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TLV-TWA
Threshold Limit Value – Time Weighted Average
Acceptable average exposure of the concentration of a harmful
substance on the basis of an 8h/day, 40h/week work schedule
without adverse effects on the workers’ health.
TLV-STEL
Threshold Limit Value – Short Term Exposure Limit
Acceptable limit of the concentration of a harmful substance
which will not adversely affect the health of a worker by its spot
exposure for a duration of 15 minutes, that cannot be repeated
more than 4 times per day with at least 60 minutes between
exposure periods.
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12. Glossary
O
2
Oxygen
H
2
S
Hydrogen sulfide
COMB
Combustible gas
CO
Carbon monoxide
Air adjustment
(zeroing)
To adjusting the zero point (or 21.0% for oxygen) in clean air.
Clean air: air free from target or interfering gases, and composed
of 20.9-21.0vol% oxygen in dry conditions.
Gas atmosphere: Air containing target or interfering gases.
Span adjustment
To adjust the indicated values by using span gas.
Explosion-proof
structure
Structure of electrical equipment which will not become an
ignition source to ignite an ambient explosive atmosphere.
Intrinsically safe
structure
Structure tested (e.g. spark test) to not become an ignition source
in a flammable atmosphere due to an electrical spark or hot
surface during normal operation and fault conditions.
Hazardous area
An area in which an explosive atmosphere is present, or may be
expected to be present, in quantities such as to require special
precautions for the construction, installation and use of electrical
apparatus.
Non-hazardous area An area in which an explosive atmosphere is not expected to be
present in quantities such as to require special precautions for
the construction, installation and use of electrical apparatus.
%LEL
Concentrations of combustible gas given in terms of percent of
the lower explosion limit.
vol%
Gas concentrations given in terms of percent of cubic volume.
ppm
Gas concentrations given in terms of millionth part of cubic
volume.
LEL
Lower Explosive Limit. Lowest concentration (percentage) of a
gas or vapor in air capable of producing a flash fire, or explosion
in the presence of an ignition source like arc, flame or heat.
