6
Theory
Theory of Radiation Gaging
Radiation gages operate on the principle of radiation absorption and transmission.
A beam of gamma radiation is directed from the source holder, through the vessel and its process material,
and onto the surface of the detector.
Radiation which is not
absorbed
by the material through which it passes, is
transmitted
to the surface of the
detector.
Process measurement is possible because the amount of radiation
absorbed and transmitted
is predictable.
The absorbed radiation is directly related to the level of process material in the vessel while the transmitted
radiation is inversely related to the level of process material in the vessel.
Therefore, an
increased process level results in a decrease of transmitted radiation.
Since the radiation that's not being
absorbed
is being
transmitted
, the process level can be inferred by
measuring the amount of radiation reaching the detector at any point in time. The detector's output signal,
in counts, also
varies inversely
to the process level.
When the process level is low the detector is exposed to a maximum amount of radiation which produces a
HIGH output of counts. When the process level is high the process material "shields" the detector and
prevents radiation from reaching the detector, producing a LOW output of counts.
The X96S Microprocessor converts the detector signal to user's measurement units of level: m, mm, cm,
in, ft.
The X96S displays the output measurement range in the selected user units. The "zero" of the
measurement range represents the lowest level of interest, while the "span" of the measurement range
represents the highest level of interest.
Reduction of the signal "noise" due to radiation statistics is handled in the stage of signal processing known
as digital filtering. Digital filtering is a form of statistical averaging used to smooth, or dampen, random
radiation as well as process-related noise. Increasing the digital filter’s “time constant” decreases signal
noise.
Dynamic tracking permits the gage response to temporarily by-pass the digital filter. This is helpful in
some processes where sudden or drastic step changes in process must be observed in their true, or
unfiltered, state.
Software also compensates for the decay of the radioactive source activity. On-going adjustments are made
automatically for the rate of decay, or source half-life.
