Operating Manual

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When using the elliptical exposure technique, the images of the weld on the source side
and on the film side are shown separately, next to each other. The distance between two
weld images has to be approximately one weld width. This requires a certain amount of
source offset (O), relative to the perpendicular through the weld. The offset can be calcu-
lated with the following formula :
In which:
w = width of the weld
f = distance from source to the source side of the object, measured perpendicularly
D
e
= external pipe diameter
O = Offset distance
Perpendicular technique
Alternatively, the perpendicular technique can be used if the elliptical technique is not
practical, (see fig. 3-18). This is the case when, for instance, pipes of different wall thick-
ness are joined or a pipe is joined to a 45 °/ 90 ° bend.
Three exposures equally divided over the circumference are sufficient for 100% coverage.
Fig. 3-18. Perpendicular double wall-double image technique
Film Film
O= 1.2
.
w
.
f/D
e
18.4 Determination the depth position of a defect
The depth position (d) of a defect can be determined by the parallax-method, as shown in
figure 4-18. The radiograph is exposed from two opposite angles. The required quantity of
radiation is equally divided over positions A and B. Only one film is used.
The shift in defect image on the film (G in mm) is a measure for the depth position; the
shift of the source (A to B in mm) and the source-to-film distance (H in mm) are important
data. The depth position is calculated with the formula: d = (GxH) / (AB+G).
Another, much more complex method of depth determination is stereo-radiography, by
which two separate films are exposed which are viewed simultaneously via mirrors.
However, this method is rarely used.
Fig. 4-18. Determining the depth position of a defect
= defect shift
source shift