Fluke Biomedical 440RF - Manual (Page 21)

Operation
Chamber Corrections
2
2-13
Additional Equilibrium Wall Material
For effective energies above 250 keV, additional equilibrium wall material should be used. When the
energy response correction curve was generated, a ¼ inch thick plastic plate was used. The plate area
was approximately the same area as the magnesium window and it was placed directly in front of the
window.
Any plastic may be used. It is not critical.
Temperature and Pressure
Air, at standard temperature and pressure, is the ionized medium in our exposure rate definition. The
density of air directly determines the ionization rate with all other factors held constant. The density is, in
turn, a function of barometric pressure and Kelvin temperature. Since the ion chamber is not sealed,
correction must be made for temperatures and barometric pressures different from those used during
calibration.
Standard temperature, 0° C, is rarely encountered in practice. The Model 440RF/D is calibrated and
corrected to 22° C and 760 mmHg. The instrument will automatically compensate for temperatures and
pressures different from those used during calibration using a pressure transducer and temperature
sensor located inside the case. Correction factors for changes sensed by these elements are stored in
memory. They are also software controlled.
Figure 2-9. 440RF/D Energy Response Correction Curve
2.8 Low Level Measurements
Non-zero and Fluctuating Indications
If the instrument is set to the 1 mR/h or the 3 mR/h range and there is no external radiation, the meter will not show a
zero indication; but will continuously fluctuate slightly upscale from 0. Occasionally, the needle will move to the ½
mR/h range on the scale. This is to be expected in an instrument with this sensitivity and response time. The needle
fluctuations in the 1 mR/h or the 3 mR/h range can be attributed to four sources.
NOTE
Correction Fac
tor (Mul
tiply reading by)