Environnement AS32M, Technical Manual, Page: 102 / 186

Environnement
S.A
AS32M
Duplication prohibited
DECEMBER 2013
3–64
3.4.6 NO2 INTERNAL PERMEATION BENCH (OPTIONAL)
The scale point check can be performed using a permeation bench equipped with a NO
2
source.

Principle:
The NO
2
used is maintained in a state of liquid/vapour phase equilibrium inside a closed cylindrical
tube fitted with a polymer diaphragm. Due to the differential partial pressure of the gas on either
side of the diaphragm and the diaphragm "permeability", gas is diffused towards the outside of the
tube. The weight of the gas diffused by unit of time, called "permeation rate", depends on several
parameters: nature of gas, thickness, surface and nature of diaphragm, partial pressures of gas on
either side of diaphragm, temperature.

Implementation :
The permeation tube is placed in a block which is thermostat-controlled to 50 °C (± 0,1 °C) and
continuously ventilated with a scavenging gas whose flow is limited by a restrictor (0,2 mm) at
about 17 l/h. Filtration of the scavenging gas is provided by a filtering columns assembly
connected to the zero air inlet of the analyzer. The outlet of the permeation bench (PTFE plug) is
connected to the "span gas" inlet of the analyzer. When this inlet is selected, the sample flow rate
of the analyzer is added to the ventilation flow rate and the analyzer measures the concentration
generated by the bench.

Operation :

The internal bench is selected using the "span" key, or automatically on a calibration cycle.
Once stabilized, the NO
2
concentration measured by the analyzer must be compared to the
concentration recorded on the test report.

Since NO
2
permeation is very sensitive to the quality of the scavenging air, the state of filtering
columns should be periodically checked.

The permeation bench being continuously ventilated, a filtering column containing 450 cm
3
of
vegetal activated carbon should be connected to the "pump" outlet of the analyzer in order not
throw outwards the NO
2
generated.

When setting into service or following an extended shutdown period, the stabilization time is
about 24 hours.

The autonomy of the permeation tube supplied with the analyzer is about 16 months. In case of
non-use of the analyzer, the tube must be removed from the bench, replaced in its original
packaging with absorbent bags and stored in a cool and ventilated place.

To keep the best possible accuracy, it is required to periodically check the characteristics:

Diluting air flow rate of the bench,

Permeation rate of the tube (theoretically constant).
It is required to perform a differential weighing on a
10th mg
precision scale, a
100th mg
precision scale
will be better.