Has anyone tried to test the difference of sensitivity when the cover is on or off their GMC080? try it and you will see a MAJOR difference in sensitivity especially with the M4011 tube. Here are some test result averages over 5 minutes with and without the cover at the same distance from the tube: I am using GeigerBot and a data cable to my iphone.
I'm sure that having the cover on will effect the counts. It's another layer of substance to stop the rays from getting to the anode of the tube. I just leave the M4011 tube attached by the clips, soldered to wires, soldered to the board. That way when I want more sensitivity I just pull it out and it hangs by the wires.
I had an interesting time at a local kitchen/bath supply store. Found a few types of granite counter top samples showing ~50CPM. Seems the darker granite with the reflective spots were 'hotter' than the lighter colored ones. BTW, the sales associate was pleased as punch to have me checking things for her.
Anyway.. I noticed fewer counts when the M4011 was inside the box, than when it was out..