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4.0
OPERATION
4.1 Working principle
The gas sample is taken in through the fumes probe, by a diaphragm suction pump inside the instrument.
The measuring probe has a sliding cone that allows the probe to be inserted in holes with a diameter of 11 mm to
16 mm and to adjust the immersion depth: the fumes picking point must be roughly in the centre of the flue
section.
The gas sample is cleaned of humidity and impurities by a condensate trap and filter positioned along the rubber
hose that connects the probe to the analyser.
The gas components are then analysed by the electrochemical sensors.
Oxygen (%O
) is measured with an electrochemical cell that acts like a battery which, over time, is apt to lose
2
sensitivity.
The toxic gases (CO, SO
deterioration being intrinsically lacking of oxidation processes.
The electrochemical cell guarantees high precision results in a time interval of up to about 60 minutes during
which the instrument can be considered very stable. When measurement is going to take a long time, we suggest
auto-zeroing the instrument again and flushing the inside of the pneumatic circuit for three minutes with clean air.
During the zero calibrating phase, the instrument aspirates clean air from the environment and detects the cells'
drifts from zero (20.95% for the O2 cell), then compares them with the programmed values and compensates
them. The pressure sensor autozero must, in all cases, be done manually prior to measuring pressure.
The values measured and calculated by the microprocessor are viewed on the LCD display which is backlit to
ensure easy reading even when lighting is poor.
4.2 Measurement cells
The measurement cells are electrochemical cells made up of an anode, a cathode, and an electrolytic solution,
which depends on the type of gas to be analysed. The gas penetrates the cell through a selective diffusion
membrane and generates an electric current proportional to the absorbed gas. Such current is measured,
digitalized, temperature-compensated, processed by the microprocessor, and displayed.
The gas shall not be at a pressure such to damage or destroy sensors. The maximum estimated allowed
pressure is ±100mbar gage.
The response times of the measurement cells used in the analyser are::
O
=
20 sec. at 90% of the measured value
2
CO(H
) =
50 sec. at 90% of the measured value
2
CO
=
50 sec. at 90% of the measured value
NO
=
40 sec. at 90% of the measured value
NO
=
50 sec. at 90% of the measured value
2
SO
=
50 sec. at 90% of the measured value
2
It is therefore suggested to wait 5 minutes (anyway not less than 3 minutes) in order to get reliable analysis data.
If sensors of poison gases are submitted to concentrations higher than 50% of their measurement range for more
than 10 minutes continuously, they can show up to ±2% drift as well as a longer time to return to zero. In this
case, before turning off the analyser, it is advisable to wait for the measured value be lower than 20ppm by
intaking clean air.
The CO sensor can be protected from high gas concentrations through the dilution function which allows for a
wider measurement range of the sensor without overcharging the sensor itself.
4.3 Connecting the fumes probe
The fumes picking probe is made up of an INOX steel tube with a plastic hand grip and an internal K-type
thermocouple (Ni-NiCr) for measuring the fumes temperature up to 800°C. The probe is connected to the
analyser through a double flexible hose, a filter group and a compensated cable for the thermocouple. The
polarized connector of the thermocouple is to be connected to the special outlet on the lower side of the
instrument. It is not possible to perform a wrong connection thanks to the different width of contacts. Connect the
shorter tube of the probe to the filter group (fine dust/condensate trap) which, in turn, shall be connected to the
central connector of the instrument marked with letter "A". Connect the longer tube, ending with a male
connector, to the negative pressure input of the instrument marked with letter "P-". The different diameter of
connectors does not permit any wrong connections; this permits to avoid any damages to the instrument.
4.4 Condensate trap and fine dust filter
The sample gas to be analysed shall reach the measurement cells after being properly dehumidified and purified
from the residual combustion products. To this purpose, a condensate trap is used, which consists of a
transparent polycarbonate cylinder placed along the rubber hose of the sampling probe. Its purpose is to
decrease the air speed so that the heavier fine dust particles can precipitate and the vapour in the combustion
gases can condensate.
The condensate trap must be always kept in the vertical position in order to prevent condensate from touching
K900000000SE 022628A0 071013
, NO, NO
) are measured with electrochemical sensors that are not subject to natural
2
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