First, the reason for the dip switch settings is to provide adjustable exposure levels depending on the speed of the film being used for processor quality control. Usually mammography film is used for processor quality control and there are many different mammography films on the market differentiated by speed and contrast characteristics.
According to the recommendations of the American College of Radiology (ACR), the speed of an x-ray film is measured at the step closest to (but not above) 1.20 Optical Density. In setting up a processor, establishing quality control AIM values for the first time, the customer must average all 21 steps of five films exposed and processed over five days (one each day). From the average obtained, they find the step number that yields a density closest to 1.20 in density. Then they read that step number every day from their exposed and processed strip to obtain the speed value for that day. The speed value is plotted on the QC chart.
It is not required that the step closest in density to 1.20 be at a certain step but through the years step #11 has become the desired speed step. Therefore, it is not uncommon for customers to want to adjust the dip switch settings on the back of the sensitometer to result in step #11 being closest to 1.20 OD. It is important to remind customers that they do not have to achieve 1.20 exactly on their 21 step strip. They only have to find the step closest to 1.20.
Exposure from the sensitometer increases from exposure setting 1 to setting 7. However, most x-ray film will utilize only steps 2, 3, or 4. The sensitometer arrives at the customer site on setting #3, which is considered the normal setting. Depending on the speed of the film being used, the customer would use setting #2, #3, or #4 to achieve a density closest to 1.20 at step 11.
The slower the film, the higher the exposure setting needed. The faster the film, the lower the exposure setting needed.
When a customer asks what dip switch setting they must be on, there are many variables involved. Therefore, even if they tell you what film they are using, the answer is not simple. The processing chemicals and developer temperature they are using can vary from facility to facility. The customer has to experiment. Tell the customer to expose three films, one each on setting (#2, #3 and #4). Of course, the film used in this experiment needs to be the film they will be using for quality control and all three films need to come from the same box. Process the films through the same processor and determine which setting yielded a density closest to 1.20 at step #11. This is the dip switch setting they should continue to use.