Yes, you figured it out - a snuff box with a membrane cap on it. If the cap threaded onto the box it would be a better device, but with it glued together it is just a throwaway. The indicator solution should never evaporate away, but biofilm would eventually plug up the membrane pores unless it is cleaned regularly with peroxide or bleach. If the unit is left in the light forever I assume the dye in the indicator would fade, otherwise it should last a long time.