Vaisala GMP343 - Manual (Page 19)

Chapter 2 __________________________________________________________ Product Overview
VAISALA_______________________________________________________________________ 17
Sampling System
The GMP343 flow-through model has no diffusion filter inside the
measuring chamber and the optics of the flow-through model cannot be
cleaned by the user. Therefore the sample gas must always be filtered and
dried before it is pumped to the measuring chamber. A hydrophobic
diffusion filter before the inlet of the measuring chamber is needed in order
to prevent particles and water from the surroundings from getting into the
sensor. The diffusion filter needs to be exchanged often enough to provide
a sufficient flow.
In humid environments it is important to avoid water condensation inside
the measuring chamber. This can be avoided by drying the sample air. The
most common method of drying the sample air is cooling and re-heating
the air. A simple system may consist for example of a cooling coil and a
water trap which are either cooled or located in a cool environment,
followed by a re-heating system. The idea is to get the moisture in the
sample to condensate on the walls of the copper tube, trap this water and
then lower the relative humidity by heating the sample. If the temperature
inside the measurement chamber is essentially higher than the
surroundings the cooling coil and the water trap can be simply located
outside the chamber. For re-heating, the heat generated by a pumping
system may sometimes be adequate, meaning that no additional heater is
needed. A simplified schema of a sample system removing particles and
moisture is illustrated below.
All sampling system components are commercially available.
Figure 3 Components of the Sampling System
CAUTION Liquids or acidic vapors may seriously damage the sensor.