(Rt) Exposure Time-charts

EXPOSURE TIME / CHARTS:

( R. Holten )

EXPOSURE TIME CALCULATIONS The quality of radiographs and consequently the detection of flaws in materials depends to a considerable extent, on the density of image. Thus, the selection of exposure required to produce the radiography density for best revilement of detects is necessary. The radiographic exposure time for a particular set up can either be calculated directly or read off from exposure charts. Commercial X-ray machine are generally supplied with exposure charts. For gamma source, exposure charts can be prepared by the user or exposure can be calculated by using below equation.

n = job thickness expressed in number of HVLs. FF = Film Factor SFD = Source Film Distance [ cm ] SS = Source Strength [ Ci ] RHM = Roentgen Hour per Meter

The exposure charts are prepared by exposing a step wedge having different thickness, as shown in figure below, to different exposure doses. Five or six different densities corresponding to each thickness are obtained. The exposure for obtaining a given density (for e.g. 2.0), for each thickness are then found out. A graph is then plotted between thickness and exposure in Roentgens (in logarithm scale) for gamma rays or mA-mins for Xrays. This graph is called exposure chart.

Step Wedge

Sometimes, the exposure chart for gamma rays is plotted between curie × hrs (Exposure factor) Vs, thickness. In this chart, following factors are kept constant. 1. Radiation source

2. Film screen combination

3. Density of radiograph

4. Source to film distance ( SFD)

5. Processing conditions Then for the given object thickness, the exposure factor can be read off from the chart and exposure time calculated by the formula:

EF = Exposure factor (in Curies × Hrs .) for SFD =D 1 D2

= actual source to film distance

C

= source strength in curie

After knowing the exposure time for one metal, say steel, the exposure time under the same conditions for another metal of equal thickness can be calculated using the radiography equivalence factors. Exposure charts for steel with lr-192 and Co-60 are shown in figure below. Gamma-Ray Exposure Charts Manual Processing

Manual Processing

Automatic Processing

Automatic Processing

Iridium 192 Lead screen density 2,5 SFD=1m

Iridium 192 Lead screen density 2.5 , SFD = 1 m

Cobalt 60 Lead screen density 2,5 SFD=1m

Cobalt 60 Lead screen density 2.5 , SFD = 1 m

X-Ray Exposure Charts