Baker Hughes Panametrics Aurora H2O, User Manual

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1.3 Theory of operation
The Panametrics Aurora H2O (tunable diode laser
absorption spectroscopy) 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, coalescing filter, coalescer flow
control valve, 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:
A = In (
I
o
) = SLN
I
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, 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 photo detector,
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 ppmv (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.
Since only light comes in contact with the sample of
natural gas, and all of the wetted materials are made
of non-corrosive and inert materials, this technology
does not exhibit the drift associated with gas contacting
sensor-based hygrometers. The diode laser emits low
energy light, and therefore the system will not ignite
the gas. The complete system is rated explosion-proof,
and the Aurora H2O provides very fast response time.
Once the absorption cell (see Figure 1 below) is purged,
the response time is a matter of a few seconds.
Photodetector
Optical window
Gas outlet
Tunable diode laser
Hermetically sealed housing
Stainless steel
absorption cell
Gas inlet Mirror
Figure 1: Laser absorption cell, basic elements
The control of the laser, power supply and signal
conditioning circuitry are housed in an explosion-proof
transmitter enclosure. A backlit, three-parameter, LCD
display provides digital indication of user-programmable
parameters. The Aurora H2O has three user-programmable
(4-20 mA) analog outputs and two programmable digital
ports that may be configured as either RS-485 or RS-232
with modbus protocol. The analyzer has an auxiliary analog
input (4-20 mA) input that is used for connection to an
optional process pressure transmitter. Measurement of the
process pressure enables the Aurora H2O to display and
transmit the process dew point. AuroraView software is
provided and enables remote readings, programming
data logging and data logging with a personal computer.
The
Aurora H2O hygrometer is calibrated against an
NIST (or other national metrological institute) traceable
reference dew/point generator and hygrometer. Each
system is supplied with a certificate of calibration with
functional test data.
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