NARDA ELT-400, Руководство по эксплуатации

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Annex B: Exposure STD mode ELT-400
B-2
Limit value curve frequency dependency
The frequency response describes the degree to which the field strength at
a given frequency f1 may differ from that at another given frequency f2 for
the limit value to be achieved in both cases. The limit value curve can thus
also be interpreted as an attenuation curve.
In “Exposure STD” mode, this (attenuation) curve is simulated by several
first-order filters. The relevant corner frequency is taken from the standard.
The abrupt transitions at the corner frequencies are smoothed by the filters,
leading to discrepancies of up to 3 dB. These deviations are accepted by
ICNIRP (“Guidance on Determining Compliance of Exposure to Pulsed and
Complex Non-Sinusoidal Waveforms below 100kHz”, Health Physics
Vol. 84 No. 3, 2003) since they correspond to the biological effect of the field
on humans.
The filters in the ELT-400 are switched to correspond with the relevant
standard as well as to the selected exposure range.
Example: Limit values for 50 Hz and 150 Hz are taken from ICNIRP 1998.
The corresponding reference values are 100 µT and 33.3 µT, i.e. the
permitted values differ by a factor of 3. If the field at 50 Hz is attenuated by
the filter during the measurement by a factor of 3 compared to the field at
150 Hz, both displayed values will be the same when the field strength
reference value is reached.
Frequency measurement is not required, since the filter converts the
frequency information into the appropriate attenuation. This also avoids the
uncertainty in measuring the frequency of a signal of unknown shape (e.g.
harmonics, interference frequencies, etc.).
Note: This method has long been used with noise level meters.
Limit value curve field strength dependency
Since the frequency dependency of the reference values is fixed by the
filters, only the level dependency for the selected exposure range needs to
be scaled appropriately. The instrument must indicate the value 100 %
when the limit value is reached.
This scaling is done during manufacture of the ELT-400. The display is
adjusted accordingly at any given frequency. Switching the exposure level
e.g. from “General Public” to “Occupational” takes place in the same way.
You therefore do not need to know the reference values at all.