Optidew Series User’s Manual
14
97430 Issue 2, March 2017
OPERATION
3.3 Operational
Functions
3.3.1 Operating
Principle
The system operates on the chilled mirror principle, whereby a gas sample is passed
over the surface of a polished mirror contained within the open sensor housing. At
a temperature dependent upon the moisture content in the gas, and the operating
pressure, the moisture in the gas condenses out on the surface of the mirror.
An optical system is used to detect the point at which this occurs. This information
is used to control the mirror temperature and maintain a constant thickness of the
condensation layer on the mirror surface.
The system operates by illuminating the mirror with an LED. The light re
fl
ected back
is measured by a photo detector and this amount of light is recorded as a reference
point. As moisture builds up on the mirror the level of light re
fl
ected will decrease. By
comparing this signal with the reference point at any time, the system will be able to
control the Peltier drive circuit to either heat or cool the mirror in order to maintain the
desired condensation
fi
lm thickness on the mirror surface.
At an equilibrium point, where the evaporation rate and condensation rate at the surface
of the mirror are equal, the mirror temperature, read by a Pt100 platinum resistance
thermometer embedded in the mirror, represents the dew point.
3.3.2 Operating
Cycle
Figure 4
Operating Cycle
At initial switch-on, the instrument enters a DCC cycle for 4 minutes. This heats the
mirror 20°C (36°F) above the sensor temperature. This ensures that all moisture is
driven off the surface of the mirror.
The mirror is maintained at this temperature for the DCC duration. During the DCC cycle,
DATA HOLD
fi
xes the Channel 1 mA output at the value read before DCC commenced.
DATA HOLD typically lasts 4 minutes from the end of a DCC cycle, or until the instrument
has reached the dew point. This procedure is in place to prevent any system which is
connected to the outputs from receiving a ‘false’ reading.
After the DCC period has
fi
nished, the measurement period commences, during which
the control system decreases the mirror temperature until it reaches the dew point. The
sensor will take a short amount of time to settle on the dew point. The length of this
stabilization time depends upon the temperature of the dew point. The end of a DCC
cycle resets the interval counter, meaning that another DCC will start (by default) in 4
hours time. Once the measurement is stable, DATA HOLD will release, and the Channel
1 mA output will resume normal operation.
