A situation in which the cyanuric acid stabiliser has built up through continuous use of stabilised sanitisers. This slows down the rate at which hypochlorous acid forms in solution. There are two possible remedies. Either introduce fresh water into the pool or use unstabilised chlorine such as Sodium or Calcium Hypochlorite.

This is a term that implied that there could be so much cyanuric acid (stabilizer or conditioner) in the water that the chlorine would be all "locked up." This is not true.

a somewhat archaic term used to describe the presence of high levels of combined chlorine. Combined chlorine is the difference between the Total Chlorine reading and the Free Chlorine reading. Ideally, it should be less that 1 PPM. High combined chlorine content can result from the presence of nitrogen containing wastes such as urine and sweat. Many people view any situation as chlorine lock, where lots of Chlorine has been added and there is little improvement in the chlorine readings. This is not chlorine lock, but simply a case of needing to add more because of the pool's high demand for chlorine. More algae and debris = more chlorine required