two more specifically if one or two of the systems shown in fig3 5 draws substantially more power than the others either the solar cells 37 or the battery 39 may be dedicated to the unit s with a higher power consumption with the remaining power source shared among all three systems in the alternative one of the two power sources e g solar cells 37 may serve as the principal power source for all three units and the other source used as a back up source as will now be apparent the invention provides all of the advantages of the invention disclosed in the above referenced u s patent and in addition provides an instantly recognizabe visual indication of the range state of each parameter moreover the visible led indicators 23 29 can usually be viewed from a distance greater than that required to read the individual parameter displays 20 22 so that a user can visually check the relative safety of the water by means of a simple visual inspection from poolside as a result pool water maintenance is simplified and made more precise through use of the invention although the above provides a full and complete disclosure of the preferred embodiments of the invention various modifications alternate constructions and equivalents will occur to those skilled in the art for example although the invention has been described with reference to led indicators 23 29 other types of indicator lamps which are readily visible in the water environment such as bright sunlight may be employed.