7 DEVIANT SETUPS
7.1 Passive DRYSTIK (ADS-1)
Use of a 12” passive DRYSTIK is not really a deviant
setup but rather a supplement to the standard setup.
Th
e DRYSTIK may be installed with the membrane
tubing upstream of the desiccant and the purge line
between the RAD7 outlet and the aerator.
Great care should be taken to ensure that no liquid or
foam enters the membrane tubing. Water inside the
DRYSTIK can, at best, temporarily disable it and at
worst destroy it.
With normal, clean water, the DRYSTIK placed
vertically above the aerator and with 12” of tubing
between the two, there should be no problem. But if
the water sample is particularly foamy, the DRYSTIK
should not be used in the system until it is
determined the setup is such that no foam will climb
up into it.
With the 12” DRYSTIK installed the RAD7/RAD
H
2
O system will behave normally in every respect
except that the desiccant will last about
fi
ve times
longer before it needs to be regenerated or replaced.
WAT 40 protocol will give readings of the radon in
the water when 40mL vials are used and similarly
WAT250 when 250mL vials are used.
7.2 Large Drying Unit
A large “laboratory” drying unit, as used for 2-day
protocol monitoring, may be used with the RAD H
2
O
but it increases the volume of air in the system, so
reducing the concentration of radon in the loop a
ft
er
aeration of the sample. To accommodate the change
in air-loop volume a multiplying factor of 2.0 must be
applied to the RAD H
2
O reading.
Th
us a reading of
radon in the water of 300 pCi/L taken with a
laboratory drying unit in the setup instead of a small
drying tube, the radon concentration in the water was
600 pCi/L.
Th
e multiplying factors for 40mL vials and 250mL
vials are su
ffi
ciently close to the same that only one
fi
gure needs to be remembered.
Th
e multiplying factor of 2.0 was derived from a
series of experiments performed at DURRIDGE
Company.
Th
e precise factor for any setup depends
also on the choice and length of tubing.
For the most reliable assessment users should
perform their own experiments with their own setup.
Collect a number of equal samples - say six at least.
Be very careful in the sample taking to be sure they
are all indeed the same. Analyze half the samples with
a standard setup and the other half with the deviant
setup to determine the average multiplying factor.
Corrections for sample decay over the period of the
experiment should be applied.
At the end of each analysis a big proportion of the
radon will be in the drying unit. It is necessary to
purge this out of the system before the start of the
next reading. To that end, the drying unit and RAD7
must be purged for at least ten minutes a
ft
er each
measurement.
Please note that by increasing the air volume the
sensitivity of the system is reduced. With a large
drying unit installed instead of the small drying tube
the sensitivity is halved.
Th
us the lower limit of
detectability is doubled and the uncertainty of any
reading is increased by SQR(2) or by a factor of 1.4.
7.3 Oversized Dome
Some RAD7s have high-gain modi
fi
cations installed,
one of which may be an oversized measurement
chamber, or dome.
Th
is will increase the volume of
the air loop.
For an otherwise standard setup, the multiplying
factor to compensate for the oversized dome is 1.2. If
the large dome setup also uses the laboratory drying
unit instead of the small drying tube, the multiplying
factor will be 1.68.
7.4 Extended Cycle Time and Cycle Count
A
ft
er choosing the preset protocol WAT 40 or
WAT250, depending on the size of vials used, both
the cycle time and cycle number (Recycle) may be
increased to give more counts and hence higher
sensitivity to the radon-in-water measurement.
Th
e pump will, in any case, stop a
ft
er 5 minutes,
which is long enough to aerate the sample.
Th
e
fi
nal
reading will be the same as for standard protocol
except that it will be more precise. So no multiplying
factor is required.
Section 7
Deviant Setups
27
