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N o chemical knowledge or special technical ability is necessary. H alf a m inute’s study of the simple directions will prepare an inexperienced operator to make the tests. The self contained pocket unit encourages frequent tests because of the extreme' sim plicity o f procedure: it is byilt for the practical man;HS:its compactness, durability and accuracy appeal to the supervising official and plant operator alike as .a scientific, workmanlike outfit to meet a daily need. Illuminator Unit M an y times chlorine residual tests must be made at night and in some cases at locations where light is poor at any time. T o facilitate such tests and to provide laboratory accuracy under conditions of darkness or poor lighting, - W &T have de­veloped a self-lighted u n it,' identical with the standard unit, having, in addition, a small battery operated lamp to supply the necessary light for color comparison. A small com partm ent is added to the case o f the unit, to house battery and lamp. Current source is two standard flashlight cells and the lamp is a special miniature daylight bulb, so arranged that the light is evenly diffused over the milk-glass diffusing panel. The- advantages' o f this unit are not con­fined to its use after dark. D aylight is indeterminate, being o f one ihtensity on a bright, sunny day and something entirely different on dark, cloudy, days. W ith a constant light source such as in the Illumina­tor unit, readings are more accurate, re­gardless of the time of day, the kind of day or the location in which they are taken. Provision is made for adding the battery and light com partm ent to standard units now in use. Hydrogen Ion Control H ydrogen ion concentration can now be readily determined by colorim etric com ­parison. It is thus an easy matter to properly control feed rates of chemicals used for correcting pH values. Permanent, non-fading standards now avail­able for use in the W &T Hellige Com parator make pH determinations accurate, simple and reliable. N o longer is it necessary to prepare fresh standards or to check existing standards to be sure o f their accuracy. Color discs, each covering the full range of one indicator in steps o f 0.2 pH are available. Rem oval of one disc and insertion o f another requires only a few seconds. Where wide ranges in hydrogen ion concen­tration occur, work o f determ ination can be greatly simplified by a range finder indicator and color disc. This equipment is available for use with the W & T instrument to de­termine within what standard indicator range the pH o f the sample falls. Adding these pH indicator discs to the chlorine disc used with the Comparator, provides a dual purpose testing outfit at the extra cost of the disc and indicator only.