Testing A Confined Space Before You Open It

The most dangerous moment in a confined space entry is not the entry. It is the thirty seconds before it, when somebody lifts a cover and the atmosphere inside meets the atmosphere outside.

Before that point you have a sealed space and an unknown atmosphere. After it you have a partially diluted atmosphere, a reading that no longer describes what is down there, and a person standing over the opening.

The sequence below solves that, and it has not changed in thirty years.

One: sample through the gap, not the opening

A manhole lid, a hatch cover or an access plate is rarely airtight. There is almost always a gap around the edge, and that gap is enough.

A hand aspirated sampling adapter draws air through a narrow probe tube pushed into that gap, delivering the sample to the sensor without the cover being disturbed. So the first reading you take describes the space as it actually is, sealed, undiluted and undisturbed.

If that reading says the atmosphere is hazardous, nobody has opened anything and nobody is standing in a release.

Two: open it, then test at all levels

Once the sealed reading confirms conditions under the cover are safe, remove it. Then lower the sensor slowly through the space, testing as it descends.

The reason is stratification. Gases do not mix themselves into a uniform atmosphere just because they share a space.

Chlorine is nearly two and a half times heavier than air and collects at the bottom. Hydrogen sulphide is heavier than air and does the same, which is why it accumulates in wet wells and sumps. Methane is lighter and gathers at the top. Carbon dioxide displaces oxygen from the floor upwards.

Where the previous contents of a space are undocumented, Polytec tubes help establish what is present before anything is measured for concentration.

A single reading taken at the opening tells you about the atmosphere at the opening. It says nothing reliable about the atmosphere three metres down, and three metres down is where the work happens.

Three: put the sensor where the person will breathe

Once entry is underway, the sensor belongs in the breathing zone of the worker inside the space, not at the top of the shaft and not clipped to the person at the opening.

That sounds obvious and it is routinely got wrong, usually because the instrument is easier to watch from outside.

Why a remote sensor beats a long hose

Here is the technical argument, and it still holds.

A sample drawing instrument pulls air up a hose to a sensor that stays with the operator. That introduces two problems. The sample takes time to travel, so the reading lags behind the conditions, and the further the hose the longer the lag. And the pump and filter system are drawing contaminated air through themselves, which in a wet well or a digester is exactly the wrong thing to do repeatedly.

A remote sensor on an electrical cable inverts it. The sensor goes into the space and the reading comes back as a signal rather than as air. There is no transport delay, because nothing is being transported. And there is no pump or filter to foul.

Where the sample has to be drawn, and sometimes it does, two things matter. Use tubing suited to the gas, because chlorine and several others adsorb onto standard hose and arrive at the sensor depleted. And allow the full purge time for the hose length before trusting the reading.

And log it

A confined space entry generates a record whether anybody intends it to or not. Data logging turns that into something usable: what the atmosphere was at each stage, how it changed during the work, and whether the ventilation was doing anything.

On a wastewater plant with regular entries, that log becomes trend data, and an area monitor left running through the work adds continuous coverage alongside it. Which wells run hydrogen sulphide, when, and under what conditions, is knowledge that makes the next entry safer rather than merely documented.

Talk to us

Confined space gas testing is one of the areas where the procedure matters more than the instrument. Talk to our Technical Consultants about the spaces you enter and how you enter them, from WA, NSW and QLD.