Tuneable Diode Laser Absorption Spectroscopy (TDLAS) products use the interaction between light and the molecules in a gas stream to determine the concentration of a given substance within that gas stream. In the case of Michell’s OptiPEAK TDL600, this is specifically water concentrations within a natural gas stream. The wavelengths used are specific to water molecules and therefore laser energy causes the water molecules to vibrate and absorb energy. This effect is used to calculate the concentration of water in a background gas.
Capacitive polymer sensors provide high-accuracy, excellent long-term stability and negligible hysteresis. They are insensitive to contamination by particulate matter, are not permanently damaged by liquids and are resistant to most chemicals. A capacitive humidity sensor works like a plate capacitor. The lower electrode is deposited on a carrier substrate, often a ceramic material. A thin polymer hygroscopic layer acts as the dielectric, and on top of this is the upper plate, which acts as the second electrode, but which also allows water vapor to pass through it, into the polymer. The water vapor molecules enter or leave the hygroscopic polymer until the water vapor content is in equilibrium with the ambient air or gas. The dielectric strength of the polymer is proportional to the water vapor content. In turn the dielectric strength affects the capacitance, which is measured and processed to give a relative humidity measurement. By also measuring temperature a dew point or absolute humidity value can also be obtained.
The ceramic metal-oxide moisture sensor is constructed using state-of-the-art thin and thick film techniques. Its operation depends on the adsorption of water vapor onto a porous non-conducting "sandwich" between two conductive layers built on top of a base ceramic substrate. The active sensor layer is very thin - less than one micron and the porous top conductor that allows transmission of water vapor into the sensor is even thinner. 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.
The Quartz Crystal Microbalance (QCM) technology for moisture measurement is based on monitoring the frequency modulation of a hygroscopic-coated quartz crystal with specific sensitivity to water vapor. Bulk adsorption of water vapor onto the coated crystal causes an increase in effective mass, which reduces the resonant frequency of the crystal, in direct proportion to the water vapor pressure. This sorption process is fully reversible with no long-term drift effect, giving a highly reliable and repeatable measurement.
Michell's chilled mirror dew-point hygrometers are precision instruments for critical measurement and control applications. A miniature polished stainless-steel mirror is cooled by a solid plate Peltier thermoelectric heat pump until it reaches the dew point of the gas under test. When this temperature has been reached, condensation forms 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 dew-point temperature of the gas.
The fundamental nature of this method means that chilled mirror instruments can be used as either extremely reliable and stable field instruments or as laboratory reference standards for the calibration of other devices.