Fluke Biomedical 10100AT - Manual (Page 51)

Theory of Operation
Ion Chambers
3
3-5
3.10.2 Air Density Corrections
The dosimeter corrects exposure and exposure rate readings and adjusts the displayed ion chamber
calibration factor according to the temperature and barometric pressure settings chosen while in the Air
Density screen, if applicable. The reference calibration conditions are:
Temperature: 22° C, 71.6° F, 295.2° K or 20° C, 68° F, 293.2° K
Sea Level Pressure: 760 mm Hg, 29.92 in Hg, 1013.25 mB or 1013.25 hPa
The calculations and methods are based on the following formulas:
calculated exposure air density corrected exp. or
or exposure rate
X
correction factor
=
exp. rate displayed
air density sea level pres.
temp. setting
converted to Kelvin
correction factor
=
pressure setting*
X
RefTemperature
* If chosen unit is in terms of altitude relative to sea level:
54041 – feet sea level effective
54041 + feet
X
pressure
=
pressure setting
16471.7 – meters sea level effective
16471.7 + meters
X
pressure
=
pressure setting
3.11 Electrometer/HV Isolator/Ranging Amplifier
The Model 35050AT Dosimeter is equipped with a triaxial-BNC ion chamber input connector. This
connector serves a dual purpose. First it supplies the 300 V bias potential to the ion chamber. It is this
bias potential which causes the radiation induced ionization of the air within the ion chamber to result in
the collection of an electrical charge. As its second function, the connector provides a guarded path for
the extremely small ionization charge that is produced within the ion chamber to be carried to the
electrometer measurement circuit. It is the primary function of the Electrometer / HV Isolator / Ranging
Amplifier to transform these small electrical charges to signals which can be easily measured by the A-D
converter.
The electrometer is a dual range resettable integrating amplifier. Both the reset and the integrator
ranging functions are performed by reed relays under control of the microcontroller. By dosimeter
convention, and for operator safety, the outermost conductor of the triaxial interconnect system is
maintained at instrument case (or ground) potential. This arrangement requires that the electrometer’s
input be elevated to the 300 Volt bias potential above ground for proper ion collection.
An analog isolator performs the functions of powering the electrometer at this elevated potential and
returning the signal produced by the electrometer to ground level. Under control of the microcomputer, a
final analog ranging amplifier stage provides three voltage gains (X1, X10 and X100).
This combination of a dual range electrometer with a three range amplifier results in 6 possible ranging
combinations. The system has been set up so that the middle pair of ranges are completely overlapping.
Thus, the result is a signal-conditioning block that can be ranged over 5 decades under control of the
microcomputer. The most sensitive range has a full scale of 10 pC and an effective resolution of 5 fC.
Similarly, the least sensitive range has a full scale of 100 nC and an effective resolution of 0.5 pC.
In order to eliminate the need for resistive feedback elements and the associated low level switching, all
current measurements are derived by measuring the quantity of charge integrated over a 1 second
interval. Thus, five decade current measurement ranges are derived. The most sensitive range has a full