I have one too, it has been in service since 1971 and still dandy.
Selenium cells give up reactivity with exposure to light, so keep it with a dark cap over the cells when you're not using it. Anyhoo I compare its readings with my other light meters and no significative deviation is happening, but there are a number of factors involving accurate reading apart the cells itself, so better check its readings often, and do not ask so much to this meter under low light conditions!