Impedance Moisture Sensor Technology

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.

The Dark Spot principle showing the annulus ring on light being reflected from the mirror surface.

This article supplied by Michell Instruments Ltd.

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