These DPD No 1 high quality Tablets are for PHOTOMETER use only. They test for Chlorine, Bromine and Chlorine Dioxide in your Swimming pool or Hot Tub. All our Tablets have at least 2-3 years expiry date from the day you buy. Confused about your hot tub water chemistry? Our guides answer the most Frequently Asked Questions! Why do I need to test my hot tub / spa / swimming pool water chlorine levels ? Why do I need to test my hot tub / spa / swimming pool water pH levels ?