December 2008

The impedance hygrometer infers water vapor

Moisture measurement for the natural gas transmission sector is important.

Accurately tabbing the water content in can save thousands of dollars in natural gas transmission operation, custody-transfer quality assurance, and storage recovery costs by preventing moisture, corrosion, and inefficient fuel usage.

A solid sample conditioning system can save wasteful over-drying processes, extend the life of a moisture sensor, and ensure accurate moisture measurements.

The impedance hygrometer works in hydrocarbons that consist of hydrogen and carbon atoms or in halogenated hydrocarbon, amides, esters, and silicon oils. It measures the water content of a sample by means of probe whose electrical impedance is a function of the vapor pressure of moisture in the fluid. Impedance is the opposition to the flow of alternating current (electrical resistance).

The probe is an aluminum strip anodized to form a porous layer of aluminum oxide. A thin coat of gold covers the aluminum oxide. Leads from the gold and aluminum electrodes of the probes connect the sensing element to the measuring circuitry.

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As the sample gas passes over the probe, the water vapor that exists in the gas penetrates the gold layer and equilibrates on the aluminum oxide.

The number of molecules attaching to the aluminum oxide is a function of the water vapor pressure in the sample. Each water vapor molecule adsorbed contributes a distinct increment to the total conductivity of the aluminum oxide.

The total probe impedance, which is the reciprocal of the probe's conductivity, is thus a measure of water vapor pressure in the sample.

The water vapor pressure directly determines the dew-point temperature and the moisture content of the gas.

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The probe's design is for direct installation into the process stream. However, where danger of explosion exists, the probe is in a small chamber connected to the process line with flashback arrestors located at the sample inlet and outlet.

The instrument works on all gases that are not corrosive to the probe and that will not spontaneously polymerize on contact with the probe materials. On liquids, the instrument works only in fluids with a moderate solubility for water vapor. Thus, it is not suitable for measurement in polar liquids such as the alcohols.

Use this type of probe and system in cyclohexene, diethyl ether, liquefied natural gas, ethane, propane, butadiene, butane, styrene, and propylene.

The analyzer requires recalibration about twice a year and must not visit excessive temperatures (160°F, 70°C) or excessive flow velocities.

An aluminum oxide impedance hygrometer can cost anywhere from $2,000 to $20,000.

Nicholas Sheble (nsheble@isa.org) writes and edits Automation Basics. The source for this article and the schematics is IEH: Process Measurement and Analysis, Fourth Edition; CRC Press and ISA Press, 2003