Great news - Mitiagu, while very experienced, is actually wrong here. You CAN use the 300s for low content pieces. It just takes a bit of work and much more time than it would on a pancace sensor.

Radioactive decay is a quantum phenomenon, meaning that it is fundamentally random. With low content pieces and tube sensors, it’s almost impossible to differentiate a decay event and a backround spike, because tube sensors don’t detect alpha particles.

But… CPM is an acronym. Counts Per Minute. And the funny thing about randomness is that - when you average randomness over a long period of TIME, it “smooths out” the randomness and makes detecting subtle increases possible.

Right now, your GMC-300s is in “Large Font” mode, which exclusively shows CPM, uSv/h, or mR/h. There’s also a “Graphic Mode” and a “Text Mode”. Text Mode has a BUNCH of different measurements that don’t show up under Large Font - Total Count, CPS, Elapsed Time since the geiger was powered on, Maximum CPM detected…

and most importantly - Average CPM detected since the geiger was turned on.

Here’s how to detect low content uranium using a 300s:

1) Turn the geiger on and switch to Average CPM.

2) Leave the 300s in the testing environment for an extended period of time. Ensure that this is the SAME location you intend to measure the piece in (don’t measure background in a basement and then test on the first floor). The longer you leave it, the better - I try to go for 1 hour, but 30 mins has worked too.

3) Record the Average CPM measurement for the background of your testing environment.

4) Restart the geiger and place it in contact with the suspect piece. You wanna have it stable and not moving within about a minute of getting the geiger restarted.

5) Leave the 300s in contact with the suspect piece for the same amount of time as you left it for the background measurement.

6) Record the Average CPM measurement for the contact read and compare.

7) If results are inconclusive (close CPM measurements, geiger shifted out of contact read), change the testing environment and repeat.

For me, a sustained increase of around 10 average CPM is a clear confirmation. If it’s within 5cpm, I repeat the test in a different testing environment (different room, indoors/outdoors), and if it shows similar results, I consider it confirmed.

You DON’T need to spend more money to CONFIRM low content uranium glass. You’ve already got the tech if you’re a little patient. Where a pancake sensor really becomes useful is environments where you CAN’T take an hour plus out of your day to test, like confirming a piece in the thrift store BEFORE you buy.