We used this sensor extensively in the past, as it is the only one we know of that can be immersed directly in organic liquids (although only the more nonpolar ones). It is also unaffected by most organic vapors when used in the gas phase, but its lower sensitivity and tendency to drift make it no longer our choice for this, compared with the LiCl sensor.
The sensor and corresponding hygrometer are manufactured by the German division of Endress & Hauser (Maulburg). In our studies, we have used the model WMY 270 Hygrometer with type DY20 probes. This Hygrometer is an old model and is no longer advertised by Endress & Hauser. In 1988, it was still possible to obtain replacement DY20 probes from Endress & Hauser UK, on special order (cost about US$ 1200, EUR 1000). Our experience in the use of these sensors in organic reaction mixtures is described in ref. 19. Endress & Hauser replaced these models with the Hygrolog WMY 770 Z and DY 43, DY 63, or DY 73 sensors. These operate on the same measurement principle, but the probes now include integral signal processing electronics, so that the meter reads directly in humidity units. We have not tested these sensors ourselves, but believe the primary measuring element would work as well as the old type. There is a problem, however, because we often use the sensors outside their specified range. The manufacturer's literature states a maximum dew point of 20°C, whereas biocatalytic reaction mixtures above ambient temperature will often exceed this. We have found that the older sensor does, nevertheless, give useful readings outside the nominal range. However, we are told that the electronics on the newer types will not pass on any out-of-range signal to the meter.
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