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

Figure 9-2. Hydrometer.

reading is taken at the point where the top of the meniscus intersects the stem of the hydrometer (fig. 9-2).

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