Principles Of Operation; The Orion; Principles Of Calibration; Explosive Limits - MSA ORION Manual

Four-Head Gas Detector: MSA Orion
2.5

Principles of operation

2.5.1

The Orion

A pump draws the gas sample into the instrument through a sampling wand. Filters
remove water and dust particles. A set of four sensors then test the gas for
combustibles, oxygen content, carbon monoxide and hydrogen sulphide.
Combustible gases are tested by a catalytic sensor. See
sensor', on page 36
Oxygen and toxic gases are measured by electrochemical sensors. See
'Oxygen and toxic gas sensors', on page 37
The Orion is protected by a series of three filters:
• a water stop filter in the wand,
• a dust filter located on the rear of the Orion,
• an internal water stop located in the body of the Orion under the dust filter.
2.5.2

Principles of calibration

Calibrating the Orion involves these concepts:
a) The instrument is first zeroed. This means that the response of the instrument in
clean air is registered.
b) Calibration gas of a known concentration is applied to the instrument. The
response to this known concentration is registered.
c) The difference between the zero gas response and the known concentration
response is the span. Instrument readings for concentrations from zero to this
known concentration are set according to this span.
An unknown concentration of a gas is determined by comparing the response of the
instrument to the unknown, with the span value.
2.5.3

Explosive limits

To sustain combustion, there must be an appropriate mix of fuel (in this case a
flammable gas) and oxygen. If the mix is too lean and there isn't enough fuel, the
reaction can't sustain itself. If the mix is too rich and the fuel starts to displace the
oxygen, the reaction will once again stop.
The Lower Explosive Limit (LEL) is the lowest percentage of fuel in the mix for
combustion to take place.
The Upper Explosive Limit (UEL) is the highest percentage of fuel in the mix for
combustion to take place.
D.I. Rec-ommended Practice
FILE: W:\NB\RP HMRU\Orion\Frame Ver 02\Orion.fm
NOTE
If necessary, the Orion can be used without the wand and sampling
hose.
for details.
A.3: 'Combustible gas
for details.
Version 02 Issued February 2011
© Copyright State of NSW (Fire and Rescue NSW) 2011
11
A.4: