Two independent things limit how much of a girth weld a single exposure can record. The first is simply how much film is there: a cassette of diagnostic length L laid on a pipe of outside diameter D covers an arc of 360 x L / (pi x D) degrees. The second is beam obliquity: away from the perpendicular the beam passes through 1/cos(phi) times the nominal wall, and once that ratio exceeds what the density band and IQI will tolerate the film is no longer diagnostic even though it is still exposed.

Which one bites depends on the technique. On a panoramic crawler shot the beam is perpendicular everywhere, obliquity is not a limit at all, and film length alone decides how many cassettes go round the joint – typically two or three overlapping strips on a large-diameter line. On an external shot the obliquity limit is far more restrictive than film length, so the arc is set by the thickness ratio and the film is longer than the diagnostic area it carries.

Diagnostic length is not physical film length. Both ends carry identification, location markers and the overlap that proves adjacent films joined without a gap, and none of that counts as evaluated area. Deduct the overlap and identification allowance before dividing into the circumference, and always round the film count up – a joint short by half a film is a joint not radiographed.

Location markers must be placed so that the evaluated coverage can be demonstrated on the film itself, which is what turns this arithmetic into an auditable record rather than an estimate.

Worked example

Pipe outside diameter	168.3 mm
Film / cassette length	360 mm
Overlap and identification allowance	25 mm
Permitted through-wall thickness ratio	1.1
Circumference	528.73 mm
Diagnostic film length	335 mm
Arc covered by one film	228.09 deg
Films to wrap the joint	2
Arc allowed by beam obliquity	49.24 deg
Governing arc per exposure	49.24 deg
Exposures for full coverage	8

Circumference = pi x 168.3 = 528.73 mm. After a 25 mm allowance the diagnostic length is 335 mm, which is 360 x 335 / 528.73 = 228.09 degrees of arc, so two films wrap the joint. Obliquity at a 1.1 ratio only allows 2 x acos(1/1.1) = 49.24 degrees, so obliquity governs and eight exposures are needed for full coverage.

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