Larson Davis System 824, Reference Manual

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11/8/00 Using ID Setting Files 7-17
0 to 255 dB
Exposures at higher levels than the overall criterion level
will result in a 100% dose in less time; or, if exposure con-
tinues, the overall dose will exceed 100%.
Criterion Time - (LOG)
The Criterion Time determines how long exposure to the set
TWA level can occur before 100% dose occurs for the over-
all measurement.
Crnt Exchange - ( SSA , ISM,
LOG)
In the ISM instrument the Crnt
Exchange setting is a submenu to the
SLM menu selection. The Crnt Exchange rate setting sets the exchange rate
(sometimes called the “doubling rate”) that calculates the
current TWA. The exchange rate reflects the relationship
between the permitted exposure levels and the exposure
duration. Selecting an exchange rate of 5 dB, for example,
means that 5dB may be added to the permissible exposure
level every time the exposure duration is cut in half. This
setting provides the following four options:
This setting affects the System 824’s
secondary or current buffer.
3dB - The Crnt Exchange defaults to the 3dB
option. This provides a L
eq
time weighted average in
accordance with European common usage in addition
to compliance with U.S. Environmental Protection
Agency (EPA) regulations.
4dB
-
The 4dB option results in the L
DOD
time
weighted average which complies with U.S. Depart-
ment of Defense (DOD) regulations.
5dB
-
The 5dB option results in the L
OSHA
time
weighted average which complieswith U.S. Occupa-
tional Safety and Health Administration (OSHA) reg-
ulations.
6dB
-
The 6dB option results in the L
Avg
time
weighted average which provides a convenient repre-
sentation of the long-term average noise level as
opposed to the RMS noise level.