PAUL NESDORE
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
Figure 1. Giant outdoor cylinders at Oxygen
Service Company plant store liquid oxygen,
nitrogen and argon at 250 psi. As needed,
liquid “gas” goes to rooftop vaporizers,
then down to filling stations on main floor
where gas is pumped into standard cylinders for delivery to customers. Company
moves more than 100,000 cylinders/year
Figure 2. Electrochemical gas detectors
continually measure oxygen levels at floor
and ceiling in Oxygen Service Company
laboratories. Readout stream is fed to
wall-mounted monitors in quality control
chemist Bruce Nasser’s office.
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 O percentages.
2
As long as oxygen levels are
between 19. 5 and 21. 5 the LEDs glow
green. If O levels drop, the LEDs turn
2
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.
G&I
PAUL NESDORE IS GASES & INSTRUMENTA-
TION 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@sensorelec
tronics.com.
