AMMTIAC Quarterly, Vol. 2, No. 2 - Advanced Materials ...

System Expenses
At approximately twice the cost, typical CR systems are more
expensive to purchase than film processing systems. In general,
depending upon system configuration, conventional CR
systems approach a purchase cost of $125,000. It is important
to note that these costs are dropping, while the cost of the
film-based system is staying the same or increasing. Regarding
recurring costs, each IP costs approximately $550 to $700,
nearly equal the cost for 100 sheets of film. An important difference
is that these plates can be reused up to 5000 times.
The IPs require occasional cleaning and other maintenance.
While climate control for the IP storage is not necessary, moisture
can be a considerable problem. Moisture and the presence
of dirt and grime will shorten the life of the IP.
There are two pieces of support equipment used for traditional
radiography methods that will still be needed in a CR
system. These are film identification units and some type of
storage cabinets for CD-ROMs.
Figure 3. A Closer View of the Inspection Results.
curve. Standards for accepting and rejecting inspection results
are still being developed. Although many recurring expenses are
reduced, some still exist and the up-front costs of CR are substantial.
Finally, the complexity of CR systems warrants careful
consideration on where they will be located by those planning
to implement the systems.
The Learning Curve
Conventional film-based radiography has well established procedures
for carrying out radiographic techniques. These procedures
include the amount of radiation, length of exposure and
resulting image quality. Because, as previously noted, CR typically
requires less radiation and shorter exposure time, operators
need to learn how to optimize the parameters in order to
achieve acceptable results. In addition, the spatial resolution of
the images (i.e., how coarse or fine the image is) affects the
interpretation of the results. There is concern that until training
and standards are well established and coordinated, images
potentially could be over-analyzed, and anomalies that would
have been acceptable under wet film processing may now cause
the part being inspected to fail the test.
Standards for Accepting or Rejecting Results
Current accept/reject standards are based on film as well as
the proven history of how defects will appear in a film-based
system. Changing the capabilities of the imaging system also
changes the predictability of results. Further research is needed
to compile a catalog of digital results which are subject to
an analytical process that addresses probability of detection
(POD), probability of failure, desired or expected service life,
etc. This type of work is still in the early stages of progress.
Pending new standards, CR will not be accepted for certain
types of inspections.
System Location Requirements
The intended installation location should be carefully planned
and evaluated prior to proceeding with the implementation due
to the complexity of CR systems. The temperature of the housing
facility should be stable, and there should be no heat
sources (including direct sunlight) within close proximity. As
noted above, moisture and excessive dust and corrosive gases
would also degrade system performance; humidity and ventilation,
therefore, should be considered. Constant vibration and
shock must be avoided as well.
COMPUTED RADIOGRAPHY SYSTEMS IN NAVY REGION
NORTHWEST
CNRNW acquired CR systems (see Figure 4) through the
PPEP for three separate Navy activities. Two activities received
their systems in 2003 and have used them for several informational
inspections with favorable results. Based on those results,
CNRNW requested an additional system from PPEP for a
third activity; it was installed in early 2006.
Figure 4. An NDI Inspector Uses an IP Reader and Workstation.
http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 2, Number 2 13