6150AD5/6 just has the additional function that it will indicate changing conditions by an increase of
Note that the time required until the 5% error limit has been achieved, and thus the flashing stops,
depends on dose rate and the sensitivity of the detector currently in use. It may range from a few seconds
Calculating the mean value starts when the instrument is switched on and is not limited in time. It starts
again when a probe is connected or disconnected. The 6150AD3/4/5/6 additionally allows to clear the
mean value and thus to start its calculation again. Moreover, the 6150AD5/6 is also able to indicate the
standard deviation of the mean value.
We shall shortly explain the meaning of the relative standard deviation for users who are not very
familiar with this subject. Assume you have measured some value (say, 100) with some relative standard
deviation (say, 3%). This does not mean that the »true« value (the value you would have obtained when
measuring infinitely) will certainly be within the range 100 ± 3%, that is within the range 97 to 103. It
only means that the true value will be within that range with a certain probability. Theory of statistics
within one standard deviation are:
within two standard deviations are:
within three standard deviations are:
Applied to the 100 ± 3% example this means that the true value is within the range of 97 to 103 with a
probability of approx. 68%. In other words, there is a probability of (100-68)% = 32% that the true value
is outside that range. Taking three standard deviations (3
3% = 9%) instead of one results in a range of
100 ± 9%, that is 91 to 109. The probability that the true value is within that range is as high as 99.7%;
the probability that the true value is outside that range is as small as (100-99.7)% = 0.3%. It depends on
the application how accurate a value is required. Common practice is to specify two standard deviations
as measuring accuracy. This is sometimes called a »two sigma confidence level« or »95% confidence
interval«, saying that the true value is with a probability (confidence) of 95% within the specified error
(the theory of statistics uses the Greek letter
»sigma«, to denote the standard deviation).
The 6150AD remembers the dose rate maximum value, which can be displayed with the help of the
function key. The maximum value can be useful after a job like, for example, measuring the spatial
distribution of a radiation field.
Determination of the maximum value starts when the instrument is switched on. It starts again when a
probe is connected or disconnected. The 6150AD3/4/5/6 additionally allows to clear the maximum value
and thus to start its determination again.
The 6150AD permanently accumulates dose, which can be displayed with the help of the function key.
With the 6150AD1/2, dose is always zero after the instrument has been switched on, and after a probe is
connected or disconnected. The 6150AD3/4/5/6 has a non-volatile dose memory, see next section.
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