Pressure Measurement
Taking the total pressure of an aerosol that has a partial pressure of air is a bit of a challenge. Just using an
ordinary gauge will give lower pressure than the original one in the can, since some of the trapped air enters
the entry fittings and Bourdon tube of the gauge. In effect, the head space has been enlarged. Using a prepressurized
gauge will give a higher, more accurate pressure, but still, some air will be lost into the adapter
tube up to the toggle valve. The most accurate pressure is obtained by using a gauge three consecutive times.
For example, if pressures are 60.0, then 59.0 and then 58.0psig, it follows that the “0th” or original pressure
was 61.0psig. The can should be shaken between readings for re-equilibration and the can pressure should be
within the middle third of the dial range.
Oxygen Aerosols
Of all the ingredients in tropospheric air, oxygen is the most chemically active. It is, of course, vital to life. On
roads leading from China to the 18,000 foot highlands of Tibet, stores and gas stations offer large aluminum
aerosol cans containing about 125psig of oxygen gas, since oxygen deprivation can cause older people to fall
asleep en route to the Himalayas. The cans are returnable, but for very little money, since the shopkeepers say
they cannot determine if any of the 5–6 grams of oxygen have been used.
Extra oxygen refreshes the brain and tired muscles. On that basis, weary shoppers in Tokyo’s Ginza District
can go to an “oxygen bar” for a few deep inhalations of either pure or 85% oxygen. Their energy restored,
they can then continue their shopping tour. Take-away aerosols are available, but bar patrons usually get their
oxygen from a large cylinder. Payment is ostensibly high for the sterile nose-piece.
Oxygen as an Oxidizer
In the early days of the U.S. aerosol industry, two-piece and threepiece
tinplate cans were filled inverted with the product pre-chilled
to about -40°F using a massive ammonia or HCFC-22 refrigeration
system. The heavy Freon or Genetron propellant vapor displaced
all the air from the head space. The cans were hand fitted with bottoms,
which were sealed by means of a Panama Double Seamer or
equivalent device. They were then hot water bathed.
Similar techniques are currently used in the filling of beer and
beverages. Wine bottles are filled almost to the top, and the tiny
amount of air is displaced with a stream of carbon dioxide. It is well
known that air (oxygen) can degrade the odor and taste of these
drinks, so it is rigorously excluded. Once a bottle of wine is opened,
oxygen gets into the product that remains after the first round of
drinks. Degradation, including off-odors, taste changes, translucency
and resin formation can begin after only a few minutes, usually
making the wine less enjoyable.
By about 1957, the aerosol industry left cold filling behind
and adopted T-t-V, and then under-the-cup (U-t-C) gassing
instead. For the first time significant amounts of air (oxygen)
remained in their plain interior tinplate cans. Problems began
with serious corrosion, affecting truckloads of DDT-containing
insecticides. Traces of oxygen and water were the culprits. The
industry rapidly developed both lined cans and vacuum crimping
equipment. Numerous corrosion inhibitors were utilized.
A problem with oxygen is that it is slightly soluble in water
and organic liquids. Simply stirring them or pouring them in
air is sufficient to have them rapidly absorbing the gas to near
saturation levels. Oxygenated water is fairly corrosive to iron,
tin, chromium and many other divalent or polyvalent metals,
slowly converting them to such oxides as Fe2O3, SnO2 and
Cr2O3. The rate of oxidation of various substances, such as
ascorbic acid, is hugely increased by the presence of transition elements like
iron, but especially traces of copper.
Sometimes the oxidation is instantaneous and even reversible, as in the
case of N,N-dimethylaniline, which turns deep yellow with a stream of air or
oxygen. However, upon flushing out the oxygen with a stream of nitrogen, it
again becomes colorless.
(Top) Oxygen deprivation can cause
older people to fall asleep en route
to the Himalayan Mountains, so gas
stations offer trekkers large
aluminum aerosol cans containing
about 125psig of oxygen gas.
(Bottom) Oxygen bar in Japan.
March 2018 SPRAY 37