mru AirFair OPTIMA 7 NDIR, User Manual

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MRU GmbH USER MANUAL OPTIMA 7 - NDIR
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5.2 Electrochemical measured principle
The oxygen content of the sample gas is measured with a 2 electrode electrochemical
sensor.
Toxic gases like carbon monoxide (CO), nitrogen oxide (NO), nitrogen dioxide (NO
2
), sul-
phur dioxide (SO
2
), and hydrogen sulphide (H
2
S) are measured with 3 electrode sensors.
The electrochemical sensors are based on gas diffusion technology.
The advantage of this technology is that the signal generated is direct proportionally and
linear to the volume concentration (% or ppm) of the analysis gas components.
The 3 electrodes are: S (sensing electrode), C (counter electrode) and R (reference elec-
trode).
When the gas being measured contacts the sensing electrode, it reacts on the electrode surface
either through oxidation (for example CO, SO
2
, NO) or reduction (like NO
2
, and Cl
2
).
Example: CO sensor:
CO reacts at the sensing electrode as follows:
CO + H
2
O -> CO
2
+ 2H+ + 2e-
and at the counter electrode, oxygen from air will be re-oxidized to water:
½ O
2
+ 2H+ + 2e- -> H
2
O
The sensor supplies a constant current signal (
µ
A-range) that is monitored and analyzed. The cur-
rent intensity is dependant upon the volume concentration of the sample gas, while other parame-
ters like temperature and cross sensitivity are calculated by the analyser.
5.3 Principle of the IR-bench
A regulated infrared source (IR) provides light in the range of 2 to 8 microns (µm). The infrared
light is directed through the sample cell to the optical detector. The source is electronically modu-
lated with no chopper wheels and no moving parts.
The residual infrared energy, not absorbed by the sample gas, is guided to the optical detector. It
passes an optical narrow band pass.
Infrared light passed through the filter is collected by a pyro-electric detectors. It produce a voltage
that is proportional to light intensity.
Unlike electrochemical sensors, the NDIR measurement shows no ageing due the gas concentra-
tion measured.
It’s lifetime is only limited by the lifetime of the IR source.
Pos.
1 Sample gas
2 Particle filter
3 Sensing Electrode
4 Reference Electrode
5 Counter Electrode
6 Connection pin
7 Electrolyte reservoir
8 Electrolyte
9 Capillary diffusion barrier