Testing the alarm with a super low source consisting of a uranium glass figurine. My background averages about 2,000 counts per minute. which matched to my ludlum 192 that would be about 3uR/hr. so it seems ever 1,000 cps = 1.5 uR/hr. This works nicely as a fixed portal alarming solution to alert on any unusual elevation of background. It would alarm in the presence of a check source 24 feet away.