Thermo Scientific APEX 100 User Manual page 305

Factors Affecting the Sensitivity of the Detector
Sensitivity capabilities of different detectors used in different applications will vary. A
smaller aperture is capable of detecting smaller pieces of metal. Product effect may also
interfere with the detection capability. It is dangerous to rely on a corporate standard to
determine and test the detector's operation.
Ideally, each detector should have its own standards of operation and a corporate
outline should be used only as a maximum allowable guide. Sample sizes should be
selected so that they are clearly detectable (peak signal size of approximately double
the detect level), when compared to the signal produced by the product or other
interfering signals. If samples are established which are very close to the product signal,
frustration on the part of operators can lead to a lack of confidence in the detector's
operation.
The detector should be tested at its normal operating speed. Test pieces should be
inserted so that they travel through the detectors at the product's normal rate of flow.
This ensures that the detector and reject device responses are accurately tested.
The detector is not equally sensitive to all types of metal. Depending on the type of
product and application, there can be three typical metal groups, which will produce
three different levels of detection, as follows.
• Ferrous—typically, any magnetic metal is the easiest to detect.
• Non-ferrous—any good electrical conducting metal such as aluminum, copper,
brass, and so forth.
•
Stainless steel—the 300-series stainless steels, which are non-magnetic, tend to
be the most difficult metals to detect.
Test samples used should contain spherical metal contaminants. Any other shape will
produce a different size signal depending on its orientation as it passes through the
detector. This can lead to inconsistent results. In addition, any test procedure
established must allow for the test product to be completely rejected by the reject
device. The reject device will tend to be the most likely point of failure in any detector
system.
Care must be taken so that if the detector or reject device fails to operate correctly, the
test sample can still be recovered. This can be achieved by the use of a second valve or
a removable catch grid, which can operate as a fail-safe device, catching the test sample
and allowing product to flow through. Contact Thermo Fisher Scientific for assistance—as
described in the "Contacting Thermo Fisher Scientific" section at the end of the manual.
Note: If insertion testing is difficult to achieve, "side of the pipe" method may be used
for frequent sensitivity testing. Adjustment to the test sample size should be made to
compensate for the increase in sensitivity of the metal detector as you near the side of
the aperture. Under most circumstances the metal detection will be approximately 0.5 to
1.0 mm more sensitive as you reach the edge of the aperture. If this method is used it is
highly recommended that you use reject confirmation to ensure the correct performance
of the system.
Testing Schedule
The user must decide how often the detector should be tested. If the test procedure can
be designed to be simple, it will help ensure that the test will be performed more
Thermo Scientific APEX User's Guide Page 305