PST Michell Instruments S8000 -100, User Manual, Page: 9 / 68

S8000 -100 User Manual
Michell Instruments 1
INTRODUCTION
1 INTRODUCTION
The S8000 -100 is a high-precision laboratory reference instrument for the measurement
of moisture content in air and other gases. Relative humidity and other calculated
parameters based on dew point, pressure and temperature of the sample gas can also
be displayed. Gases can be sampled at a maximum pressure of 0.06 MPa (0.6 barg /
8.7 psig).
The S8000 -100 is capable of measuring dew points as low as -100 °C (-148 °F), and
up to a maximum of +20 °C (+68 °F).
1.1 Operating Principle
The system operates on the chilled mirror principle, whereby a gas sample is passed
into the sensor housing and flows over the surface of the chilled mirror contained
within. 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, and this information
is used to control the mirror temperature and maintain a constant thickness of the
condensation layer on the mirror surface.
A light-emitting diode (1) provides a light beam of constant intensity which is focused
by a lens system (2) to become the incident beam on the mirror surface (3), flooding it
with a pool of light.
Before the light beam reaches the mirror (3), a beam splitter (4) directs part of the
beam via a lens system (5) onto a sensor (6), which monitors the intensity of the LED
light and provides a feedback loop to keep this at a constant level.
Two sensors (7 and 8) monitor the light level reflected by the mirror. One of these
sensors (7) measures the light level due to the reflected incident beam, and the other
(8) measures the degree of light scatter due to the formation of water/ice on the mirror
surface. Each sensor has its own optical lens system (9 and 10) to concentrate the
reflected light onto the sensor.
The output from each of these sensors is compared and then used to control the drive to
a Thermo-electric cooler (11). Dependant on the result of this comparison, the control
system will cause the heat pump (11) to either heat or cool the mirror (3) in order to
maintain the desired condensation film thickness on the mirror surface.
At an equilibrium point, where the evaporation rate and condensation rate on the surface
of the mirror are equal, the mirror temperature, read by a Pt100 platinum resistance
thermometer (12) embedded in the mirror, represents the dew point.