Friday, June 8, 2012

Geiger Counter Continued

I investigated the large variation observed in the graph of June 6. While babysitting the monitoring software, I noticed that occasionally the software would report bursts of over 200 CPM, even though the Geiger counter itself never measured such a spike. Two "faulty minutes" in one hour could easily explain the 400 clicks variation. Therefore, I first tried different serial/USB converters (settings were always as specified by the GC manufacturer), since low quality converters are notorious for messing up signals. But I noticed faulty burst even for expensive converters. Then I tried my only computer that still has a serial port. Now, no more faulty bursts occurred during several test runs.

After that, I redid the 24 hours experiment. The results are shown below. A much more stable graph, reporting around 700 CPH, or .1 μSv/hour, which translates into about .9 mSv/year.

I am glad I am now able to perform more reliable measurements. If anyone has suggestions on debugging the serial/USB convertors, please let me know.