Atmospheric Testing for Confined Spaces

Your nose cannot protect you in a confined space. Some deadly gases have no odor at all, and hydrogen sulfide can knock out your sense of smell just as it reaches dangerous levels. The only way to know what is in the air is to test it with a working instrument, in the right order and in the right places. This is for people who test the air, including entry supervisors, attendants and experienced entrants. Reading it does not make you qualified. You need training on your own meter and your employer's procedures.

What You Will Be Able to Do

• Explain the limits for oxygen, flammable gases and common toxic gases.
• Test a space in the right order, at the right levels.
• Bump test a meter, and say how that differs from calibration.
• Spot a reading that looks safe but is not.
• Set up ventilation that works, and retest it.
• Explain why inerted spaces need supplied air.

What Counts as a Hazardous Atmosphere

OSHA's confined space rule treats the air as hazardous if any of these are true:

As a rough check for dust, OSHA notes that a cloud that hides objects 5 feet or more away may be at the limit.

Oxygen

Normal air is about 20.9 percent oxygen. The levels below are approximate, and people react differently.

Low oxygen is hard to notice, since the body does not sense it well. Oxygen drops when it is used up by rust, rotting material, fire or welding, or displaced by gases such as nitrogen, argon or carbon dioxide.

High oxygen is dangerous too. Above 23.5 percent, clothing and hair catch fire easily. It usually comes from a leaking oxygen line or torch hose. Never use pure oxygen to freshen the air in a space.

A Small Drop Can Mean a Big Problem

Air is about one-fifth oxygen, so anything that pushes air out lowers oxygen by only one-fifth as much. That makes a small oxygen change worth a hard look.

Worked example: The meter shows oxygen at 19.9 percent, which is inside the acceptable range.

1. Normal oxygen is 20.9 percent, so the drop is 1 percent.
2. 1 / 20.9 = about 0.048, so about 4.8 percent of the air has been displaced.
3. That is about 48,000 parts per million (ppm) of something else.
4. If that gas is carbon dioxide, it is above the IDLH level of 40,000 ppm.

A reading of 19.9 percent is not a green light on its own. Ask what took the place of the air, and test for it.

Flammable Gases

A gas burns only between two limits. Below the lower explosive limit (LEL), the mix is too lean. Above the upper explosive limit (UEL), it is too rich. Too rich is not safe, because fresh air can bring it down into the burning range. You will also see the terms LFL and UFL, which mean the same thing.

Your meter reads flammable gas as a percentage of the LEL, not as a percentage of the air.

Worked example: Methane has an LEL of 5 percent gas in air.

1. A reading of 100 percent LEL means 5 percent methane.
2. A reading of 10 percent LEL means 0.5 percent methane.
3. At 10 percent LEL, the space is at OSHA's limit and workers must leave.

Many employers set an alarm lower than that, such as 5 percent LEL.

Common Toxic Gases

Hydrogen sulfide forms where organic matter rots without air, so you find it in sewers, wastewater plants, oil and gas work and paper mills. You can smell it at very low levels. At around 100 ppm, it can deaden your sense of smell, so the odor seems to go away just as danger is rising. At high levels, one or two breaths can knock you down.

Carbon monoxide comes from engines, generators and torches. It takes the place of oxygen in your blood. A gas engine near the blower intake can fill a space with it.

Nitrogen is not toxic, but it is a big danger. It has no odor, and it is used to purge vessels. A person breathing nearly pure nitrogen can pass out with no warning at all.

Your alarm settings may be lower than OSHA's limits. Know the settings on your meter. Other toxic gases depend on the space. Check what it held, what is nearby, and what you plan to do inside. A standard four-gas meter does not see everything.

The Four-Gas Monitor

A typical monitor reads oxygen, LEL, H2S and CO. Each sensor can be fooled or worn out, so:

• Bump test before each day's use. This is a short burst of test gas that shows the sensors and alarms respond. It does not adjust anything.
• Calibrate on the maker's schedule, and after a failed bump test. Calibration adjusts the meter against a known gas.
• Keep sensors in good shape. They wear out with age. Silicones, lead compounds and high gas levels can damage them.
• Warm it up in clean air and let it zero there, not at the opening of the space.
• Keep records of bump tests and calibrations.

Follow the maker's instructions and your employer's procedure.

How to Test a Space

1. Bump test the meter. Do this away from the opening, in clean air.
2. Do not enter. Test from outside, using a sampling pump and hose or by lowering the meter on a line.
3. Test in order: oxygen, flammable gases, toxic gases. OSHA requires this order. Many flammable sensors need enough oxygen to read correctly, so the oxygen reading tells you whether the others can be trusted. Flammable gas comes next because an explosion is the quickest way to hurt the most people.
4. Test top, middle and bottom. Do not stop at the first good reading.
5. Allow time. A sampling hose needs time to draw gas. Check the maker's figure, often a couple of seconds per foot of hose, and wait at each level.
6. Test in all directions in large spaces, including corners and side pockets.
7. Record results, times and the tester's initials on the permit.

Why Levels Matter

Gases layer by weight, especially in still air.

The middle level is where a worker's head may be, but the bottom is often where the worst air sits. Sludge is another source. Stirring it during work can release trapped gas after your first test passed.

Testing During the Entry

Conditions change. Welding, painting, cleaning, sludge and leaks can all change the air. In construction, OSHA requires continuous monitoring unless the employer shows that the equipment does not exist for that hazard or that periodic testing is enough. In general industry, the rule is to test as needed to show that conditions stay safe, and continuous monitoring is good practice. Keep a meter with the entrant in the breathing zone. If an alarm goes off, everyone leaves, and the source is found before anyone goes back in.

Ventilation

The standard method for clearing bad air is mechanical ventilation with a ducted blower that runs continuously.

1. Pick a clean air source. Put the intake upwind, away from engines, exhaust and vents.
2. Blow fresh air to where people will work. Run the duct close to the bottom, so that air sweeps through the whole space.
3. Avoid short circuits. If the duct ends near the opening, air can just go back out.
4. Use the right blower. If flammable vapors could be present, use a blower rated for that hazard, bonded and grounded.
5. Calculate the purge time. Allow for duct bends and length, which reduce airflow.
6. Retest with the blower off or running, as your procedure says, before entry.
7. Keep it running during the whole entry. If it stops, everyone leaves.

Worked example: A tank holds 2,000 cubic feet, and your blower moves 500 cubic feet per minute through its duct.

1. One air change takes 2,000 / 500 = 4 minutes.
2. If your procedure calls for 5 air changes, that is 4 x 5 = 20 minutes.
3. Retest. The reading decides, not the clock.

For fumes made during the work, such as welding, add local exhaust near the source.

Inerting

Inerting means replacing the air in a space with a gas that does not burn, such as nitrogen, to prevent fire or explosion. It creates an IDLH atmosphere with almost no oxygen. Entry into an inerted space needs supplied-air or self-contained breathing equipment, with a trained team and an escape plan. It is a specialized job, and your site must have a procedure for it.

Remember that nitrogen from a process can leak into other spaces and pool in low spots, even if nobody planned to inert them.

Common Mistakes

Check Your Understanding

1. Your meter reads oxygen 20.9 percent, LEL 0, H2S 0 and CO 0 at the top of a pit. Is it safe to enter?

Not yet. You have tested only one level. Test the middle and bottom, wait for the sensors, and confirm the rest of the permit.

2. An H2S alarm sounds at the bottom of a sump, then the rotten egg smell fades. A coworker says it has cleared. What do you say?

No. Fading smell can mean your sense of smell has been knocked out. Trust the meter, leave and ventilate.

3. Why test oxygen first?

OSHA requires that order, and low oxygen can make flammable sensors read wrong, so you need to know that first.

4. A vessel was purged with nitrogen last week. It has no smell. A coworker wants to lean in to read a tag. What should happen?

Stop him. Test first. Nitrogen has no smell, and a few breaths can be enough.

5. Your blower's duct is 20 feet long with three bends. Can you use the rated airflow on the label?

No. Bends and length reduce airflow, so use the reduced figure from the maker's chart, then retest.

Key Takeaways

• Acceptable oxygen is 19.5 to 23.5 percent, and flammable gas must stay at or under 10 percent of the LEL.
• Test oxygen, then flammable gas, then toxics, at top, middle and bottom.
• Bump test before each day's use, and calibrate on the maker's schedule.
• A small oxygen drop can mean a large amount of another gas.
• Use continuous ducted ventilation from a clean source, and never use oxygen.
• Inerted spaces are IDLH and need supplied air.