I also tested the SBM-20 tube on this device.
Conversion factor for SBM-20 tube.
Russian SBM-20
60Co 1/150 = 0.00664
226Ra 1/198 = 0.00504
Please note that conversion factor varies depending on the tube sensitivity at different isotopes, Cobalt60 and Radium226
Nevertheless, based on my testing compared to the local university lab readings, and based on what Libellum (the company that manufacture the kit) declares here:
http://www.libelium.com/wireless_sensor ... _counters/
the most appropriate value for SBM-20 tube whit this circuit seems to be 0.00277