Impedance Moisture Sensor Technology

[Ate Shey]

This is article supplied by Michell Instruments Ltd.
Email: info@michell.co.uk
Website: www.michell.co.uk

Modern impedance dewpoint sensors are typically constructed
using state-of-the-art thin and thick film techniques. Operation
of the sensor depends upon the adsorption of water vapour
into a porous non-conducting "sandwich" between two
conductive layers built on top of a base ceramic substrate.

The active sensor layer and the porous top conductor, that allows
transmission of water vapour into the sensor, are engineered very
thinly.  Therefore the sensor responds very rapidly to changes in
applied moisture, both when being dried (on process start-up)
and when called into action if there is moisture ingress into a
process. Despite this extreme sensitivity to changes in moisture
content, the Impedance Moisture Sensor can be incredibly rugged
due to the nature of its construction. To protect the sensor further
against contaminants and pipe swarf it recommended that the
sensor is housed in a protective sintered stainless steel guard.



Exploded view of a ceramic impedance moisture sensor

Chilled Mirror Technology
Chilled Mirror Sensors are fundamental in their method of operation.
A miniature polished metal mirror is cooled by a solid state Peltier
thermoelectric heat pump until it reaches the dewpoint of the gas
under test. When this temperature has been reached, condensation
will begin to form on the mirror surface.

An electro-optical loop detects that condensation is forming, by
a reduction in the intensity of light reflected from the mirror surface
and through the control electronics of the cooled mirror instrument.

This modulates the cooling power applied to the Peltier. The mirror
surface is then controlled in an equilibrium state whereby evaporation
and condensation are occurring at the same rate. In this condition
the temperature of the mirror (measured by a platinum resistance
thermometer) is equal to the dewpoint temperature of the gas.



Chilled mirror measurement principle:
single optics detecting system 2 stage Peltier heat pump

Hydrocarbon Dewpoint Technology
The Dark Spot optical principle utilised for hydrocarbon dewpoint
detection is radically different from that of any other chilled mirror
instrument. Sensitivity of the order of 1ppm (molar) of condensate
enables the analyser to detect the almost invisible films of condensate
which are characteristic of hydrocarbon gases at dewpoint, due to
their low surface tension.

The optical surface is the key element of the sensor cell and
comprises an acid etched, semi-matt surface with a central conical
shaped depression. A well collimated beam of visible red light is
focused onto the central region of the optical surface. In the dry
condition most of the incident light beam is reflected from the
optical surface to form an annulus ring of light.

Optical detection is made of light dispersed. As hydrocarbon
condensates form on the optical surface during a measurement
cycle its optical properties are modified - the reflected light
intensity of the annulus ring increases and there is a dramatic
reduction in the scattered light intensity within the dark spot
region. It is this secondary effect which is monitored and interpreted.

[Power768]

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