FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

5. QUALIFYING THE IMAGE
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In comparing two methods of producing radiographie images, the basic consideration
is the qualification of the image against a specified standard.
Using a given exposure geometry or technique, an image must be qualified
before it is interpreted. Qualified images meet sensitivity and density
requirements; they are also properly identified for purposes of traceability.
Sensitivity of the film is assured with the use of properly placed image
quality indicators (IQI's). Placed next to the weld at specified increments,
image quality indicators assure the inspector that image contrast sensitivity
and resolution are adequate to visibly detect discontinuities in accordance
with the governing standard. In the United States, the standard API 1104 1
or ASME^ hole penetrameter is the image quality indicator. "2%-2T" sensitivity
Is a common requirement of the film Images. The penetrameters are
chosen with regard to pipewall thickness and weld reinforcement. "2%" refers
to the thickness of the penetrameter as related to the thickness of the steel
that the radiation must penetrate. When "2T" is specified, it is required
that the "2T" hole in the penetrameter appears in the radiograpic image.
In Europe, wire "DIN" image quality indicators are the standard. The ability
to resolve wires of decreasing diameter on a given specimen relates directly
to increasing image sensitivity. A lesser known image quality indicator is
called the CERL IQI. It is composed of three parts; the first part is a
flexible step wedge which is used to measure thickness sensitivity. The
second and third parts are metal wires which are closely spaced and dimensionally
graded in geometric progression. The unsharpness of the image is
measured by the least discernable spacing between two wires. CERL IQI's
might be best suited for measuring the resolution and contrast sensitivity of
television type radiographie images, where the resolution and sensitivity are
partially a function of the pixel density that comprises the image.
The density or relative darkness of an image is a second consideration in
qualifying a radiograph. Density is measured on a scale called Hurter-
Dreffield (H&D) units. Lighter film obtains lower valued H&D units while
darker film obtains higher values. For film radiography, the range of
acceptable density falls approximately between 1.8 and 4.0.
Most specifications require that identification of weld images appear on the
radiograph so that the image'can be traced back to the weld. Improper identification
can be cause for rejection of the radiograph. Currently, lead
numbers and flashing are the primary means for identifying radiographs.
6. INSPECTION TIME
The inspection cycle time is the time commencing with the arrival of the pipe
at the inspection station and ending with the final interpretation decision
on weld quality. In conventional film radiography, the following sequence of
events occur during the cycle time: