HYDROMETERS237.223Figure 9-1.–Read tbe lower line of the meniscus when measuringliquidsthermometer, your eyes should be level with the top ofthe liquid column in the capillary tube; otherwise, thereading may be off as much as 2 or 3 degrees. This erroris due to the refraction of the cylindrical magnifier thatis built into the capillary tube.Most Navy photographic labs have metal, dial typeof thermometers made of corrosion-resistant steel. Theyhave a long, thin metal stem, or rod, with a circular dialindicator at the top. The action of this thermometer isremarkably fast, and the dial is easy to read.The accuracy of all lab thermometers should bechecked regularly against one of known accuracy, suchas a Kodak process thermometer.Another measuring device used in photography isthe hydrometer. A “hydrometer” is used to determine thespecific gravity of a solution. A specific gravity checkis one of the first tests to verify the dilution of a solution.When the same chemicals are used and when the samequantity of chemicals and an equal volume of water areused each time, the resulting liquid is approximately thesame specific gravity each time. This is a characteristicof that particular solution when all specific gravitymeasurements are made at the same temperature.The specific gravity should stay within an upper anda lower limit as determined by the manufacturer for eachsolution. Variations beyond the upper limit-indicating adenser or heavier liquid-suggest that more than theprescribed amount of one or more of the ingredients hasbeen used, an ingredient foreign to the solution has beenadded, or not enough water was added to the solution.Measurements that fall below standard limits mightindicate that something has been left out, that a foreignchemical has been substituted, or that more than thecorrect amount of water was added.The silver content of a fixing bath increases as thebath becomes exhausted. This causes the specificgravity of the solution to rise. Hence, in addition totesting the consistency of chemical solutions, specificgravity tests may be used to check the amount of silverin the fixing bath. A hydrometer used for this purposemust be calibrated in grams of silver per liter of solution.A hydrometer consists of a hollow tube with anenlarged lower section, or float, topped by a narrowstem. The lower section is weighted, so the hydrometerwill float in liquids with its stem protruding from thesurface. The stem is graduated with marks that are usedto indicate the density of the liquid in which thehydrometer floats. When the density of the liquid is high,it supports the hydrometer more easily, so less of thestem is submerged. Less dense liquids allow thehydrometer to sink deeper.Hydrometers are commonly graduated in terms ofspecific gravity. Specific gravity is the ratio of thedensity of a substance to the density of distilled water.However, hydrometers designed for special purposeshave different types of graduated scales. An example isthe hydrometer that is used to check the silver contentof a fixing bath.Because of the effects of surface tension andcapillary action, a meniscus is formed at the interfacebetween the solution and the hydrometer stem. The9-3
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