Time Weighted Average; Combustible Gas Sensor - MSA ORION Manual

36
A.2

Time Weighted Average

The TWA is the concentration of gas a worker can be continuously exposed to for an
eight hour working day, repeated over five days for a forty hour working week.
The equation below shows how the Orion calculates your time weighted exposure.
To calculate TWA:
Orion TWA reading =
Example 1: Assume the Orion has measured a 2 hour exposure of H
concentration of 16 ppm:
Orion TWA reading =
The published TWA for H
across an eight hour day gives an Orion TWA reading of 4 ppm, the actual exposure
is 16 ppm – above the published STEL of 15 ppm for H
respiratory protection.
Example 2: Assume in a 10 hour operation, the Orion has measured a CO
concentration of 45 ppm for 3 hours and 30 ppm for 7 hours:
Orion TWA reading =
The TWA for carbon monoxide published by HSIS is 30 ppm. This worker requires
respiratory protection.
A.3

Combustible gas sensor

Combustible gases are detected by a catalytic sensor. This kind of sensor relies on
these concepts:
• the gas can be burnt
• different combustible gases have different flammability characteristics
• an increase in the temperature of a coil carrying a current results in a change in
the electrical resistance of the coil.
The gas to be tested is passed over a heated metal coil. The coil burns the gas,
resulting in an increase in temperature of the coil. This change in temperature causes
a change in the electrical resistance of the coil, which is measured.
The greater the concentration of gas, the more gas is burned, resulting in a greater
change in temperature and electrical resistance.
The result is interpreted and displayed on the LCD as a percentage of LEL.
D.I. Recommended Practice
FILE: W:\NB\RP HMRU\Orion\Frame Ver 02\Appendix.fm
(Exposure time in hours) x (Exposure concentration in ppm)
(2 hours) x (16 ppm)
8 hours
= 4 ppm
S is 10 ppm. While the two hour exposure averaged
2
(3 hours x 45 ppm) + (7 hours x 30 ppm)
8 hours
= 43 ppm
Version 02
© Copyright
Four-Head Gas Detector: MSA Orion
8 hours
S. This worker requires
2
Issued February 2011
State of NSW (Fire and Rescue NSW) 2011
S
2