10. Time Constant and Standard Deviation
Just like any other instrument the 6150AD needs a certain integration time to keep the standard deviation
to a reasonable limit, that is to prevent dose rate indication from fluctuating too much even under stable
conditions. Consequently, if dose rate changes in a sudden step, dose rate indication will not immediately
follow that step, but will slowly approach the new value. The time required until the instrument has
followed about two thirds of that change is called »time constant« (to be more precise: the time constant
is the time it takes until a fraction of e
-1
= 0.368 of the step is remaining). The 6150AD uses a time
constant of eight seconds, which provides a fairly stable dose rate indication at reasonable response time
(other versions with a floating time constant are also available). Time constant and relative standard
deviation of dose rate indication are linked to each other as shown in the diagram below. The x-axis is the
pulse rate, and below the x-axis you can find the dose rates of the various detectors corresponding to that
pulse rate.
0.7
7
70
700
µSv/h 6150AD1/3/5
0.06
0.6
6
60
µSv/h 6150AD2/4/6/-18/-t
3.5
35
350
3500
µSv/h 6150AD-15
0.1
1
10
100
nSv/h 6150AD-b
1)
0.06
0.6
6
60
S
-1
6150AD-17
0.08
0.8
8
80
S
-1
6150AD-19
0.1
1
10
100
S
-1
6150AD-0
Note, however, that the 6150AD performs more than the diagram suggests. The time constant shown in
the diagram only applies to small changes in dose rate. If dose rate changes strongly, the 6150AD turns
off its time constant intermediately and jumps to the new value within one or two seconds. After that the
time constant will be active again to smooth dose rate indication. You may ask how strong a change in
dose rate must be to cause such a fast response. There is no simple answer to that question. It depends on
both the initial and the final value of dose rate; a detailed discussion would go beyond the scope of this
manual. Just trust the 6150AD’s microprocessor that it will make that decision in the best way according
to the rules of statistics.
1)
For the Scintillator Probe 6150AD-b, the time constant applies as illustrated, whereas the standard deviation does
not
,
because the Scintillator Probe 6150AD-b does not output statistically distributed counting tube pulses like the other
detectors do.
1
10
100
0,1
1
10
100
1000
Relative Standard Deviation [%]
.
1
10
100
Time Constant [seconds]
.
Standard Deviation
Time Constant
03/2004
Operating Manual 6150AD
Page 35
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