GE CRx25P Brochure Scaricare il pdf (Pagina 37)

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8.2 Film type selection

Most procedures and codes of good practice for the performance of industrial radiography

base the choice of type of film for a specific application on the EN or ASTM classification

systems. For weld inspection, when one is attempting to detect small cracks, a film of class

C2 or C3 would be specified. For the examination of castings or general radiography a film

of class C4 or C5 would normally be used. For small component inspection, where the

image might be viewed under magnification to reveal small details, a film of class C2 or

possibly even a single emulsion film of class C1 would be desirable.

In megavoltage radiography, because most equipment have a very high radiation output,

class C3 films can be used for objects of great wall thickness. This has the advantage that a

high film gradient can be achieved.

8.3 Film sizes

Film sizes in industrial radiography are to a large extent standardised according to

ISO 5565. Non-standard sizes are possible. Standard film sizes and metal screens are

supplied separately, but can also be supplied vacuum-packed so that the risk of film faults

is considerably reduced. For weld inspection there is so-called Rollpac film strip on the

market which is available on a roll together with the lead screen. For very large projects the

strip film can even be pre-cut to suit a particular weld length or pipe/vessel circumference.

8.4 Handling and storage of unexposed films

The conditions under which unexposed films are handled and stored play a very important

role in the final quality of the exposed film. Recommendations for handling and storage

are contained in, for example, ASTM E1254. “Pre-exposure” as a result of background radi-

ation must be avoided as it causes unacceptable fogging of the film.

If films are to be kept for a longer period, the following storage conditions must be

adhered to:

• background radiation levels below 90 nGray

• temperatures below 24˚C

• relative humidity levels below 60 %

• away from X-ray film chemicals

• preferably stacked on edge

In the long run, minor fogging will occur to films stored. Background fog to a maximum

density of 0.3 is considered acceptable.

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GE CRx25P Brochure Pagina 37