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Technical reference
B.1
Principle of operation
SITRANS TDL is a single laser analyzer that utilizes tunable diode laser (TDL) absorption
spectroscopy. The central unit sends a laser light through the process which is detected by the
receiver unit mounted on the opposite side of the process. When a gas of interest is present in
this process, it will absorb the laser light. The optical power detected in the receiver unit will
depend on the concentration of the gas, temperature, pressure and optical pathlength
according to Lambert's Beer law. Thus in order to determine the gas concentration, the analyzer
requires information regarding temperature, pressure and optical pathlength. Temperature and
pressure signals can be provided by analog inputs, via industrial communication protocols or as
manual values via Webserver or HMI. Pathlength needs to be provided by the user via HMI or
Webserver.
In the SITRANS TDL analyzer, the laser wavelength is specifically chosen to match the
fingerprint region of the particular gas of interest and is being continuously scanned over
the absorption lines(s). Since full spectral information is recovered with very high spectral
resolution the analyzer remains immune to foreign gas broadening and is immune to cross-
interferences from dust and any other gas constituents in the process.
SITRANS TDL by design can operate in several wavelength regions from near infrared to
Mid-Infrared.
B.2
Calibration and calibration certificate
The analyzer is calibrated using certified gas sample in the factory. One of the key parameters of
the analyzer is its limit of detection (LOD) which determines the lowest measurable
concentration. This quantity is estimated for each application and is determined for each device
before shipping to our customers. For every SITRANS TDL analyzer, we determine the limit of
detection during a 12-hour cycle in a climate chamber. The analyzer is placed on a test gas cell
and a zero sample is introduced. A zero sample is either nitrogen or dry air to purge out the
constituent of interest. The climate chamber temperature is then varied between -20°C to +55°C
and the concentration signal is logged, as well as other important analyzer parameters. Based
on the concentration reading during the temperature cycle, the limit of detection is calculated
using the following equation:
LOD = 2 * standard deviation (gas concentration)
Limit of detection is usually quantified in ppm x meter units. This allows to easily recalculate
the limit of detection for arbitrary path length. For example: Limit of detection for standard
carbon monoxide application at room temperature is LOD 0.2 ppm x m. If the process path
length is 2.5 meters the actual limit of detection at this distance will be:
LOD (@ 2.5 meters) = LOD/Process path length = 0.2 ppm x m/2.5 m =
0.08 ppm
Before shipment, each analyzer is accompanied with individual calibration certificate, where
all calibration and performance information are given.
SITRANS TDL
Operating Instructions, 05/2023, A5E51429088_AA
B
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