a. Oxygen deficiency tests should be performed with an approved and calibrated oxygen
deficiency indicator. Operation of the indicator instrument should be in accordance with the
b. For the detection of "toxic gases," one of the simple effective colormetric detectors
(color changes to indicate concentration) may be used. Hydrogen sulfide can be detected at
concentrations as low as one part in 1,000,000. These detectors can be obtained commercially,
and a universal test kit is available which will detect concentrations of carbon monoxide, hydrogen
sulfide, and numerous other gases. An approved portable unit should be used to measure the
amount of combustible and toxic gases in the manhole atmosphere.
188.8.131.52 Elimination of Combustible or Toxic Gases
Never enter a manhole until test results indicate that the manhole is free
of combustible or toxic gases.
a. If tests made upon removing the manhole cover indicate that the atmosphere is
satisfactory, the manhole or vault may be entered and worked in. Additional tests should be made
when each crew begins work; the test interval should not exceed 8 hours. When the manhole is
covered with a tent or tarpaulin, the test interval should not exceed 2 hours. Place the tent or
tarpaulin so that an opening is left in the covering for ventilation.
b. If more than the allowable trace of gas is found on the initial test, ventilate the manhole
or vault with a power blower for a minimum of 10 minutes, then make a second test with the
blower running. If the test is satisfactory, the manhole or vault may be entered. Make this test
away from the direct blast of the blower. If gas is again found on the second test, continue to
ventilate the manhole with a power blower until the test is satisfactory.
c. Work can then be started in the manhole, provided adequate power blower ventilation
is continued. There should be enough ventilation to hold the quantity of gas in the manhole to an
allowable value until the work has been completed and the cover is replaced. While working in a
manhole being ventilated with a power blower because of previous gas detection, test the
atmosphere every hour. If the blower stops, leave the manhole at once and do not re-enter until
ventilation has been restored and the atmosphere test is satisfactory. Operate the blower outside
of a manhole tent or tarpaulin.