Most of the radiation devices measure radiation units accurately . The digital ones have to have a timer. The ones that have a clicker or flash light do not have a display on which to read- but they are also generally accurate. Its like buying a shaving machine- even the cheap ones will do the job- but the more expensive ones will do it better and come with more bells and whistles. So it all depends on your budget- you can buy the Ferrari of radiation devises such as the Terra Dosimeter or you can buy the Hyundai such as the Nuka alert.
The size of the Geiger Muller tube is an important consideration. For example the one in the key chain sized detectors is usually quite small. So the point to note is that even the small ones will do the job but they wont have a bigger sample to work with as the larger devices. However with smaller units you may have to approach the object emitting the radiation more closely for it to detect accurate radiation units compared to the bigger devises.
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