Figure 28 Mirror Contaminaton Warning Symbol - Michell Instruments Optidew User Manual

3. Particulates- Particulates – particulates passing over the sensor can build up on the mirror
over time. This can cause a loss of mirror reflectivity. DCC will compensate for this by taking
into account anything on the surface of the mirror when resetting the optical condition,
however if the problem becomes too severe, the 'mirror contamination warning' symbol will
be displayed in the Sensor Status display.
4. Sample temperature – consider the difference between the sample temperature and the
dew-point temperature. Make sure that the sensor you are using has the cooling capability
to make the measurement (see section 4.5. for further information). If the sensor does not
have the necessary cooling capability, then you should consider an extractive system so the
sample can be cooled prior to measurement.
5. Sample pressure – If you are interested in readings in terms of ppm
the sensor is positioned in an environment of known pressure. You can then either enter this
pressure into the Optidew via the 'Inputs' screen (see section 3.2) or connect a pressure
sensor directly to the point of measurement (see section 2.6).
Extractive
If the sensor will be mounted into a sample conditioning system, then the above points are
still of relevance, but the following should also be considered:
1. Extraction point – make sure that the chosen extraction point is representative
of the process, i.e. that the sample of interest is flowing past the extraction point,
and it is not being pulled from a dead volume.
2. Enclosure and sample line heating – if the sample has a dew point greater
than ambient temperature, then all components upstream of the sensor will need
to be heated to at least 10°C above the sample dew point to ensure the water
remains in vapour phase.
3. Sample block flow path – the sensor block must be configured with gas inlet
and outlets installed in the side ports. The top is either blanked or used to install
a pressure transmitter.
If replacing an old Optidew installation then be aware that using the
old sensor block will result in poor response speed at low dew-points,
as it does not allow enough flow across the mirror.
Michell Instruments

Figure 28 Mirror Contaminaton Warning Symbol

Optidew User's Manual
or g/m Ensure that
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