If in a bath on a blue line there is a photo-element, it automatically gives a signal when water reaches a dark blue line. And water delivery stops.         In hospitals, sanatoria there are medical procedures, for example, pearl baths and others. On one nurse - at once 15 baths!  The nurse can't be in time and water pours out in vain.  As well in the apartment will pleasantly relax in a bath, but not to receive then the huge account for water.