The zirconium-oxide measuring principle is used for O2
measurements in especially combustion processes for use of
control and optimisation of the process. The zirconium
oxide probe has a platinium-coated ceramic measuring
element that can conduct oxygen ions at temperatures
typically above 600 °C. In the measuring element, one side
is filled with atmospheric air with a known O2-
concentration (20.9 vol%), and on the other side is the O2-
concentration to be measured. Since the partial pressure of
O2 is different from one side to the other, the O2-
molecules will try to establish a equilibrium by moving
from one side to the other. The movement of O2-molecules
makes a difference in voltage which is equivalent to the O2-
concentration in the process.
Laser Type Gas measuring principle
The LaserGas measuring principle TDLAS (Tuneable Diode
Laser) is a comparatively new measuring principle. It is
used for in-situ measurements of for instance O2, CO2, CH4,
H2S, H2O, HCl, HF and NH3 gasses. The LaserGas instruments
are often used for applications where short response times
are essential or for applications where the gasses to be
measured are complicated to measure in a conventional
extractive analysis system.
LaserGas measurements are performed by laser light to be
sent across the exhaust gas duct by a specific wavelength.
The absorption of the light sent into the process is an
indicator for the concentration of gas to be analysed. The
wavelength is thoroughly selected for the specific gas so
that interference from other gasses is
The LaserGas measuring principle is very reliable even in
high temperatures and dust concentrations. By means of
automatic adjustments it is possible to have a reliable
measurement even if the transmission of light is only at 2-
Paramagnetic Type Gas measuring principle
Some gasses such as O2 have paramagnetic characteristics
due to non-paired electrons. This feature can be used by
letting the gas pass through a magnetic field in a gas
analyser. When O2 passes through the gas analyser, the
magnetic field is activated and this energization will be
in direct ratio to the O2-concentration of the gas that is
led through the analyser.
Normally 2 types of paramagnetic measuring cells are used.
One type, dump-bell, consists of N2-filled bulbs mounted
onto a thin wire in a magnetic field (the measuring cell).
The bulbs are bent off more or less from centre of the
measuring cell dependent of the O2-concentration. The bend
is proportional to the O2-concentration. The other type,
thermo magnetic, consists of a heated wire placed in a
magnetic field (the measuring cell). When O2 is led through
the measuring cell it is dragged towards the magnetic field
which gives a cooling of the heated wire and thus a change
in resistance. The change of resistance in the heated wire
is proportional with the O2-concentration
Electrochemical Type Gas measuring principle
The electrochemical measuring principle is often used for
measurement of O2 in extractive gas analysis systems.
However, it can also be used for measurement of CO, NO and
SO2 from small transportable gas analyser systems. The
measurement is made by letting for instance O2 pass through
a gas selective membrane, which only allows for the gas to
be measured to pass through. The O2-molecules diffuse into
an electrolyte solution and are converted to H2O. This
reaction is provoked by a gold cathode in the electrolyte.
By the anode, which is made of lead, lead oxide is produced
by means of the electrons that are released when O2 reacts
and is converted to H2O. The migration of electrons between
the anode and the cathode represents the amount of O2
converted to H2O, and it is thus an indicator of the O2-
concentration in the process.
During the latest years the use of the electrochemical
measurement principle has become more common. This is
partly due to the fact that the electrochemical cells have
been developed continuously so that they have now become
both accurate, stable and with long service lives. Besides
the measurement of O2 by the electrochemical measuring
principle is less expensive than measurement of O2 using
the paramagnetic measuring principle.
NDIR (Non Dispersive Infrared) Type Gas measuring principle
ared light (IR & NDIR)
Many gasses absorb infrared light extremely well, which
makes the use of infrared light (IR) very useful for the
analysis of many gasses such as CO, CO2, NO, SO2 and CH4.
One of the IR measuring principles is the NDIR (Non
Dispersive Infrared) principle. The measurement is made by
a gas flow which is led through a cuvette where the IR
light source and an optical filter has been placed by one
end of the cuvette and a detector has been placed by the
other end. The IR light source sends out a scattered IR
light and the wave length of the light that is sent through
the gas in the cuvette is determined by the optical filter
that has been placed between the light source and the
cuvette. Different kinds of wave lengths from IR light are
used for analysis of different gasses. The absorption of
the light that is send into the cuvette is an expression of
the concentration of the gas to be analysed. The amount of
light passing through the gas is measured by the detector
at the other end of the cuvette.
As many gasses absorb well in the IR area, it is often
necessary to compensate for interfering components. For
instance CO2 and H2O often initiates cross sensitivity in
the infrared spectrum. As many measurements in the IR area
are cross sensitive to H2O it is some times not possible to
analyse for instance SO2 and NO2 in low concentrations
using the infrared light principle. In such cases, it is
necessary to use the UV light principle instead.
Ultraviolet (UV) Type Gas measuring principle
Ultraviolet (UV) light is often used for the analysis of
NO, NO2 and SO2. Often, when the UV measuring principle is
used it is actually the NDUV (Non Dispersive Ultraviolet)
principle. The measurement is made by leading a gas flow
through a cuvette where the UV light source and the optical
filter have been placed at one end of the cuvette and a
detector has been placed at the other end. The UV light
source sends out a scattered UV light, and the wave length
of the light that is led through the gas in the cuvette is
determined by the optical filter installed between the
light source and the cuvette. Different kinds of wave
lengths of UV light are used to analyse different gasses.
The absorption of the light that is sent into the cuvette
is an expression of the concentration of the gas to be
analysed. The amount of light passing through the gas is
measured by the detector at the other end of the cuvette.
The UV measurement principle is generally not as cross
sensitive towards other gasses such as CO2 and H2O as these
do not absorb well in the UV area. This is why the UV
measuring principle is very useful for the analysis of
gasses in low concentrations of especially SO2 and NO2.
FTIR type gas measuring principle
The FTIR measuring principle is a measurement with IR
light. Contrary to NDIR with a narrow wave length area by
means of an optical filter, the scan area of the IR wave
length by use of the FTIR measuring principle is large. The
principle of FTIR is that the gas to be analysed is led
through a cuvette with an IR light source at one end that
is sending out scattered IR light, and a modulator
that "cuts" the infra red light into different wave
lengths. At the other end of the cuvette a detector is
measuring the amount of IR light to pass through the
Like the NDIR measuring principle it is the absorption of
light at different wave lengths that is an expression of
the concentration of gasses to be analysed. By data
processing, Fourier Transformation mathematics is used to
turn the measured absorption values into gas concentrations
for the analysed gasses. As the light, when using the FTIR
measuring principle, is modulated into many different wave
lengths, it is possible to analyse many different gasses in
the same instrument; such as CO, H2O, SO2, NO, NO2, HCl,
Using the above measuring principle also produces a much
larger data material (as compared to the conventional NDIR
principle), from where the concentrations of the different
gasses can be measured. The large data material supply
excellent calibration curves and correlation values, thus
providing very reliable analysis-results.
Chemiluminescense (CLD) type gas measuring principle
The chemiluminescence, CLD, measuring principle is used for
analysis of NOX. The principle is that a gas flow is led
through a reaction chamber in which NO reacts with O3
(ozone). The NO2 produced from this process reaches an
excited condition and the molecules are sending out red
light. In the reaction chamber a photo detector is placed
to measure the amount of red light.
To be able to measure NOX as the total sum of NO and NO2, a
NO2 to NO converter is installed into the CLD analyser so
that NO2 is converted into NO before it is led into the
reaction chamber to react with O2. The delivery of the
converter is for some CLD analysers optional.
Some CLD analysers have an integrated ozone generator;
others need an external ozone generator. The CLD measuring
principle is often used for analysis of NOX by accredited
measurements as well as within research and development.
The CLD measuring principle is often much more expensive
than the IR and UV measurement of NOX.
NOx Converter type gas measuring principle
For some applications it is desirable, often for economic
reasons, to measure NOx by means of an NO gas analyser in
combination with an NO2 to NO converter.
The principle of the converter is that the gas passes
through the heated catalyst material which turns NO2 into
NO. Afterwards the gas is led through the NO analyser and
the total NOx concentration is measured. The NO analyser
used for this purpose is either an IR or UV gas analyser.
Flame Ionisation (FID) type gas measuring principle
The flame ionisation measuring principle (FID) is used for
analysis of hydro carbons, also called TOC and UHC. The
analysis of TOC with the FID measuring principle is done by
leading the gas into a combustion chamber where the gas is
burned off in an H2-flame. In this way the hydro carbon
molecules are decomposed and the concentration of c-atoms
is analysed by the detector. The FID measuring principle is
not able to differentiate between the different hydro
carbons but measures all c-atoms as a whole.
The FID analyser can be supplied either as a "heated"
or "cold" version. Non-heated FID analysers are not
applicable for emission measurements and other measurements
in warm and wet flue gasses. This is due to the fact that
hydrocarbons are water soluble and will be washed out in
the analysis system prior to the actual analysis.
This is why it is very important that the FID analyser used
for measurements in warm and wet flue gas is warmed up to a
temperature above the dew point of the flue gas.
Flow Measurement, Ultrasound type measuring principle
Flow measurement in the flue gas using the ultrasound
measuring principle has become more common during the last
5 to 10 years. When the flue gas flow is measured by
ultrasound, a sender and receiver unit, both with
ultrasound transducers, are placed across each other on
each side of the flue gas duct in an angle of 45° (or
another well-defined angle between 30° and 60°) in relation
to the direction of flow.
The difference in time for a sound impulse to pass the duct
with the flow direction and against the flow direction is
used for the calculation of the flue gas flow. Ultrasound
flow measurement is especially used in processes with hot,
wet gasses that might cause blockage of the pipe and
corrosion of materials placed inside the process. The
sender and receiver units are placed on flanges on each
side of the duct with no parts directly in the process.
In RTU Panel we are having several Analogue & Digital
Inputs /Outputs. We are recently facing some problems over
Analog inputs , which are not updating properly (But
Digital Input are updating normally) Can any one tell me
from where the problem is creating , we have checked PLC
CPU card, Analog Input card & we reconfigured ,But still
the issue is there.
1. Sometimes Analog values are updating & sometimes not.
2. Analog values are not updating normally , showing some
3. Total dead (0 value)
Analog values are Level & flow measurement from the field
(But in field the instruments are working normally &
Displaying the values normally in local display unit)
This problem we are facing in our Water Plant.
If any one experienced this same problem or having any
ideas related to our issue PLS send your valuable