Avoid External Obstructions; Avoid Source Cross-Talk - Vega FiberFlex LFXG-H Operating Instructions Manual

Installation
If the vessel has a central agitator, the source holder and detector can
mount to the vessel on an arc other than a diameter, so the beam of
radiation does not cross the agitator. You can avoid other obstructions this
way.

Avoid external obstructions

Any material in the path of the radiation can affect the measurement. Some
materials that are present when the gauge initially calibrates pose no
problem because the calibration accounts for their effect.
Examples:
• Tank walls
• Liners
• Insulation
However, when the materials change or you introduce new ones, the gauge
reading can be erroneous.
Examples:
• Insulation that you add after calibration absorbs the radiation and causes
the gauge to erroneously read upscale.
• Rapidly changing tank conditions due to material buildup. Regular
standardizations compensate for slowly changing tank conditions due to
material buildup. See the "Calibration" chapter for information on
standardization.

Avoid source cross-talk

When multiple adjacent pipes or vessels have nuclear gauges, you must
consider the orientation of the source beams so each gauge senses
radiation only from its appropriate source.
The best orientation, in this case, is for the source holders to be on the
inside with radiation beams pointing away from each other.
30 LFXG-H Installation and Operation Guide