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To users of high-purity bulk nitrogen, hydrogen, dry air or other specialty gases, it is critical to stable operations that those gases are free of contaminants such as moisture. Specifications for maximum moisture allowed are often as low as the parts-per-million and even parts-per-billion levels.


In the past specialty gas suppliers and users have relied on electrochemical cells, gravimetric and other technologies to measure trace amounts of moisture in specialty gases in process streams. Gravimetric technology provides excellent results, but it can be slow, expensive, and require a lot of expertise to run.


Although electrochemical devices may perform with acceptable accuracy at first, the confidence level in their measurements soon becomes low, due to drift, the inability to read high concentrations of moisture. There is a tendency for those sensor probes to become desensitized by the gas streams they are measuring.


Specialty gas manufacturers must certify the contents in ppm or ppb to their customers. This is why reliability of measurement is so important.


Now a new H2O moisture and analyzer technology is available that employs tunable laser diode (TDL) spectroscopy to provide highly accurate and virtually instantaneous measurements of trace amounts of a particular gas (H2O in this instance). By its nature, the TDL based gas detection method is not susceptible to aging affects, making its factory calibration a timeless constant. The result is a very low maintenance analyzer that does not require consumables or scheduled calibration.


In effect, this new technology offers the best of both worlds in measurement: accuracy and speed.


Developed by SpectraSensors (Rancho Cucamonga, CA), TDL-based analyzers have revolutionized the measurement of gases in petrochemical streams such as natural gas. The technology employs a simple measurement that uses a fundamental principle: Molecules vibrate when excited by light at specific wavelengths. Therefore, if you can detect how much light is absorbed at these wavelengths, you can precisely measure the concentration of a given gas.


The system consists of a cell that the sample gas flows through, a tunable laser diode that emits a specific wavelength of light through the gas, an optical detector, and software to analyze and output the results.


The technology, widely considered a new standard in trace gas measurement, is extremely fast and the results are highly reliable. Furthermore, this "TDL" sensor does not become desensitized or drift over time.