Confined Space Safety
Selecting gas detectors for confined space entries - Part I
Without proper gas detec tion, hazardous atmos
tection instruments:
pheres may significantly
indirect reading and direct reading.
affect the health of an en
With indirect read
try team. Many airborne contaminants cannot be detected by smell or vision and can only be measured with special ised equipment. Data gathered in the late seventies and early eighties indi cated that 65% of all those who died in
confined spaces were unaware that the space they were entering was a poten tial hazard. Over 50% of confined space
ing instruments, samples must be sent to a laboratory for analysis; direct reading instruments provide their infor mation at the time
CATALYTIC COMBUSTION GAS SENSOR .FLAME ARRESTER
.HOUSING
COMPENSATING
BEAD
deaths occur to the rescuers and over one
of sampling. Since the primary objec tive of gas detec
third of the fatalities occurred after the
tion in confined
space was tested and declared safe and the gas detector was removed. Depending on its sensor configura tion, proper gas detection equipment can help identify the hazard and protect your
space operations is to immediately warn the entry team of adverse atmospheric changes, direct reading instruments are the only safe source of sampling infor
BEAD
Figure 1
mation. There are a number of direct
reading portable gas detectors on the Selecting a gas detector should be market today. based on the hazard faced. Unfortu Gas detectors come in a variety of nately far too many purchasers make one sizes, shapes, colours and sensor con of the largest and most crucial single figurations. For confined space work, equipment expenditures without really it is necessary to monitor for oxygen understanding what they are buying. deficiency/enrichment, combustible Sensors and their capabilities are the sin gases and toxics. Therefore an instru gle most important factor when choos ment capable of dealing with these three ing a gas detector, yet more often than issues is necessary. not, decisions are based on size, price, Gas detection manufacturers produce bells and whistles and other such fea instruments with a variety of sensor con tures that have nothing to do with the figurations. One should be fully aware instrument's detecting abilities. of the different sensor types and the ca There are two categories of gas de- pabilities of each before purchase. Just because catalogues or advertisements President, Canadian Safety Equipment say that their instrument covers these ^President, Safetyscope three main parameters doesn't mean that it will provide enMETALLIC OXIDE SEMICONDUCTOR(MOS) try teams with the required protec workers.
tion.
Never as
sume that instru .SiNTERED STAiNLESS STEEL
CUP
.NOBEL METAL WIRE
26
chambers which each contain a coiled
wire filament. One chamber is designed to allow air to enter it, and its coil is
coated with platinum or palladium. The other chamber is sealed to prevent air from entering and the coil is not coated. Both coils are heated (to temperatures of 500°F or higher). When combusti ble gases are exposed to the coil they will ignite and raise the temperature of the bead even higher. The temperature increase and the change of the coil's electrical resistance is displayed as"per cent LEL".
These sensors offer good linearity, and can react to most combustible gases. Flowever, as resistance change per per cent LEL is quite small, they work bet ter in concentrations between 1,000 and
what you can rea sonably expect to find in your par ticular space. that the instrument
Figure 2
Combustible Gas Sensors
a) Catalytic Combustible Gas Sensors. These sensors look for explosive atmos pheres. They detect combustible gases by causing an actual combustion of gases within the sensor chamber. (Fig ure 1). A catalytic sensor consists of a flame arresting material, encasing two
50,000 PPM. They do not measure trace amounts of gas (under 200 PPM) and therefore are of no use in determining toxic levels. The disadvantages are: • they must have a minimum of 16% oxygen content in the air to work accu rately, • the sensor can be damaged by lead or
2. You must ensure
.NICKEL PINS
SENSOR TECHNOLOGY
ments will work for
tant criteria:
.HEATER COIL
these hazards.
specific applica tions without ap plying two impor 1. You must know
SENSOR.
.TEMPERATURE
SENSING.
you are going to purchase/use is ca pable of detecting
silicone,
•the readings can be affected by humid ity and water vapour condensation, •they respond poorly to low energy hy drocarbons such as oil vapours, • they are not recommended for use in
Environmental Science & Engineering, November 1997