Combustible Gas Indicator Use
A CGI will be used to monitor the work zone. The MSA Model 260 Combustible
Gas and Oxygen Alarm is a hand-carried, battery-operated instrument.
It is used to sample atmospheres for combustible gases or vapors and
oxygen content and warn the user when predetermined concentrations of
either are reached.
The monitor will only detect combustible gases and vapors in air. It
will not indicate the presence of combustible airborne mists or dusts
such as lubricating oils, coal dust, or grain dust.
The lack of a response on this meter does NOT guarantee that
the environment is safe. If the presence of combustible mists or dusts
is suspected, the area will be immediately evacuated until further
guidance is received from the Operation Section Chief.
The CGI, as with the PID will not read concentrations of specific toxic
compounds. The CGI operates by combusting flammable components of the
vapor/gas sample on the surface of a platinum filament that increases
the temperature of the filament. The rise in temperature increases the
electrical resistance, reducing the current through the detector that
is measured through a potentiometer. The change in current is indicated
by an increase in the deflection of the meter. The meter reads 0-100
percent of the Lower Explosive Limit (LEL). When concentrations are above
LEL, the meter will indicate greater than 100 percent. With most CGIs,
the meter will return to 0 when concentrations are greater than the Upper
Explosive Limit (UEL). If CGI measurements in the work zone area are
equal to or less than 10 percent of the LEL, work can continue with caution
using continuous monitoring; if the work area measurements exceed 10
percent of the LEL all operations must cease, and the area must be evacuated
and permitted to ventilate. The CGI will be set to alarm at 10 percent
of the LEL, and will be positioned as close as possible to the source
during sampling. In addition, readings will be taken directly at the
borehole every 15 minutes. The Operation Section Chief, will direct the
Incident Safety Officer to periodically check the CGI readings for the
area to determine if the work may proceed. The instrument sensitivity
can be reduced by the following compounds: selenium compounds, silicon
compounds, and volatile heavy metals such as tetraethyl lead. High humidity
may reduce sensitivity. Halogenated hydrocarbons corrode the filament.
The sensitivity of the CGI varies with different vapors and gases so
it is only truly accurate when measuring the calibration gas.