4.8 Dose Alarm (6150AD3/4/5/6 only)
The 6150AD3/4/5/6 provides dose warning, which the 6150AD1/2 does not. As soon as dose reaches the
dose alarm threshold, the instrument automatically switches to the »dose indication« state and issues dose
alarm in both audible and visible form. The alarm tone is an intermittent one (four sounds per second,
twice as fast as with dose rate alarm), and the flashing loudspeaker symbol and the flashing bar graph
take the visual part. The only way to acknowledge the dose alarm is to leave the »dose indication« state.
You can do so using the function key, or by pressing the signal key, where the latter will automatically
return to the ground state »dose rate indication«.
Apart from several fixed dose alarm thresholds the 6150AD3/4/5/6 also provides a user programmable
threshold, see section 12.2.
As soon as dose increased by a certain amount, the instrument will repeat dose warning. We shall call this
repeated alarm a »post-alarm«. The dose increment required for a post-alarm shall be called »post-alarm
increment«; it is a certain percentage of the dose alarm threshold. A post-alarm appears exactly like the
first alarm. Every time the dose increased by one post-alarm increment, the instrument issues a new
post-alarm, thereby repeatedly reminding the user that the dose exceeds the threshold. The concept of the
post-alarm is explained in more detail in the following numerical example:
Dose alarm threshold:
250 µSv
Post-alarm increment:
2% (2% of 250 µSv = 5 µSv)
Dose alarm at:
250 / 255 / 260 / 265 / ... µSv
In the above example, the first dose alarm occurs when dose is equal to or greater than 250 µSv.
Post-alarms will occur every time dose catches up with the next value in the sequence of post-alarm
thresholds. This sequence is only limited by the dose range of the detector in use.
You can set the post-alarm increment to one of these values (see section 12.3):
2% / 5% / 10% / no post-alarm
where 2% is factory setting. The post-alarm increment applies to the internal tube and to all probes. If set
to »no post-alarm«, only one dose warning will occur when the dose reaches the alarm threshold.
The post-alarm increment will be rounded off to full micro-Sieverts if necessary. For example, 10% of
94 µSv will be rounded off to 9 µSv, and 10% of 95 µSv will be rounded off to 10 µSv. Although such
»odd« thresholds are unlikely to occur, the cannot be excluded because of the programmable threshold.
Also, the post-alarm increment can never be lower than 1 µSv or greater than 50 mSv. With thresholds
lower than 15 µSv, the minimum value of 1 µSv causes that each further µSv will trigger a post-alarm
regardless whether the post-alarm increment is set to 2% / 5% / 10%.
Should dose alarm and dose rate alarm occur simultaneously, dose alarm and dose post-alarm take
precedence over the dose rate alarm. You will not notice the dose rate alarm until you acknowledge the
dose alarm by leaving dose indication.
When using the instrument with a low dose alarm threshold in a strong radiation field, post-alarms may
repeat so frequently that it is impossible to move the instrument to some state other than dose indication.
Of course, the first choice remedy would be to leave that radiation field. But there is another possibility:
acknowledge the post-alarm by keeping the signal key depressed. As long as you keep the signal key
depressed, you will be able to move to the other states using the function key as usual.
You can also use dose thresholds with pulse probes. Everything just mentioned also applies to this
category of probes, if you replace »dose« with »pulse count« and »1 µSv« with »1 pulse«. For example,
you could use a pulse count threshold if you would like to perform a measurement until a certain pulse
count corresponding to a certain accuracy is obtained; the warning would then signal the end of the
measurement.
Page 12
Operating Manual 6150AD
03/2004
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