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Mixers
Chemical Mixing Rules

Photography (Basic) - Introduction to photography and other graphic techniques
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weaken a developer and cause it to underdevelop and sometimes  stain  film.  Too  little  agitation  during  mixing may cause the powdered chemicals to settle to the bottom of the mixer and form hard lumps. When these lumps of chemicals are undissolved and undetected, they can clog pumps and plumbing during transfer from the mixer to the storage tank. These lumps can also cause the solution to be less active. Agitation mixers circulate solutions through a pump that causes a stirring action. There are several types of agitation  mixers  available.  These  include  large  capacity models for preparing large volumes of solutions and small models for making small amounts of solution. Impeller  Mixers Impeller mixers provide thorough, rapid mixing, but they must be used with care to prevent frothing or foaming and introducing air into the solution. The solution must be mixed so a minimum amount of air is drawn into it. When the shaft is placed in the center of the container, the impeller causes a whirlpool effect that introduces excessive amounts of air into the solution. Furthermore,  when  the  shaft  is  in  the  center  of  a container, there is very little agitation in the bottom- center area of the container and undissolved chemicals pile up directly beneath the end of the shaft (fig. 9-5). Avoid bumping the shaft or impeller on the sides or bottom of the mixing vessel. This procedure may bend the mixer shaft, and a bent shaft produces excessive vibrations  that  can  ruin  the  motor  bearings. WEIGHTS AND MEASURES 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. 9-6







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