METHOD OF MEASUREMENT:
WAVELENGTH-MODULATED TUNABLE DIODE LASER
SPECTROSCOPY (WMTDL)
THE BARRACUA MODEL 4010LX
utilizes a state-of-the-art, next generation technology
called Wavelength-modulated Tunable Diode Laser Spectroscopy (WMTDL) for fast, accurate
and highly reliable moisture measurements. This proven technology is based on the
Beer-Lambert Law
.
HOW WMTDL WORKS
The
Beer-Lambert Law
states that the amount of light absorbed by the natural gas sample is
proportional to the amount of water vapor in the path of the laser. Hence, by measuring the
amount of absorbed light and the total pressure, the concentration of moisture in a particular
sample can be accurately determined.
The measurement in a
MODEL 4010LX
is made by tuning the laser wavelength rapidly 2
times per second, back and forth, across the narrow absorption line for a single vibrational/
rotational mode of a water molecule. The number of water vapor molecules in the path of the
laser is determined by the ratio of the detected signal when the laser wavelength is on the
absorption line to the signal when it is off the absorption line. Any variances in the laser
intensity, laser detector gain, obstructions in the optical path or other changes are the same
on and off the absorption line and will cancel out when the ratio is taken – thus, keeping the
measurement stable (in calibration) if variances occur.
Over longer periods of time, the laser wavelength can drift. A standard feature of the
MODEL
4010LX
Moisture Analyzer is that it tracks the original location of the background methane
absorption. Using AMI’s
COMMAND CENTER
Software
, the customers can easily check for
any drift in the laser wavelength and, if necessary, correct it with a simple click of a button.
(Learn more about this in the Command Center Software overview
3
