While we included the basic dimensions in the datasheet for the GGreg20_V3 Geiger counter module, these are not all the measurements we are occasionally asked about.

For makers who want to purchase the GGreg20_V3 Geiger counter module without a tube (because they already own a compatible tube or plan to source one elsewhere), this small technical note details the dimensions of the tube mounting on our module’s PCB.

As you can imagine, having the option to buy just the module can be very convenient for some. We have always provided this option (on our website, Etsy, and previously on Tindie) — allowing you to select and order only the components you need right now. Over time, we received feedback, gained experience, and discussed this matter with our customers.

Note: The GGreg20_V3 was originally designed for the SBM20 tube, but its Soviet-era stock quickly ran out, so we specifically chose the even better J305 tube as a replacement. This means the standard SBM20 will also fit the module, as it was the tube we originally designed it for.

It turns out that the Chinese market offers J305 tubes with varying dimensions. This is why we want to explain exactly which J305 tube (or similar) will fit the GGreg20_V3.

We provide data for the J305 tube that we currently supply with the GGreg20_V3. We use the data provided to us by our J305 tube supplier from China.

Further on, we include a technical drawing with dimensions and a few photos to clearly illustrate the setup.

We also provide a photo of several J305 tubes from the latest batch, which clearly shows that while J305 tubes have minor size variations, they can be considered identical for our purpose. In several years of working with the J305, we have only encountered two initially defective tubes, as well as one tube that significantly differed in size from the rest of the batch and from the dimensions specified in the datasheet.

We hope this publication will help you understand which tube to choose so that it fits the module, particularly in terms of length.

Good luck!
IoT-devices team