Panametrics Aurora H2O, User Manual

Page: 14 / 170

Chapter 1. Features and Capabilities
2 Aurora H2O User’s Manual
1.3 Theory of Operation
The BH Aurora H2O ( TDLAS ) hygrometer for natural gas is a system designed to continuously monitor the moisture
content in natural gas. It fundamentally measures the partial pressure of water vapor (water in the gas state), and
with the simultaneous measurement of pressure and temperature, provides readings and both analog and digital
signal transmission of user-selectable moisture parameters including dew point temperature, volume ratio and
absolute humidity (lbs/MMSCF or mg/m
3
) in addition to temperature and pressure.
The Aurora H2O is supplied with an integrated sampling system which includes an optional pipe-mounted liquid
separator and pressure reducing valve/regulator, and the following components mounted within a stainless steel
enclosure: isolation valve, liquid membrane filter, sample cell control valve, second pressure reducing
valve/regulator, and flow indicator (rotameter), as well as an optional electrical heater and thermostat.
The fundamental water vapor pressure measurement is based on the
Beer-Lambert Law :
where:
A
= Absorbance
I
= Light intensity transmitted through a sample gas
I
o
= Incident light intensity
S = Absorption coefficient*
L = Absorption path length (a constant)
N = Concentration of the water vapor in the absorption cell
* The absorption coefficient is a constant for a specific gas composition at a given pressure and temperature.
The concentration of the water is directly related to the partial pressure. At certain specific frequencies/wavelengths
of light, light energy will be absorbed by water molecules. As the concentration of water increases, the absorption
also increases. The Aurora H2O sweeps the diode laser output across a narrow band in the near infrared spectrum
and, by measuring the light intensity with a photodetector, is able to provide a direct indication of the partial
pressure of water. The partial pressure, multiplied by 10
6
and divided by the total pressure, yields the volume ratio in
ppm
v
(parts per million by volume).
The laser diode is housed in a hermetically sealed and dry housing. The light is transmitted through a window made
of proprietary transparent material. The light travels through a stainless steel cell and is reflected off a gold-plated
mirror and returned to a photo detector, where the light intensity is measured.
A In
I
o
I
----
 
 
SLN = =