Vaisala DM500 - Manual (Page 23)

Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 21
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the functionality of the product.
DM500 Principle of Operation
DM500 is an automatic, continuously controlling condensation
hygrometer designed to measure dewpoint temperatures from -75 ºC
to +60 ºC, depending on the model. The system consists of the
DMP501 dewpoint sensing unit and the DMI500 user interface unit.
The measured gas flows through the sensing unit, where it contacts a
temperature controlled quartz surface. The quartz surface is monitored
for the presence of condensation. The presence or absence of
condensation is sensed using surface acoustic wave (SAW)
technology. In the measuring mode, the temperature of the quartz
surface is varied until condensation exists in equilibrium. Temperature
of the sensor element is measured with a temperature sensor (1/3 DIN
B, Pt100, 4-wire) bonded on the backside of the quartz substrate. This
measured value is used to accurately output the dew/frostpoint
temperature.
Figure 5 DM500 Operating Principle Illusrated