increase the life expectancy of the pump. Air will
remain stationary in the RAD AQUA for four
minutes before moving to the desiccant where it waits
another four minutes before entering the RAD7.
While in the RAD AQUA, the stationary air may
approach equilibrium with the water thus inhibiting
further radon transfer from the water to the air. It
will be about 15 to 20 minutes before that parcel of
air returns to the RAD AQUA. We can therefore
estimate that the response time of the system will be
increased by about 20 minutes if the pump is set to
AUTO
Having the pump on AUTO would normally be
associated with having the RAD7 cycle time of 30
minutes or longer. So an extra 15 to 20 minutes on
the response time will not be excessive.
2.2.5 RAD7 Mode
In AUTO (the default) mode, the RAD7 will
automatically switch from SNIFF mode to NORMAL
mode a
ft
er three hours into the run.
Th
is is to take
advantage of the additional counts provided by the
214-Po decays that will, by then, have approached
equilibrium with the (steady) radon concentration in
the measurement chamber.
For slow, long-term measurements with long cycle
times AUTO mode for the RAD7 is appropriate.
Th
e
RAD7’s response time will be a couple of hours or so.
Th
e RAD AQUA can be running with a low water
fl
ow rate and the RAD7 pump can be on AUTO also.
For fast response, however, it is essential to force the
RAD7 to stay in SNIFF mode (
Setup, Mode,
Sniff
[ENTER]
). It will then always count only the
218-Po decays, giving it a 13-minute, 95% response
time.
2.3 Long Term Measurement
2.3.1 Desiccant
As set up, above, the system will continue making
measurements inde
fi
nitely.
Th
ere are, however,
various resources that are being used up in the
process, and which must be replenished.
Th
e most
obvious is the desiccant. A new, or regenerated,
laboratory drying unit will normally last for about ten
days of continuous use in a temperate climate. In this
application, however, it is receiving saturated air and,
therefore, will be hydrated more quickly. When the
remaining length of blue (dry) desiccant is less than
one inch, the desiccant should be replaced. Please see
the RAD7 manual on desiccant regeneration. If the
desiccant is not replaced, and the humidity in the
instrument rises above about 20%, then the
sensitivity drops o
ff
and the reading is lower than the
true value.
2.3.2 Memory
Th
e capacity of the internal memory of the RAD7 is
1,000 records or cycles. If each cycle is half an hour,
that would be data for 500 hours, or just over 20 days.
Every time the desiccant is changed, therefore, all the
stored data should be downloaded to a PC, backed
up, and erased from the RAD7 memory.
Section 2
Measurement Procedure
11
