Chapter 2 __________________________________________________________ Product Overview
VAISALA _______________________________________________________________________ 21
- AC mains power supply
- RS-485 serial line
- LAN and WLAN interfaces
- data
logger
module
with real time clock
- additional analog output module
- alarm
relay
module
Operating Principle
The DMT345/346 transmitter incorporates the Vaisala DRYCAP
®
sensor, which uses an operating principle based on changes in
capacitance as its thin polymer film absorbs water molecules together
with a combined temperature measurement with a PT100 resistive
temperature sensor.
The DMT345/346 transmitter measures water vapor directly and gives
thus accurate results of moisture in the process. In DMT346 the sensor is
cooled down with a cooling set which makes it possible to use a polymer
sensor in very high temperatures, for e100 ... +350 °C. With this
technique, no complicated sampling systems are needed. The cooling set
is thermally isolated from the process itself. The temperature of the
sensor is cooled down which increases the sensor saturation rate.
Sensor saturation rate and temperature are then measured, and the
transmitter calculates the dewpoint and the mixing ratio on the basis of
these readings.
Note that for DMT346, SSR and temperature values don't represent the
process itself as they are measured after cooling. Dewpoint and mixing
ratio, however, are independent of temperature and remain unchanged.
The dewpoint measurement range is -25 ... +100 °C.