What Are We Measuring with This Gadget?
This gadget is measuring radiation in “counts per minute” (CPM), which at this writing is the most commonly used increment for sharing DIY radiation counter data on Pachube. Each time a subatomic particle ionizes the gas molecules in the detection tube, thus closing the circuit, Arduino registers that as one count.
We use CPM because we can’t assume that DIY radiation detectors are calibrated to an official standard. The detector tube used in this build claims to have a factor that can convert counts per minute into sieverts, a unit of radiation measurement commonly used by scientists. But since most of us don’t have access to the kinds of laboratory facilities that would allow us to confirm this calibration, counts per minute are the best units to use.
Taken over weeks, months, and years, CPM give us a useful qualitative measurement of radiation levels, rather than a quantitative measurement. That is, the readings can tell us if the radiation level changes dramatically, such as jumping from 50 to 150CPM. A significant increase like that might be worth looking into, even if we don’t know exactly what it means in sieverts.