Parts-per million (ppm) is a common quantity used in many areas of math and science. It can be somewhat difficult for some to understand because it is not a true unit. In fact, ppm is a unitless quantity. It is analogous to percent. Percent can refer to just about anything. Percent means part-per-hundred, per cent (cent meaning hundred). ppm is an exactly analogous quantity, but it is one part per million instead of one part per hundred. ppm is a ratio between two numbers that have the same units. Consider the example of a 5% sales tax. For every hundred dollars I spend, I must pay 5 dollars in sales tax. So:
5 dollars/100 dollars = 5%
I would much rather pay a 5 ppm sales tax:
5 dollars/106 dollars = 5 ppm
We do not usually refer to money in ppm, but we could. Ppm is more often found as a concentration, for example, ppm by mass or ppm by volume (sometimes referred to as ppmv). In nuclear magnetic resonance spectroscopy ppm can be used to describe the amount of chemical shift in frequency (Hz/MHz). This post focuses on the use of ppm as a measure of concentration.
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Friday, July 30, 2010
Friday, July 23, 2010
I am enrolled with a health plan through which I get my medications from Medco®. On June 25, my doctor sent in a prescription for Patanol®, an allergy medication for the eyes. As of July 10, I had received no word on the prescription.
On Saturday, July 10, I called Medco to find out what had happened. After navigating their inane voice-mail system, I was able to speak to a human being. I achieved this amazing feat by repeatedly saying "representative" to all of the voice-mail prompts.
The woman on the other end told me that my prescription had stopped for some unknown reason, but that she could restart it manually and that I should expect it to arrive in 8-10 days. She told me that I could go to my local pharmacy and that Medco would pay for a 7-day supply to hold me over. It sounded too good to be true.
Saturday, July 17, 2010
Snake-oil salesmen are alive and well. People will buy nonsense if it is marketed correctly. I enjoyed the review of the wine clip on the Dan's Data Website. This device supposedly makes wine better by pouring the liquid through a magnetic field. The review does a good job of explaining the nonsense.
- Dan's Data: Dan's Data: Review of the wine clip
Friday, July 9, 2010
The question of how to convert from one set of units to another comes up from time-to-time, and I think it might be helpful to have a few short posts that simply address unit conversion. This post addresses conversion to and from wavenumbers (cm-1) (also called reciprocal centimeters, inverse centimeters or Kaisers). A previous post What is Infrared Radiation (IR)? addresses the concepts behind this unit. The unit is proportional to frequency, and can be considered a unit of frequency or of energy.