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Update: 4/6/15, 6:00 P.M. - Geiger Counters are like Rain Gauges...

Imagine a rain gauge, typically a cylinder about 1" in diameter, graduated in inches of rainfall, just like a ruler standing up.  So if we get a 1" rain, the rain gauge will read 1", of course.  But how much rainwater did the rain gauge actually collect?  Given our 1" diameter rain gauge, and the equation for the volume of a cylinder, that works out to .78 cubic inches of rain collected.   Now, suppose I want to build my own rain gauge, except that I use a 1.75" diameter cylinder, yet graduated equally in inches, the same as the store-bought rain gauge.  So then I get another 1" rain, and my home-built rain gauge still reads 1" on the graduated cylinder, which means I built it correctly.  But because I used a larger diameter cylinder for my home-built rain gauge, I actually collected 2.4 cubic inches of rainwater, or about 3 times as much as collected by the 1" diameter store-bought rain gauge.  So both rain gauge designs are equally accurate, except that in arriving at their respective rainfall measurements, they collected different amounts of actual rainwater.

The same principle holds true with Geiger counters.  So often, I receive queries as to why one station has such lower or higher CPM readings than another.  Just like rain gauges, not all Geiger counters are created equal.  So to put a station/Geiger counter's CPM reading in context, it is necessary to understand its design, which in the case of a Geiger counter, revolves around its sensor, or Geiger-Mueller tube.  Like a rain gauge, a GM tube is basically a cylindrical device of specified diameter and length.  Many are what I call a "standard tube" which is typically about 2 or 3" long and maybe 3/8" in diameter, and which tubes emit background radiation of about 10 CPM at sea level.  This contrasts with "pancake tubed" models which are often 1.75" in diameter, and generate radiation counts of about 30 CPM at sea level, or 3 times that of a standard tube.  And just like the rain gauge, while the pancake tube collects about  3 times more radiation counts than the standard tube, their actual radiation measurement in standardized units like mR/hr or uSv/hr will be about the same, say, .01 mR/hr or .1 uSv/hr, in this example.