Thermo Scientific APEX 100 User Manual page 301

Typical causes of intermittent electrical loops include loose bolts on the framework or on
the detector's mounting, corrosion of metal work, broken welds, open hinged doors, and
broken or rubbing contacts.
Interference can be overcome by opening the conductive path with an insulating pad or
closing the path by welding or tightening the bolts so that it cannot become intermittent.
The source of such loop interference can be very elusive and difficult to locate. The
larger the aperture in the APEX, the greater the high-frequency leakage out of the
aperture and the greater the possibility of trouble from loops.
Metal Objects Near the Search Head
The metal detector is very efficiently screened and metal near the top, bottom, and ends
will not significantly affect the detector performance. However, metal positioned close to
the aperture can cause interference problems, if it moves or vibrates. The area close to
the aperture, which should be kept metal free, is known as the metal-free zone. This
metal-free zone is dependent on the aperture dimensions and the sensitivity setting of
the detector. The search head is sensitive to moving metal. It is not always possible to
discriminate if the metal is passing through the head or moving outside the head. Metal
in the metal-free zone may cause the detector to be sensitive to vibration.
If the APEX vibrates or is accidentally bumped, it will cause relative movement between
the detector and the offending metal, which could trigger the detector.
Electrical Interference
It is always good practice to suppress electrical interference at its source and, if trouble
is experienced, the offending source should be located and suppressed. If this is not
possible, it may be necessary to operate the APEX at a reduced sensitivity.
A relay in the metal detector is often used to control solenoid valves, power contractors,
and similar electrical devices. When these devices are switched off, the collapsing field of
the winding generates a wide-band interference, which may be picked up by the search
coils in the head and cause false triggering.
A capacitor of suitable capacity and working voltage connected across the offending
device and grounded as close to the device as possible, will often provide satisfactory
suppression.
Pneumatic Air
Connect incoming air to the detector's air-regulator fittings using a coiled copper tube or
2
air hose. Adjust the incoming air pressure to 30–40 PSI (2.1–2.5 kg/cm ) for pneumatic
2
devices, or 40–80 PSI (2.8–5.6 kg/cm ) for air rejects.
Thermo Scientific APEX User's Guide Page 301