Thermo Scientific RO-20, Operating Manual

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Ro-20 RB-XXX-XXXXXX E 3-3
Löw 08.10.2008
Note The silica gel desiccant should be changed as soon as it shows clear or pink crystals.
The amount of ion current produced depends upon the intensity of the radiation field and the
density of the air in the chamber. The density depends on the temperature and pressure of the
air. A temperature- sensing compensation circuit corrects for the change in ion current due to
temperature changes, leaving only the pressure effects to be considered external to the
instrument.
Table 2-1 and Table 2-2 show correction factors that are useful when the elevation
at calibration is different from the elevation during usage. Normal atmospheric pressure
variations at any one location are small enough to be ignored.
A voltage gradient must exist across the ion chamber to move the ions and produce the ion
current to be measured. All of the chamber structure is maintained at chassis potential, but the
center electrode disk and all the remaining circuitry is biased at –36 volts relative to the
chassis. Three 12-V alkaline batteries provides this voltage. Except for insulation leakage and
the very few microamps taken during battery check, the alkaline battery (3ea 12V) is
essentially open circuited and will last indefinitely.
3.2.2 The Circuit
The first stage of operational amplifier A101 is connected to the ion chamber. (See Figure 3-3
and Figure 3-4 .)Also connected to the chamber are the elements in the feedback loop,
consisting of two high- value resistors, a reed relay, and a capacitor. The switching in the loop
is done by two reed relays. One of the relays is used to short out the components so that any
voltage offset may be adjusted out with the Zero Knob. That relay is closed when in the two
battery-check positions (to provide maximum battery load), and in the Zero position of the
Function Switch. It is open in the other Function Switch positions
Figure 3-3: Detector board component layout