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Mixing, Testing, and Storing Equipment
pH Meters

Photography (Basic) - Introduction to photography and other graphic techniques
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HYDROMETERS 237.223 Figure 9-1.–Read tbe lower line of the meniscus when measuring liquids thermometer, your eyes should be level with the top of the liquid column in the capillary tube; otherwise, the reading may be off as much as 2 or 3 degrees. This error is due to the refraction of the cylindrical magnifier that is built into the capillary tube. Most  Navy  photographic  labs  have  metal,  dial  type of  thermometers  made  of  corrosion-resistant  steel.  They have a long, thin metal stem, or rod, with a circular dial indicator at the top. The action of this thermometer is remarkably fast, and the dial is easy to read. The accuracy of all lab thermometers should be checked  regularly  against  one  of  known  accuracy,  such as a Kodak process thermometer. Another measuring device used in photography is the hydrometer. A “hydrometer” is used to determine the specific gravity of a solution. A specific gravity check is one of the first tests to verify the dilution of a solution. When the same chemicals are used and when the same quantity of chemicals and an equal volume of water are used each time, the resulting liquid is approximately the same  specific  gravity  each  time.  This  is  a  characteristic of  that  particular  solution  when  all  specific  gravity measurements are made at the same temperature. The specific gravity should stay within an upper and a lower limit as determined by the manufacturer for each solution.  Variations  beyond  the  upper  limit-indicating  a denser  or  heavier  liquid-suggest  that  more  than  the prescribed amount of one or more of the ingredients has been used, an ingredient foreign to the solution has been added, or not enough water was added to the solution. Measurements that fall below standard limits might indicate  that  something  has  been  left  out,  that  a  foreign chemical has been substituted, or that more than the correct amount of water was added. The silver content of a fixing bath increases as the bath becomes exhausted. This causes the specific gravity of the solution to rise. Hence, in addition to testing the consistency of chemical solutions, specific gravity tests may be used to check the amount of silver in the fixing bath. A hydrometer used for this purpose must be calibrated in grams of silver per liter of solution. A hydrometer consists of a hollow tube with an enlarged lower section, or float, topped by a narrow stem.  The  lower  section  is  weighted,  so  the  hydrometer will float in liquids with its stem protruding from the surface. The stem is graduated with marks that are used to indicate the density of the liquid in which the hydrometer floats. When the density of the liquid is high, it supports the hydrometer more easily, so less of the stem  is  submerged.  Less  dense  liquids  allow  the hydrometer  to  sink  deeper. Hydrometers are commonly graduated in terms of specific gravity. Specific gravity is the ratio of the density of a substance to the density of distilled water. However,  hydrometers  designed  for  special  purposes have  different  types  of  graduated  scales.  An  example  is the hydrometer that is used to check the silver content of a fixing bath. Because  of  the  effects  of  surface  tension  and capillary action, a meniscus is formed at the interface between  the  solution  and  the  hydrometer  stem.  The 9-3







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