ENMET ISA-44-RALE-OD - Manual (Page 8)

ENMET Corporation ISA-44RALE-OD/ISA-44RAHE-OD/ISA-44E-OD
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1.3 Calibration Explained for the ISA-44RALE-OD
The alarm settings of the ISA-44RALE-OD are adjusted and set (calibrated) at the factory to respond within specified
time limits to certain concentrations of gas. These concentrations are shown on the red calibration stickers on the display
plate of the instrument. Unless otherwise specified, the ISA-44RALE-OD has two alarm settings for carbon monoxide
(10 and 20 ppm CO) and one for oxygen (19.5% oxygen by volume). The terms "ppm" and "% oxygen by volume" are
described below.
Often used to indicate toxic gas concentrations, the term "ppm" refers to "parts per million". This term describes the
concentration of a gas as a proportion of the total volume of an atmosphere. The concentration 20 ppm CO indicates that
out of a sample of air divided into one million parts (volumetric units) of equal volume, 20 parts (units) are comprised of
carbon monoxide gas, and the remainder is comprised of clean air.
Most commonly used in the safety industry to describe the oxygen content of an atmosphere, the term "% oxygen by
volume" describes the amount of oxygen measured as a percent of the total atmosphere in a space. For example, fresh,
clean air is approximately 20.9% oxygen by volume, indicating that just over one-fifth of the total atmospheric volume is
comprised of oxygen.
N OTE : That this is only a relative measurement; it does not indicate the exact volume of oxygen. For example, at high
altitudes the air is much less dense; therefore, even though 20.9% of the atmosphere may be oxygen, the thinner
atmosphere means that there is actually less oxygen than in denser sea level air.
The alarm levels of the ISA-44RALE-OD are set to meet certain OSHA requires that compressed breathing air meet at
least the specifications of grade D respiratory air as described by the Compressed Gas Association (CGA).
Table 1: Grade D Breathing Air
Grade D Breathing Air
Oxygen 19.5% - 23.5% By Volume
Carbon monoxide 10 ppm
Hydrocarbons Maximum 5 Mg/M3
Odor Free of pronounced odor
Carbon dioxide Maximum 1000 ppm
The standard alarm levels of the ISA-44RALE-OD (10, 20 ppm CO, and 19.5% oxygen by volume) monitor compressed
air for the maximum allowable concentration of CO and the minimum allowable concentration of oxygen. The 10 ppm
alarm indicates the maximum allowable level CO (as established by OSHA) is present. The 20 ppm CO alarm alerts the
use to higher concentrations of gas, which, if significantly above 50 ppm, could threaten the safety of the worker. The
19.5% oxygen by volume alarm indicates that the oxygen content of the compressed air has fallen below the minimum
requirement for grade D air. If the oxygen level continues to fall, the health and safety of the worker are endangered. At
approximately 16% by volume, the senses and perceptivity of exposed workers are affected. At 14%, degradation of the
senses is serious. At 10% the worker quickly becomes unconscious, and his life is in serious danger. Atmospheres
containing 5% or less oxygen by volume are instantly fatal.