It was Lord Kelvin who said:
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it."
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."
And, who am I to disagree? But, even the great Lord Kelvin could not have foreseen the dimensional mess that plagues environmental science.
It's important enough that I devoted a Knowledge Base article to it.
The issue involves metric-LOOKING units such as milligrams per cubic meter being used--usually by government agencies--instead of parts-per-million. The problem, of course, is that the stupid mass per volume unit is a function of temperature (and barometric pressure), and even if the number is stated to be at, for example, 20 degrees C, it is still foolhardy.
Why? It is foolish because air quality measurements must be done in the real world, not under controlled laboratory conditions. While, we can generally assume the we are operating near the standard barometric pressure of 1 atmosphere (760 mmHg), in the real world, the temperature isn't always 20 deg C, is it?
More than that, many people are unaware of the temperature dependence, and use "simplified" formulas to convert between parts-per-million and milligrams per cubic meter. The formulas are simplified in that they assume a particular temperature, usually 20 deg C, as well as a barometric pressure of 760 mmHg.
To help out the environmental community, we have posted spreadsheets to make these conversions ACCURATELY.
A separate Knowledge Base article is devoted to correction for atmospheric pressure differences.
