The dose alarm (only provided by 6150AD3/4/5/6) takes precedence over the dose rate alarm in case both
alarms should occur simultaneously.
The 6150AD permanently measures dose rate mean value, which can be displayed with the help of the
function key. The dose rate mean value is particularly useful at low dose rates, where direct dose rate
indication is subject to strong statistical fluctuations. The mean value provides, though at the expense of
measuring time, a statistical accuracy otherwise only achievable with much more sensitive and thus more
When reading the dose rate mean value two things must be observed:
The mean value is only reasonable if conditions were stable during its measurement. For example, if
a radiation field is to be measured more accurately with the help of the mean value, this radiation
field has to be constant in time, and the instrument has to remain at the same place during that
measurement. If these conditions are not met, the mean value will only represent a not very
meaningful average in time and space.
You must not overestimate the absolute accuracy of the mean value. It is true that by measuring long
enough you can bring down the statistical error to almost any small value, but this does not eliminate
other possible error sources such as calibration error, ambient temperature, energy and direction of
radiation, instrumental background, and so on. For example, if you leave a 6150AD2/4/6 to itself for
24 hours at natural background of approx. 70 nSv/h, the statistical error of the dose rate mean value
will be as low as 1 nSv/h. However, this does not mean you have measured natural background with
that accuracy. One uncertainty is, for example, the instrumental background. Although mostly
compensated, the instrumental background slightly varies from one instrument to the other; the
background remaining after compensation may amount up to approx. 10 nSv/h.
The main purpose of the dose rate mean value is to reduce the statistical error at constant measuring
conditions. A typical example is checking the instrument with a check source. This requires an accurate
reading and then comparing that reading with some reference value obtained earlier. What then matters is
the comparison of two values, not so much their absolute value. The dose rate mean value is best suited
for that purpose. Another example is measuring contamination with an external probe. Reading the mean
value without and with the object to be checked will allow to detect small increases in pulse rate (for
example from 0.1 to 0.3 pulses per second), which would be impossible or at least very inaccurate when
reading the pulse rate directly.
As measuring time increases, the mean value becomes increasingly accurate. As a rough indication for
accuracy, the digital display flashes until the statistical error has gone down to 5% (to be more precise:
until a relative standard deviation of 5% has been achieved). The various models use slightly different
methods to check for that 5% error limit. The 6150AD1/2/3/4 assumes the error not to exceed 5% as soon
as calculation of the mean value is based on at least 400 pulses. This is correct for statistically distributed
pulses originating from counting tubes. The 6150AD5/6, however, also has to process the pulses coming
from the Scintillator Probe 6150AD-b, which are not statistically distributed. Therefore, the 6150AD5/6
uses fluctuations of dose rate instead of total pulse count to calculate the standard deviation of the mean
value. Now, if conditions change, that is if dose rate changes, the standard deviation calculated from dose
rate fluctuations will suddenly start to increase, after it had been decreasing continuously. Consequently,
with the 6150AD5/6 you may notice the mean value to start flashing again after it had already been static.
Such behaviour indicates changing conditions. The 6150AD1/2/3/4 will not recognize such changing
conditions, because it only observes total pulse count. However, under constant conditions, both methods
are equivalent, and only constant conditions will give meaningful readings anyway. Therefore, the
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