SpectraSensors, Inc. is a manufacturer of optically based gas sensors for the industrial process and environmental monitoring markets. The company’s sensor products measure the absorption of laser light at specific wavelengths to detect carbon dioxide and water vapor in industrial process control and environmental monitoring applications. Such applications include non-contact measurement of moisture, carbon dioxide, and other corrosives in the energy industry, petrochemical industry, arsenic and other impurities in drinking water (Water Quality), and airborne water vapor and other atmospheric measurements from commercial aircraft for the U.S. and International Weather Services (Atmospheric).
Natural gas poses a unique situation since it can have very high levels of solid and liquid contaminants as well as corrosives in varying concentrations.
Water measurements are made in parts per million, pounds of water per million standard cubic feet of gas, mass of water vapor per unit volume, or mass of water vapor per unit mass of dry gas. That is, humidity is the amount of “vapor-phase” water in a gas. If there are liquids present in the gas, they are often filtered out before reaching a gas analyzer to protect the analyzer from damage.
Measurements of moisture in natural gas are typically performed with one of the following techniques:
* Color indicator tubes
* Chilled mirrors
* Electrolytic
* Piezoelectric sorption, also known as Quartz Crystal Microbalance
* Aluminum oxide and silicon oxide
* Spectroscopy
Other moisture measurement techniques exist but are not used in natural gas applications for various reasons. For example, the Gravimetric Hygrometer and the “Two-Pressure” System used by the National Bureau of Standards are precise “lab” techniques but are not practical for use in industrial applications.
An accurate method for determining the amount of water is the Karl Fischer titration, developed by the German chemist whose name it bears in 1935. This method detects only water, contrary to loss on drying, which detects any volatile substances.
The classic laboratory method of measuring high level moisture in solid or semi-solid materials is loss on drying (LOD). In this technique a sample of material is weighed, heated in an oven for an appropriate period, cooled in the dry atmosphere of a desiccator, and then reweighed. If the volatile content of the solid is primarily water, the LOD technique gives a good measure of moisture content. Because the manual laboratory method is relatively slow, automated moisture analyzers have been developed that can reduce the time necessary for a test from a couple hours to just a few minutes. These analyzers incorporate an electronic balance with a sample tray and surrounding heating element. Under microprocessor control the sample can be heated rapidly and a result computed prior to the completion of the process, based on the moisture loss rate, known as a drying curve.
Moisture analysis covers a variety of methods for measuring moisture content in both high level and trace amounts in solids, liquids, or gases. Moisture in percentage amounts is monitored as a specification in commercial food production. There are many applications where trace moisture measurements are necessary for manufacturing and process quality assurance. Trace moisture in solids must be controlled for plastics, pharmaceuticals and heat treatment processes. Gas or liquid measurement applications include dry air, hydrocarbon processing, pure semiconductor gases, bulk pure gases, insulating gases such as those in transformers and power plants, and natural gas pipeline transport.