DURRIDGE RAD7 Manuel de l'utilisateur - Page 8

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2.2.4 RAD7 Mode

In AUTO (the default) mode, the RAD7 will

automatically switch from SNIFF mode to NORMAL

mode after three hours into the run. This is to take

advantage of the additional counts provided by the

214-polonium 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. The

RAD7's response time will be a couple of hours or so.

2.3 Long Term Measurement

2.3.1 Desiccant

As set up, above, the system will continue making

measurements indeﬁnitely. There are, however,

various resources that are being used up in the

process, and which must be replenished. The 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 operating in a closed,

dry air loop where the only moisture entering the

loop comes from tiny leaks of ambient air. Therefore,

each Laboratory Drying Unit can be expected to last

several weeks. 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

Section 2

Measurement Procedure

For fastest response, the RAD7 can be forced to stay

Setup, Mode, Sniff [ENTER]

in SNIFF mode (

It will then always count only the 218-Po decays,

giving it a 13-minute 95% response time. However,

the improvement in overall response time is marginal

due to the 2-hour timescale for radon exchange

across the membrane tubing, and this has to be

weighed against the reduction in sensitivity (and

consequent reduction in precision) that comes with

switching from NORMAL to SNIFF mode.

replaced, and the relative humidity in the instrument

rises above about 20%, then the sensitivity drops oﬀ

and the reading is lower than the true value.

2.3.2 Memory

The capacity of the internal memory of the RAD7 is

1,000 records or cycles. If each cycle is two hours,

that would be data for 2000 hours, or just over 80

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. It is

also possible to monitor the RAD7's output remotely,

including the relative humidity, which can be used as

an indicator that the desiccant needs to be replaced.

Please see the CAPTURE manual for more

information on remote monitoring.