Fluke Biomedical 440RF - Manual (Page 15)

Operation
Measurement of Exposure Rate
2
2-7
Obtaining Readings from Indications
The instructions supplied here generally apply to high and low level measurements.
RESPONSE TIME
The measuring system requires a certain amount of time to respond to a change in exposure rate. The
response times supplied in Table 1 are times required to reach 90% of the final value. A 10% error, due
to response time, remains. If twice the quoted response time is allowed, the time error will fall to less than
1%. Therefore, in all measurements, sufficient time must be allowed before obtaining a reading from the
meter.
METER ARCS AND RANGE POSITION CORRESPONDENCE
The 0 to 10 meter arc contains 10 major and 50 minor divisions. It corresponds to the 0-1, 0-10 and 0-100
mR/h (0-10, 0-100, 0-1000 μSv/h) range positions.
The 0 to 3 meter arc contains 6 major and 30 minor divisions. It corresponds to the 0-3 and 0-30 mR/h
(0-30 and 0-300 μSv/h) range positions.
READING THE METER INDICATION
Using the formula provided in Figure 2-4, it should be relatively simple to set the decimal point and make
interpolations.
Applying Corrections To Readings
After a reading has been obtained, the operator should be able to determine if the correction factors
described in Sections 3.6 and 3.7 apply. If no correction factors apply, or if a relative indication is desired,
the readings can be accepted as final data. If there are correction factors that apply and accuracy is
required, then those factors must be applied before readings can be accepted as final data.
Correction factors are numbers, ranging around unity, which are multiplied with the reading to give a
corrected reading. They are employed to correct for errors contained in the initial reading due to some
non-standard measurement condition.
Corrections are not required when a maximum inaccuracy of 25 to 30% can be tolerated, e.g., during a
casual survey. However, when the highest possible accuracy is desired, all likely sources of inaccuracy
should be considered with corrections and compensations applied where necessary.
Arbitrary Standards
Government and/or industrial agencies may adopt a standardized measurement or set of conditions for
measurement. When this is done, it is helpful to know which correction factors will apply and which will
not apply. Generally, when a specific detector cross-sectional area and an external surface to center of
effective volume spacing is used, the geometry factors are fixed and correction factors related to
geometry are not required. Energy and air density corrections, will apply when maximum accuracy is
desired.
2.6 Source Geometry Corrections
Proximate Source
In the exposure rate definition (refer to Section 2.5, Unit of Measurement) the need for a constant
radiation intensity throughout the chamber volume or for a suitable correction factor can be perceived.
Since ionizing radiation follows the inverse square law in the same manner as visible light, radio waves,
gravitational forces, etc.; the field is relatively constant throughout the chamber only at several times the
maximum dimensions of the chamber. In Figure 2-5, a calculated correction curve is supplied for point
sources of radiation which closely approach the chamber.