The different systems of weights and measures used in chemical mixing and the relationship of the various units to one another are matters that every photographer who prepares photographic solutions should under-stand.

These days, photographic chemicals are pre-packaged and are usually published in two systems of weights and measures: avoirdupois and metric. In the avoirdupois system, chemicals are weighed in ounces and pounds and are dissolved in pints, quarts, or gallons of water. In the metric system, they are weighed in fractions or multiples of grams and are dissolved in cubic centimeters or liters of water. With a conversion table, a formula given in one system can be easily converted to the other.

Weight and Volume Conversion

Two systems of temperature measurement are used: Fahrenheit and Celsius. The Fahrenheit scale uses F as a temperature symbol. The Celsius scale uses C as its symbol. On the Fahrenheit scale 32 degrees is the freezing point of water, and the boiling point is 212 degrees. The difference is 180 degrees. The Celsius scale is 0 to 100 degrees from freezing to boiling. One degree Fahrenheit is smaller than one degree Celsius, one Fahrenheit degree being 5/9 of a Celsius degree. To convert Fahrenheit degrees into Celsius, subtract 32, multiply by 5 and divide by 9; that is, (F - 32) x 5/9 = C. To convert Celsius to Fahrenheit, multiply by 9, divide by 5, and add 32; that is, (C x 9/5) + 32 = F.

Some formulas use the word parts as a measure. They may call for two parts of one chemical, one part of another, and any number of parts of water. This is frequently done when two or more stock solutions must be combined to make the working solution. In such cases, the word parts means any convenient volume measurement may be used; however, the same measure should be used for everything required by the formula. A part may be a fluid ounce or a gallon, depending upon the total quantity of working solution needed Formulas use parts only when volume is to be measured.

The term stock solution identifies a concentrated chemical solution. A working solution is the solution used for processing. The working solution may be the same as the stock solution, but more than likely it is a diluted stock solution.

Figure 9-5. Impeller mixers.