Handbook of Solvents - George Wypych - ChemTech - Ventech!

1218 Margot Scheithauer
A portable multi-gas monitor 1312
[Bruel & Kjaer] 30 weighing 9 kg (Figure
18.1.4) enables sampling in six places up to
50 m away from the monitor. It can determine
at each measuring point the concentration
of up to a maximum 5 components
as well as the humidity. This is done by a
multi-point sampler with a dosing apparatus
controlled by system software. The de-
Figure 18.1.4. Photoacoustic Multigas Monitor 1312. vice needs calibration only four times a
year. Beyond that, the device may also be
used for determining the total C concentration.
Measuring principle (Figure 18.1.5)
1. The “new” air sample is hermetically sealed in the analysis cell by closing the inlet
and outlet valves.
2. Light from an infrared light source is reflected by a mirror, passed through a mechanical
chopper, which pulsates it, and then passes through one of the optical filters in the
filter carousel.
3. The light transmitted by the optical filter is selectively absorbed by the gas monitored,
causing the temperature of the gas to increase. Because the light is pulsating, the gas
temperature increases and decreases, causing an equivalent increase and decrease in the
pressure of the gas (an acoustic signal) in the closed cell.
4. Two microphones mounted in the cell wall measure this acoustic signal, which is directly
proportional to the concentration of the monitored gas present in the cell.
5. The filter carousel turns so that light is transmitted through the next optical filter,
and the new signal is measured. The number of times this step is repeated depends on the
number of gases being measured.
Figure 18.1.5. Measurement principle of Photoacoustic Multigas Monitor Type 1312.