3 RAD H
2
O TECHNIQUE
3.1 The Closed Loop Concept
Th
e RAD H
2
O method employs a closed loop
aeration scheme whereby the air volume and water
volume are constant and independent of the
fl
ow
rate.
Th
e air recirculates through the water and
continuously extracts the radon until a state of
equilibrium develops.
Th
e RAD H
2
O system reaches
this state of equilibrium within about 5 minutes, a
ft
er
which no more radon can be extracted from the
water.
Th
e extraction e
ffi
ciency, or percentage of radon
removed from the water to the air loop, is very high,
typically 99% for a 40mL sample and 94% for a 250
mL sample.
Th
e exact value of the extraction
e
ffi
ciency depends somewhat on ambient
temperature, but it is almost always well above 90%.
Since the extraction e
ffi
ciency is always high, we see
little or no temperature e
ff
ect on the overall
measurement.
3.2 Desiccant
Th
e RAD H
2
O requires that the desiccant be used at
all times to dry the air stream before it enters the
RAD7. If the desiccant is not used properly, the
RAD7 may give incorrect radon concentrations, or
may become damaged due to condensation on
sensitive internal components.
For water sample analysis, always use the small
drying tubes supplied, as the system has been
calibrated with these tubes. Do not use the large
drying column as its much larger volume would
cause improper dilution of the radon.
Make it a habit to inspect the RAD7 humidity
reading to be sure the desiccant is and has been
e
ff
ective through the entire measurement session. All
relative humidity readings during the measurement
should remain below 10%. In the worst case, at least
the
fi
rst two counting cycles should be below 10%. If
the relative humidity is higher than that, then the
RAD7 should be purged, see below. See the RAD7
Operator's Manual for more information on
maintaining the desiccant.
3.3 Purging the System
A
ft
er performing a water or air measurement, the
RAD7's internal sample cell will continue to contain
the radon that was measured. If this radon is still
present when you start a new measurement, it will
erroneously in
fl
uence the next measurement.
Th
is is
of special concern when the radon concentration of
the last measurement was high relative to the next
measurement. To prepare for the next water
measurement, you must remove, as thoroughly as
possible, the radon from the RAD7 and its air
conducting accessories, including the aerator head,
tubes, and desiccant.
Th
is procedure is known as
"purging the system."
To purge the system, you must have a source of
radon-free (or relatively radon-free) air or inert gas.
For most occasions ambient air is good enough, but
see below. Put the RAD7 into a purge cycle with the
"Test Purge" command, and allow the RAD7 pump to
fl
ush the clean air through the entire system for at
least 10 minutes. A
ft
er measuring very high radon
concentrations, you should purge the system for at
least 20 minutes. A purge time of 30 minutes should
be long enough to remove almost all the radon a
ft
er
measuring a sample at 100,000 pCi/L.
Be sure to allow all the hoses and
fi
ttings to
fl
ush
thoroughly by keeping them attached during the
purge cycle for at least the
fi
rst
fi
ve minutes. Also be
sure that the drying tube does not deplete its
desiccant during the purge cycle. If the depleted
(pink) desiccant gets to within 1 inch of the drying
tube outlet, replace the tube with a fresh (blue)
drying tube. A
ft
er the
fi
rst two or three minutes of
purging, you may replace the small drying tube with
the large laboratory drying unit, to conserve the small
drying tube desiccant, and continue purging the
system.
Be careful about the air you use to purge! Ambient air
may not be adequately free of radon to properly
prepare the system for a low level sample.
Th
e radon
present in the purge air will add unwanted
"background" to the next measurement. For example,
a purge air radon concentration of 4 pCi/L will give
about 4 x 25, or 100 pCi/L additional radon
concentration to the next water result (40mL water
sample).
Th
is is too much background to neglect
Section 3
RAD H
2
O Technique
16
