Testing the Atmosphere Before Anyone Enters a Wet Well or Manhole

Purpose

To make a pre-entry atmospheric test tell the truth. A single number pulled off an instrument held at an open hatch is not a test; it is a reading of the air at the hatch, taken before the instrument finished responding, at one of the several depths a body will occupy. This procedure fixes the sequence, the depths, the wait at each depth, the corrections applied to each reading, and the re-testing required after ventilation and during entry. Classification, permit issuance, retrieval, attendant duties and rescue arrangements sit in the shop's confined space program and in sibling cards.

Scope

Any pre-entry or during-entry atmospheric evaluation of a wet well, lift station, manhole, interceptor, septic tank, valve pit or below-grade vault performed by this shop, whether or not entry is ultimately made, because the test is what decides that.

The governing standard is 29 CFR 1910.146 for general industry work and 29 CFR 1926 Subpart AA for construction work. Determine which Part the job falls under before the permit is written; the shop does not get to pick.

Roles and responsibilities

Acceptance criteria, fixed before anyone opens a cover

Set here so that no reading is judged against a number invented at the hatch.

• Oxygen: 19.5 to 23.5 percent. Outside that band is a hazardous atmosphere under 1910.146.
• Flammable: below 10 percent of the lower explosive limit, after correction. The standard's own criterion; the correction is not optional, see step 7.
• Hydrogen sulfide: below the action level in this shop's written program. Set it at or below the exposure limit for the applicable Part - 29 CFR 1910.1000 Table Z-2 for general industry, 29 CFR 1926.55 for construction - and far below the 100 ppm NIOSH IDLH. A 10 ppm action level is a defensible starting point; the number is the shop's choice within that ceiling, not a code figure.
• Carbon monoxide: below the program's stated action level. A running engine, pump or generator near the opening is the usual source, and it is under your control.

Any reading that cannot be cleared is not a pass. A number that sits under a criterion but is known to be under-reading is a lower bound, and a lower bound below a limit proves nothing.

Procedure

1. Confirm the classification before the instrument comes out. A sewer, wet well or interceptor is permit-required by default because it manufactures a hazardous atmosphere continuously. A clean test does not reclassify the space; it clears an entry. Nothing in this procedure downgrades a space.

2. Verify the instrument, not the sticker. Confirm the calibration is within the manufacturer's stated interval, confirm a bump test was performed on this instrument today, and confirm sensors are fitted for oxygen, flammable gas, hydrogen sulfide and carbon monoxide. A single-gas monitor cannot execute this procedure. If the bump test was not done today, it is done now, before the cover is touched.

3. Zero in air that is genuinely clean. Your nose is not one of the instruments in this procedure and never substitutes for one: hydrogen sulfide paralyses the olfactory nerve around 100 ppm, well under the concentrations that will drop you, so the smell fading as you work is a symptom of rising concentration rather than falling. Fresh-air zero upwind of the opening and away from vehicle and blower exhaust. Zeroing downwind of the hatch teaches the instrument that the contaminated air is normal, and every reading after that is offset in the flattering direction.

4. Compute the wait time per depth and write it on the permit. Two terms, both from the instrument manual: the sensor's stated response time, and the time for the sample to travel the length of the sampling hose at the pump's draw rate. Add them, then add margin. Skipping this is why a hurried test reads clean: the instrument was still filling with hatch air when the number was written down.

5. Open the cover with the opening's own hazards controlled. Covers are heavy and pinch: use a lid hook or pick, hands off the rim, feet out from under, never fingers inside the cover seat. If the opening is in a roadway or drive lane, set traffic control to the temporary-traffic-control standard your state has adopted for that road class before anyone stands next to it. Do not lean over the opening as it vents; the first minute after a cover comes off is when the headspace unloads.

6. Test in the required sequence, at every depth a body will occupy. 29 CFR 1910.146(d)(5) sets the order: oxygen first, then combustible gases and vapors, then toxic gases and vapors. The flammable sensor is a catalytic bead that burns the sample and needs oxygen to do it, so the oxygen reading is what decides whether the flammable reading means anything. Sample at the hatch, at roughly a third and two thirds of depth, and one foot above the liquid, holding the probe still for the full computed wait at each stop. Keep the probe out of the liquid; a wetted hose or filter ends the test.

7. Apply the corrections before evaluating anything. Two, always, each on its own log line:

• Response factor. A catalytic bead sensor calibrated on one hydrocarbon reads a different one differently. Multiply the raw percent-LEL by the factor printed in that instrument's manual for that gas pairing.
• Oxygen validity. Below the sensor's stated minimum oxygen concentration the flammable reading under-reads, and hydrogen sulfide poisons that sensor family over time. Both errors run the same direction, so in a low-oxygen atmosphere the corrected flammable figure is recorded as a lower bound, written with one inequality sign, never as a value.

8. Record depth and clock time against every number. A line without a depth is not evidence, and a log without times cannot show the wait from step 4 was observed.

9. If any criterion fails, ventilate on a computed purge, not on feel. Place the blower intake in clean air upwind, never near an engine exhaust, and route the discharge away from the opening and from everyone working. Derate the blower's free-air rating using the manufacturer's curve for the duct length and bends actually rigged, compute the space volume, and run for the air changes the written program requires. Then re-test at every depth, same order, same waits.

10. Ventilation stays on for the whole entry. The space is still making gas. Securing the blower on a clean reading removes the only thing holding the atmosphere where it is.

11. Monitor continuously during the entry, and evacuate on alarm without diagnosing it. The entrant carries the instrument. On any alarm the attendant orders evacuation and the cause is investigated from outside.

12. Re-test after any interruption. A break, a shift change, an equipment swap, or the blower stopping for any reason invalidates the previous clearance. Re-test from step 6.

The filled-in artifact: one wet well, start to finish

A 6 foot diameter wet well, 15 feet deep, liquid at 15 feet, on a short-cycling pump call. Instrument manual gives a sensor response time of 15 seconds and roughly 1 second per foot of sampling hose. Hose in use is 25 feet.

Wait per depth (step 4): 15 seconds response plus 25 seconds travel equals 40 seconds. Rounded up to 60 seconds and written on the permit.

Pre-ventilation readings, in sequence, 60 seconds at each stop:

Correction line 1, response factor. The manual gives 1.6 for this gas pairing on this instrument; use your own instrument's table rather than this number. Every corrected column above is the raw figure times 1.6. At 14 feet: 6 times 1.6 equals 9.6 percent LEL.

Correction line 2, oxygen validity. At 14 feet the oxygen is 17.6 percent, below the criterion and below the flammable sensor's valid range. So the 9.6 is not a value under the 10 percent limit; it is a lower bound. Recorded as flammable at 14 feet is at least 9.6 percent LEL, cannot be cleared.

Evaluation. Three separate failures at the bottom: oxygen 17.6 against a 19.5 floor, hydrogen sulfide 130 ppm against a 10 ppm program action level and above the 100 ppm IDLH, and a flammable figure that cannot be cleared. The hatch reading of 20.9 percent and 2 ppm, taken alone, would have passed all four criteria. That gap is the entire reason this procedure specifies depths.

Purge computation (step 9). Space volume is pi times 3 feet squared times 15 feet, or 424 cubic feet. The blower is rated 800 cfm free air; the manufacturer's curve for the 25 feet of duct and one bend actually rigged gives about 500 cfm, and 500 is the number used. 424 divided by 500 equals 0.85 minutes per air change, so twenty minutes delivers about 23 changes, comfortably over the 7 the written program requires.

Post-ventilation readings, same order, same 60 second waits, blower still running:

Oxygen is inside 19.5 to 23.5 at every depth, flammable is zero raw so zero corrected, and the highest hydrogen sulfide reading is 4 ppm against the 10 ppm program action level. All four criteria met at all four depths. Note that the oxygen figure at 14 feet is now valid, so the flammable reading there is a value rather than a lower bound.

During the entry. At 11:20 the entrant's monitor alarms on hydrogen sulfide at 12 ppm while a suction bell is freed and the sludge blanket is disturbed. That is the mechanism the whole card rests on: the gas is dissolved in the liquid and in the settled solids, and mechanical disturbance releases it in seconds regardless of how clean the pre-entry test was. The attendant orders evacuation, the entrant leaves, the blower keeps running, and re-entry follows a fresh test from step 6.

Records

Canceled permits are retained under 29 CFR 1910.146(e) for at least one year because they feed the annual program review. A log that supports that review carries four things a hurried one does not: a depth on every line, a clock time on every line, raw and corrected flammable figures in separate columns rather than one overwritten number, and the instrument's asset number so a suspect reading traces back to a specific unit's calibration history. Without depths, the log cannot answer the only question the review asks, which is whether the crew tested where the body went.

References

• 29 CFR 1910 Part 1910.146, permit-required confined spaces, general industry, including the testing sequence at (d)(5) and permit retention at (e); 29 CFR 1926 Subpart AA for construction work
• 29 CFR 1910.1000 Table Z-2 for the general industry hydrogen sulfide ceiling; 29 CFR 1926.55 for the construction limit
• NIOSH Immediately Dangerous to Life or Health value for hydrogen sulfide
• Manufacturer documentation for the multi-gas instrument and the ventilation blower, for response time, response factors, minimum oxygen for the flammable sensor, and the blower's delivery curve against duct length
• See related: Why a Sewer Is a Confined Space and What That Changes; Confined Space Entry (OSHA 1910.146); What a Confined Space Permit Changes About Your Day