Radiography in Modern Industry - Kodak

Chapter 11: Process ControlUsers of industrial radiography must frequently meet requirements of density and sensitivity setforth in inspection specifications. They must also keep the radiograph rejection rate to aminimum. Control of variability in both exposing and film processing is essential if theserequirements are to be met. Although exposure and processing are the most frequent sources ofsignificant variations, other factors, such as intensifying screens and film, also contribute toradiographic variability.The exposure of industrial x-ray film to energy in the visible spectrum is not a reliable indicator ofthe process level or the repeatability of exposures to x-radiation. Therefore, exposure of thecontrol film to white light is not a satisfactory tool for controlling process variability in industrialradiography. In the procedure for controlling either the exposing unit or the film processing, orboth, described here, x-radiation is used to expose the control film.The data obtained from exposures to x-radiation can be utilized in many ways. The procedure issimple and not only reveals both exposing and processing variations but also differentiatesbetween them. It can be customized to fit specific requirements; a few suggestions for doing soare presented later. No attempt has been made to list all the measures available for reducingvariations in density inasmuch as the procedures required for adequate process control dependon the conditions in the laboratory and production specifications.Equipment And MaterialsMost of the equipment and the material needed to set up a meaningful program of processcontrol is readily available to industrial radiographers. The key items are:Electronic Direct-Reading DensitometerAccurate, precise densitometers capable of measuring diffuse density are available from dealersin photographic supplies.Calibrated Film StripThe strip is used to check the precision of the densitometer. If one is not available, a control stripon which previous readings have been recorded will suffice.Stepped WedgeThe wedge should be made of steel or the material most often tested. One step should be thickenough to permit the passage of radiation sufficient to produce a density of 0.6 to 1.0, anotherstep should be thin enough to permit passage of twice as much radiation to produce a density of2.0 or higher. Although not essential, it is helpful if the difference in the density produced byadjacent steps is uniform.FilmA box of industrial x-ray film of the type most frequently used in the normal production operationshould be reserved for the control program. Before all the film in this box has been used, a newbox should be reserved for the same purpose and the necessary data on the response of the newfilm should be obtained.RefrigeratorAfter exposure, control strips must be stored at 40°F (4.5°C) or lower. When the stock of controlfilm exceeds a six-week supply, the unexposed film should be stored in a refrigerator and nomore than a one-week supply should be removed at any one time. When the temperature and thehumidity are high (more than 75°F or 24°C and 50 percent relative humidity), unexposed filmshould be refrigerated regardless of the size of the stock.