A step-wedge phantom is used to assess the quality assurance of:

How frequently processing solutions are replaced depends primarily on the rate of use of the solutions but also on the size of tanks, whether a cover is used, and the temperature of the solutions. In most offices, the solutions should be changed weekly or every other week. The results of the step-wedge test help determine the proper frequency. Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 251