Operational Functions; Operating Principle; Operating Cycle; Figure 4 Operating Cycle - Michell Instruments Optidew User Manual

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 refl 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 refl 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

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.
14
Optidew Series User's Manual
Operating Cycle
Figure 4
97430 Issue 2, March 2017