ISCO 6700SR, Instruction Manual, Page: 171 / 190

153
6700SR Refrigerated Sampler
“When first going there, a person may experi-
ence a little ‘shortness of breath’ following
exercise.
“People in good health are not afraid to drive
over the high passes in the Rocky Mountains.
At Loveland Pass, oxygen pressure is 13.2% of
a normal atmosphere. At the top of Mt. Whit-
ney, oxygen is equal to 12.2%. Many hikers go
there, and to higher peaks as well. After ade-
quate acclimation, they may climb to the top of
Mt. Everest, where oxygen is equal to only
6.7%.
“The lowest oxygen concentrations that I have
observed in a sewer atmosphere was 13 per-
cent. It was in a sealed chamber, near sea level,
upstream from an inverted siphon on a metro-
politan trunk. A man would be foolish to enter
the chamber. Without ventilation, he might die,
but not from lack of oxygen.
“It seems unlikely that anyone has ever died in
a sewer from suffocation, that is, a lack of oxy-
gen. Deaths have often been attributed to
‘asphyxiation.’ This is a word which, according
to the dictionary, is used to mean death from an
atmosphere that does not support life. The
word has sometimes been misinterpreted as
meaning suffocation, which is only one kind of
asphyxiation.
“In nearly all cases of death in sewers, the real
killer is hydrogen sulfide. It is important that
this fact be recognized. Many cities diligently
test for explosive gases, which is very impor-
tant, and they may measure the oxygen con-
centration which usually is unimportant, but
they rarely measure H2S. Death has occurred
where it is unlikely that there was any measur-
able reduction in the oxygen concentration.
Waste water containing 2 mg per liter of dis-
solved sulfide, and at a pH of 7.0, can produce,
in a chamber with high turbulence, a concen-
tration of 300 PPM H2S, in the air. This is con-
sidered to be a lethal concentration. Many
people have died from H2S, not only in sewers
and industries, but also from swamps and from
hot springs. In one resort area, at least five
persons died from H2S poisoning before the
people were ready to admit that H2S is not a
therapeutic agent. Hardly a year passes in the
US. without a sewer fatality from H2S as well
as deaths elsewhere in the world.
“The presence of H2S in a sewer atmosphere is
easily determined. A bellows-and-ampoule type
of tester is very satisfactory for the purpose,
even though it is only crudely quantitative.
When using a tester of this type, do not bring
the air to the ampoule by way of a tube, as this
may change the H2S concentration. Hang the
ampoule in the air to be tested, with a suction
tube to the bulb or bellows.
“Lead acetate paper is very useful as a qualita-
tive indicator. It cannot be used to estimate the
amount of sulfide, but it will quickly turn black
in an atmosphere containing only a tenth of a
lethal concentration.
“Electrodes or other similar electrical indicat-
ing devices for H2S in air have been marketed.
Some of them are known to be unreliable, and
we know of none that have proved dependable.
Do not use one unless you check it at frequent
intervals against air containing known H2S
concentrations. A supposed safety device that
is unreliable is worse than none at all.
“Remember that the nose fails, too, when it
comes to sensing dangerous concentrations of
H
2
S.
“Various other toxic gases have been mentioned
in some publications. It is unlikely that any
person has been asphyxiated in a sewer by any
of those other gases, except possibly chlorine.
The vapor of gasoline and other hydrocarbons
is sometimes present in amounts that could
cause discomfort and illness, but under that
condition, the explosion hazard would be far
more serious. The explosimeter tests, as well as
the sense of smell, would warn of the danger.
Pipelines in chemical plants might contain any
number of harmful vapors. They, too, are
sensed by smell and explosimeter tests if they
get into the public sewer. Such occurrences are
rare.
“The attempt to instill a sense of urgency about
real hazards is diluted if a man is told to give
attention to a long list of things that in fact are
irrelevant.
“Be very careful to avoid high H2S concentra-
tions, flammable atmospheres, and hazards of
physical injuries. Remember that much H2S
may be released by the stirring up of sludge in
the bottom of a structure. Obey your senses in
respect to irritating gases, such as chlorine
(unconsciousness comes suddenly from breath-
ing too much). Be cautious about strange odors.
Do not determine percent oxygen in the air.
There is a danger that the result will influence
a man's thinking about the seriousness of the
real hazards. Most important, use ample venti-
lation, and do not enter a potentially hazard-
ous structure except in a good safety harness
with two men at the top who can lift you out.”