[aprssig] APRS Radiation sensor

Scott Miller scott at opentrac.org
Thu Mar 24 00:22:51 EDT 2011

> Accurate ratings in Sv units from a tube may not be obvious.  The EPA
> itself is very
> careful about it and uses CPM (particle count per minute) on their maps
> instead of using
> SI units.  Also, tube readings can be relative to a lot of environmental
> factors (not just
> calibration).  CPM alone is useful to watch trends though.

I've been asked at least 3 times in the last few days how I came up with 
a conversion for CPM to uR/hr.  In my case, since I scavenged a tube 
from an old civil defense meter, I just read off both scales on the 
analog meter and assumed that was accurate for this case.

I've read enough GM tube data sheets to see that their sensitivity 
varies considerably.  I'm not at all certain that reporting uR or Sv on 
the air is doing anyone any favors - data quality on weather stations is 
kind of questionable already since no one pays much attention to 
placement, and weather stations can at least be checked against other 
instruments fairly easily.  A 4 or 5 digit count reading would give you 
better resolution for tracking trends, without being misleading in the 


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