Unfortunately, this concept,
which is intuitively obvious to a beginning chemistry student, has
been lost on many who should know better.
In the early 1970’s, several
government agencies, including the EPA and similar groups in other
countries, began to express gas concentrations using mixed unit
parameters such as micrograms per cubic meter, or milligrams per
cubic meter. Of course, these foolishly contrived parameters are a
function of temperature and pressure, and require an asterisk
denoting the temperature and pressure at which the value was
calculated.
More often than not, however,
this information is lacking.
But, even if it were provided,
since real world gas measurements must occur at fluctuating rather
than constant temperatures and pressures, an actual user still has
to convert back to parts per-million. Either that, or he can special
order a gas detector that internally converts its ppm output to
micrograms or milligrams per cubic meter at a specified temperature
and pressure. Yes, we have supplied such units.
At Interscan Corp., we do provide
conversion spreadsheets on our website, from these mixed unit
parameters to ppm, and from ppm to the mixed unit parameters. These
are the most accurate conversion facilities available—allowing for
molecular weight, temperature, and pressure inputs—and they have
been widely downloaded. To view them, go to
www.gasdetection.com/TECH/convert.html.
Also provided from the same
gateway page are molecular weights for many compounds, and a
conversion spreadsheet for volume percent to grams per cubic meter.
In addition to being completely
impractical, the mixed unit concentration parameters have created
their own side effects. Many potential instrument users are
unfamiliar with basic chemistry and are thus unaware that these
parameters are affected by temperature and pressure. Moreover, when
concentration values are reported to us, using these parameters,
they are incorrect at least half the time.
In a recent case, a user
mistakenly thought that because in AQUEOUS solutions, milligrams per
liter is nearly equivalent to parts-per-million—of the dissolved
SOLID—the same would be true for gas mixtures.
Using this “logic,” he
communicated his requirement for monitoring 5 ppm chlorine as 5
milligrams per liter. In actuality, 5 mg / liter of chlorine in air
works out to be 1696 ppm (at 20 degrees Celsius and 760 mmHg). How
did all this happen? I have two theories, and they are not
necessarily mutually exclusive:
1. Once Canada embarked on its
conversion to the Metric system in January, 1970, American
environmental officials, knowing that our standards would be
emulated throughout the world (and the world was becoming all
Metric), wanted to put forth a good face. Thus, environmental
standards were set out in Metric-LOOKING units such as micrograms
per cubic meter and milligrams per cubic meter.
In fact, there is nothing
“non-Metric” about parts-per-million or parts-per-billion, as they
are dimensionless parameters. Therefore, this mission started out as
a fool’s errand.
2. In occupational health work,
regulatory values are promulgated for many non-gas substances, and
these are correctly stated as milligrams or micrograms per cubic
meter. Perhaps a desire for consistency across all substances,
including gases, crept into the proceedings.
Your Easy Takeaway
Our best advice is to stick with
what works: Percent, parts-per-million, and parts-per-billion. And,
if you must convert to the mixed unit parameters, use formulas that
are accurate.
FSM
