Jahresbericht 2005 - IPHT Jena

50
After long-term laboratory tests of the sensor
system with artificial and natural seawater, field
tests of the sensor system were performed by the
GKSS-Research Centre/Institute for Coastal
Research, Geesthacht on board the ferry ship
„Duchess of Scandinavia“ during a trip between
Cuxhaven (D) and Harwich (GB) [Fig. 2.26]. The
measured nitrate profile, shown in Fig. 2.27,
reflects the nitrate load in the seawater caused by
the rivers Elbe, Weser and Schelde as well as the
nitrate pollution from the North Frisian Islands
and the big harbour cities of the Netherlands and
fits well to measurements obtained by sampling
nutrient analysers on the same ferry route.
Fig. 2.26: Ferry route from Harwich to Cuxhaven.
Fig. 2.27: Nitrate content profile in the North Sea,
measured during the ferry trip in Fig. 2.26.
Further investigations mainly directed to the
improvement of the long-term stability of the sensor
were performed during a 2-week North Sea
round trip of the German research vessel
“Gauss”. On several stations the nitrate concentration
was measured by the sensor and compared
with the results of a commercial nutrient
autoanalyser [Fig. 2.28].
OPTIK / OPTICS
Fig. 2.28: Comparison between water samples
measured by sensor and by autoanalyzer.
2.2.15 Novel optical dew point sensor
(T. Wieduwilt, G. Schwotzer)
The dew or frost point is a measure of the water
content of any gas. Commercial optical dew point
hygrometers are based on the “chilled mirror”
principle. The devices consist of polished stainless
steel or platinum mirrors attached to thermoelectric
coolers (Peltier devices). The mirrors are
illuminated (e.g. with LED), and the reflected light
is received by photodiodes.
The gas stream whose humidity is to measure is
directed over the mirror surface. When the mirror
temperature falls below the dew or frost point of
the gas sample water condenses or forms ice
crystals onto the mirror surface. The reflected
light received by the photodiode is abruptly
reduced due to scattering.
The photodiode is tied into a servo loop which
controls the current to the Peltier cooler. This
enables the mirror to be maintained at an equilibrium
temperature where the rates of condensation
and evaporation of water molecules are
equal and therefore, a constant mass of water is
maintained on the mirror. The resulting temperature
of the mirror is then fundamentally, by definition,
equal the dew point temperature. A platinum
resistance thermometer (PRT) embedded
beneath the mirror surface measures this temperature.
An inconvenience of chilled mirror hygrometers
is the falsification of the measuring by contaminations
on the mirror surface. Contaminations
(e.g. organic particle) affect the light reflection
characteristics and cleaning procedures or compensation
methods are necessary. The electronic
schemes used for contamination compensation
are often sophisticated and expensive. Other
problems are the limited miniaturization and the
unsuitable manufacturing technology for mass
production.
In cooperation with BARTEC GmbH a novel optical
dew point sensor has been developed to overcome
these disadvantages.