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Why an oxygen supply company worries about low oxygen levels in its quality control laboratory.

For Minnesota-based Oxygen Service Company, monitoring oxygen levels in the laboratory helps assure employee safety. To monitor those levels, the company uses twin oxygen detectors from Sensor Electronics. The sensors must meet a number of critical capabilities. See the Sidebar, "How the Sensors Work," which describes the technology and the conditions under which the sensors must operate and consistently deliver reliable readings.

"Actually," says analytical chemist Bruce Nasser who heads up laboratory operations, "We work them backwards. We're not looking for oxygen but rather lack of oxygen in the lab. The danger isn't from varying oxygen levels per se, but rather potentially lethal concentrations of other gases. A leak of some other laboratory gas could absorb or displace oxygen, possibly endangering our staff."

Oxygen Service supplies manufacturers, welding shops, hospitals, medical and dental clinics, school and college laboratories and similar customers within a 50 mile radius around the Twin Cities. (The company also has an out-of-state branch operation.) Altogether the company moves 100,000 gas cylinders a year.

Besides oxygen, the company handles nitrogen, argon, helium, and carbon dioxide plus a dozen other inert and nonflammable gases. In addition, the company supplies acetylene (for welding shops) and propane (for forklifts and pallet jacks) housed in a separate area.

Liquid oxygen, nitrogen and argon are stored in huge vacuum jacketed vessels under 250 psi. As needed, the liquids are pumped to rooftop vaporizers, then down to filling stations where the gases go into standard-sized cylinders for delivery to customers.

Nasser's quality-control laboratory handles two functions; first, it closely checks the standard gases for purity; second, the lab blends up to six gases in specific mixtures for customer use. (Interestingly, the gases are measured by weight while mixtures are sold by volume.) Each custom-blend is carefully analyzed by a gas chromatograph to hold tight tolerances of each component gas as well as the finished mixture.

While those gases are not toxic, the danger is that a leaking pipe, valve, or fitting could flood the laboratory with an odorless invisible gas that would displace the oxygen.

That is why the two detectors monitor oxygen levels at both the laboratory floor and ceiling. Continually sniffing the ambient air, they feed real-time readings to a wall-mounted monitor where color-coded LEDs show if a dangerous condition exists, and a digital readout panel that displays actual O₂ percentages.

As long as oxygen levels are between 19.5 and 21.5 the LEDs glow green. If O₂ levels drop, the LEDs turn amber, then red. At condition red, the monitor turns on alarm lights and a screeching klaxon, warning employees to evacuate. It also turns on supply fans to flood the building with outside air. Once oxygen levels are back within the safety limits, the lights return to green. "Gas analyzers are sensitive to electrical surges, temperature swings and other factors. Depending on the setup, analyzers can drift. So they must be on a regular calibration schedule," says Nasser.

Paul Nesdore is Gases & Instrumentation Chief Editor. For more information, contact Patrick Smith, Vice President and Director of Research and Development at Sensor Electronics. He can be reached at 952-938-9486 or psmith@sensorelectronics.com.

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