Vaisala GMP343 - Manual (Page 17)

Chapter 2 __________________________________________________________ Product Overview
VAISALA_______________________________________________________________________ 15
You can also select the measurement range to optimize the performance of
the GMP343 for a certain concentration level. Selecting an appropriate
measurement range increases accuracy, since it enables the range-specific
compensation and linearization settings. For more information, see
sections
Setting the Measurement Range on page 40 and Temperature,
Pressure, Relative Humidity, and Oxygen Compensations on page 45 .
Principle of Operation
The infra-red sensor of GMP343 is based on the proprietary Vaisala
CARBOCAP
®
sensing technology. Here the pulsed light from a miniature
filament lamp is reflected and re-focused back to an IR detector which is
behind a silicon-based Fabry-Perot Interferometer (FPI). This tiny FPI is
tuned electrically so that its measurement wavelength is changed between
the absorption band of the CO
2
gas and a reference band.
Figure 2 Principle of Operation
When the passband of the FPI coincides with the absorption wavelength of
the CO
2
gas, the IR detector sees a decrease in the light transmission. The
measurement wavelength of the FPI is then changed to the reference band
(that has no absorption lines) and the IR detector sees a full light
transmission. The ratio of these two signals indicates the degree of light
absorption in the gas and is proportional to the gas concentration. It takes
2 seconds to measure this sequence and to calculate one reading.
Use of numerical filtering reduces the noise of the raw data. On the other
hand, the filtering increases the response time of the device (see
Figure 9
on page 43 ).