I have an enclosure that uses two uvb tubes at 24" in length. Recently purchase two but have a question as to determine when these tubes loss their ability to properly supply uvb. I have used my local pet store in the past to determine the "life-left" in older uvb tubes, however a recent incident at this particular store has left doubt in my mind that these tests are actually legitimate, meaning more like a sales tactic to purchase replacements, while in their store. So how can I really tell the value of these bulbs as they age? BTW...Repti-Sun 10.0 24". Thanks for any advice offered.