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Preparation important when determining gas monitor placement

August 1, 2005

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Where is the best place to position a gas monitor in confined spaces?

Answered by John Villalovos, senior application engineer, RKI Instruments, Union City, CA.

Confined spaces are those in which there is typically no air or atmospheric movement. They can range from wells or shafts to vaults, rooms, tanks, vessels or even significant depressions in the ground.

When looking for the best place to position a monitor in a confined space, several important aspects must be considered. First of all, confined spaces need to be tested or qualified before entry. Typically, this involves the use of a monitor with a pump, where a hose can be placed into the confined space while the operator remains in a known safe atmosphere, observing the readings.

Sometimes instruments have extender cables with sensors at the end of the cables instead of pumps and hoses. Or, sometimes the entire instrument is placed into the confined space and then retrieved after a period of time. This procedure has been long considered archaic by modern techniques and standards.

Confined spaces can occur at various angles from vertical (or sloping) to horizontal. One must realize that gases can have various specific weights relative to air. The most common weight measurement of gas is referred to as vapor density. Air has a vapor density of 1.0. In the absence of ventilation or atmospheric movement, gases will tend to stratify within a confined space. In these hazards, the operator must place the probe of the instrument at various vertical levels for a predetermined time before deeming the entire space safe. Some knowledge of the likely hazards that could be present will aid greatly in the testing.

In other situations, there only may be an absence of oxygen in the confined space, without any displacement by other gases. Sometimes combustion, oxidation or biological activity can contribute to or cause such a condition.

Once the confined space has been qualified and ventilated, if necessary, then the monitor can either be worn on the person (if it is small enough), or placed in the space at the best location (if it is too big to be worn). Give particular attention to the breathing zone as well as to the gases or compounds that may have been present prior to entry. Again, residual heavy gases or vapors tend to settle low while light ones rise.

An awareness of the basic characteristics of the gas hazards that may be encountered, along with sound safety protocol practices, will maximize the safe and successful outcome of any given job.



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