parts per million concentrations and percentage concentrations of anything in anything.
Conversion Table: ppm to Percent (%) or Percent Concentration to Parts Per Million
|Parts per Million - ppm||Percent - % Concentration|
Well yeah, in some cases such as measuring gas concentrations in air in an enclosed, stable condition space. Otherwise, well probably not very.
Watch out: in the field when measuring levels of gas contaminants or airborne mold spore contaminants, people are lying to you. Except under controlled environments, the true level of airborne contaminants varies constantly as building conditions, weather, equipment operations, etc. vary.
If you are given an airborne toxic mold spore concentration of 81,926.939 spores per cubic meter of air as a result of a simple "air test for toxic mold", other than a general conclusion that "the number looked pretty big" the rest, the precision is nonsense. It's a very precise number (three decimal places!) but it's almost certainly not truly accurate.
Open a window, turn on a fan, walk through the room, move the sampler, change the time of day, and the number will almost always be enormously different. Not to mention that the most harmful mold in the building might not even have been collected in the air test.
OPINION: I'd prefer to report the number above as "80,000 +/- 50,000" or something like that, based on a combination of experience, building observations, case history, procedures and equipment used. The "right" answer to such studies is "it looks big" or "it doesn't look big". Why don't your labs and experts report like this? Competition. More digits = "credibility" in the minds of some customers. "If we don't report that way we'll lose business" is what lab directors have told me at conferences. So it's our fault for not paying attention in 6th grade math class. (Thank you Miss Morton, Tuckahoe Elementary School).
Details of this problem are at ACCURACY vs PRECISION of MEASUREMENTS.
Bachrach indicates that for gas-fired equipment CO should not exceed 400 ppm in the flue gases. 
Bacharach explains that conversion burners and gas-designed equipment, if over-fired, can have flame impingement on cold surfaces and may produce CO even if they are being run with excess air and even though CO2 and O2 are at acceptable limits.
Excess air is introduced in gas combustion to assure that no CO remains in the flue products. See Matzen's FURNACES & IAQ or for an engineer's view see COMPLETE COMBUSTION, STOICHIOMETRIC for details about "complete combustion".
RW Beckett Corporation also produces combustion and flue-gas measurement equipment. However a review of literature they provided to us did not find equipment specifically for CO measurements. Beckett's tests measure oxygen percentage.
Other companies such as MSA, ISC, Monitox, and MDA also produce equipment which may be useable for these tests; these companies did not provide literature for this article.
Details are at CO DETECTION OPTIONS
Continue reading at GAS DETECTION INSTRUMENTS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
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