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Laser Hygrometer Dries up Processing Troubles

Photonics Spectra
Jan 1999
Daniel C. McCarthy, News Editor

Water may be essential to human life, but its presence can be anathema to certain controlled chemical manufacturing processes. In an attempt to control moisture in these processes, a major US chemical company is using an optical hygrometer and diode laser technology more familiar in telecommunications applications.
The hygrometer was designed by Southwest Sciences Inc. and is licensed and manufactured by Ametek Inc. The chemical company asked to remain anonymous for proprietary reasons.
"The process should have no water. Water leads to process fouling, downtime, yield loss, you name it," a senior research associate for the chemical producer told Photonics Spectra. "We'd previously been using capacitive hygrometers that suffered from a number of shortcomings, including materials incompatibility, fouling, slow response time."
The company surveyed other hygrometer technologies, among them the laser-based gas-sensing approach developed by Southwest Sciences for open-path and stack-gas-monitoring applications. "At the time we selected [this approach], it had not been used for an application of this type," the chemical manufacturer said.
The hygrometer uses optical absorption spectroscopy to measure moisture content, said David Bomse, principal research scientist at Southwest Sciences. It uses a near- infrared diode laser emitting at 1.4 µm -- a wavelength absorbed very strongly by water -- to measure the amount of optical absorption. Nitrogen bubbles through the liquid to be monitored, and the laser beam detects how much water is entrained in the nitrogen.
Other, nonphotonic technologies, such as capacitance sensors, have a slower response time, Bomse explained, and, in the event of detecting a large burst of moisture, may need to be recalibrated. Ametek's laser hygrometer can detect as little as 3 parts per billion but still can withstand large moisture emulsions without being overwhelmed. "The advantage [the chemical manufacturer] realized was faster response time and better reliability," said Bomse.
Most other optical gas sensors need data from sample and reference beams, he said. The hygrometer uses a technique called wavelength modulation that measures small absorbances with high accuracy and requires the use of only one beam. And since the measurement requires little light, a beamsplitter and fiber optic cabling enable the same laser to measure several remote locations simultaneously.

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