DURRIDGE RAD7 Gebruikershandleiding - Pagina 7

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2 MEASUREMENT PROCEDURE

2.1 Start Up

2.1.1 Temperature Probe

If you are using a Temperature Probe (sold

separately), load the temperature logger software and

connect the Temperature Logger to the PC using the

serial cable provided. Conﬁgure the logger to take

temperature readings at frequent intervals (these may

be far more frequent that the RAD7 test cycles.)

Choose the second (external) temperature sensor.

Connect the Temperature Probe to the logger and

note that when you hold the probe the indicated

temperature rises.

When everything is set, begin recording. The logger's

LED may ﬂash periodically. Once the logger has

begun running, you may remove the serial cord from

both the logger and the PC.

Warning! Make sure that previous temperature data

has been downloaded before launching the logger.

The launching process may erase previous data.

Insert the Temperature Probe into the water next to

the Water Probe.

2.1.2 Start Measurement

Switch on the RAD7 (have the printer switched on if

you are using it. The RAD7 will then print a header

for the data printout, including a review of the setup,

before it gives you a 'Test' prompt.)

Provided that the RAD7 has been set up properly (see

Test

above), at the '

' prompt, push

Test Start

right arrow, to see '

[ENTER]

push

to start the test.

2.2 Speed of Response

2.2.1 Measurement in Progress

The instrument is now measuring the radon in the

closed air loop that includes the Water Probe's

membrane tubing. With high radon activity

concentration in the surrounding water it will take

half an hour or more before there is much of a

reading, and 2 - 3 hours before you can rely on the

count rate being close to the equilibrium value. After

that you need to accumulate suﬃcient counts for the

Section 2

Measurement Procedure

[ENTER]

then the

' on the LCD, then

precision desired. For example, 100 counts would

give a reading with a standard deviation of 10%. At

very low concentrations, it may take hours, and

averaging over many cycles, to reach a suﬃciently

precise value.

2.2.2 Inﬂuencing Factors

There are two processes requiring time. One is for the

air in the closed loop to approach equilibrium with

the surrounding water and the other is for the RAD7

to respond to the changed radon concentration in the

air loop. The ﬁrst is a property of the exchange tubing

and the second is determined by the half life of the

ﬁrst daughter of radon, namely 218-polonium (Sniﬀ

mode), or the half lives of intermediary 214-lead and

214-bismuth (Normal mode). See the RAD7 manual,

Section 3 for more details.

2.2.3 Air Flow Rate

There is also a small contribution to the response

time from mixing of the radon-laden air around the

closed air loop. This depends on the duty cycle of the

RAD7's pump. However, even with the pump set to

AUTO, this delay is only around 20 minutes, which is

small compared with the 2-hour timescale for radon

exchange across the membrane tubing and the 3-hour

95% response time of the RAD7 running in Normal

mode. For this reason, it is almost always suﬃcient to

set the pump to AUTO mode, in which the air is

circulated around the closed air loop for one minute

in every ﬁve. Air will remain stationary in the Water

Probe for four minutes before moving to the

Laboratory Drying Unit where it waits another four

minutes before entering the RAD7. While in the

Water Probe, 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 Water Probe. 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 set to 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.