Confined Space Gas Detection
Confined spaces such as sewers, manholes, chambers, tanks, silos and voids can hold toxic or flammable gas or too little oxygen, with no warning to the senses. Atmospheric testing before and during entry is central to a safe system of work.
The four hazards
Oxygen deficiency, from rusting, biological activity or displacement by inert gases.
Oxygen enrichment, from leaking oxygen supplies, which makes materials burn far more readily.
Flammable gas, such as methane from sewers and landfill or fuel vapour.
Toxic gas, most often hydrogen sulphide and carbon monoxide.
The pre-entry testing sequence
1. Bump test the monitor and check it is in calibration date.
2. Test oxygen first, then flammable gas, then toxic gases.
3. Sample the top, middle and bottom of the space with a pumped monitor, allowing time for the sample to travel through the tubing.
4. Record the results on the permit to work.
5. Monitor continuously while people are inside, with a personal monitor worn in the breathing zone.
The permit to work
A permit records who is entering, the atmospheric test results, isolations, ventilation, rescue arrangements and the time limits of the entry. It should be cancelled when work ends.
The regulations
The Confined Spaces Regulations 1997 require employers to avoid entry where reasonably practicable. Where entry is necessary they must assess the risks, follow a safe system of work and have suitable emergency arrangements in place before anyone goes in. DSEAR applies where flammable atmospheres may occur.
Confined space kits and training
Our confined space kits bring together a calibrated 4-gas monitor, sampling pump, tubing and probe, with on-site training so your team can be working compliantly within days.
Hire or buy
For occasional entries, hire a pumped 4-gas monitor from £45 a week; for regular work, buying and servicing your own is usually cheaper. See the 4-gas monitor guide and our article on choosing a gas detector for confined space entry.
Common mistakes
Testing only at the opening: gases layer. Heavier gases such as H2S collect low, methane rises. Sample at every level.
Not allowing for the tube: short sampling times through a long tube give false clear readings.
Testing once: conditions change as work starts. Monitor continuously while anyone is inside.
Skipping the bump test: a monitor that has not been bump tested may not alarm.
Relying on ventilation alone: ventilation reduces the risk but does not prove the atmosphere is safe.
Oxygen readings
Normal air contains 20.9% oxygen. Most monitors alarm at 19.5% for depletion and 23.5% for enrichment. A fall of even 0.5% means something has displaced the air, so find out why before anyone enters, even if the reading is above the alarm point.
Bump testing and calibration
A bump test exposes the detector briefly to a known gas to prove that the sensors respond and the alarms sound. It takes about a minute and should be done before each day's use. A full calibration adjusts each sensor against certified gas and is normally needed at least every six months, or sooner if a bump test fails, the monitor has been dropped or it has seen a high exposure.
Our calibration service covers any brand of portable or fixed detector at standard rates by instrument type: from £40 for a single gas monitor, £50 for a 4-gas monitor, £90 for a multigas monitor and £140 for a PID. Each instrument comes back with a traceable calibration certificate.
Training your team
A detector only protects people who know how to use it. Our gas detection training covers start-up checks, bump testing, reading alarms and what to do when one sounds, plus the pre-entry testing sequence. We run on-site courses anywhere in the UK from £750 a day, or a three-hour online session from £350.
Talk to our engineers on 01489 326031 or email sales@7solutions.co.uk. We supply, hire, calibrate and train across the UK.
Frequently asked questions
In what order should a confined space be tested?
Oxygen first, then flammable gas, then toxic gases. An abnormal oxygen level affects the flammable reading, so oxygen has to be known first.
Where should I test inside the space?
At the top, middle and bottom. Gases heavier than air collect low and lighter gases high, so a reading at the opening can miss a dangerous layer.
Do I need a permit to work?
Most safe systems of work for confined spaces use a permit to work that records the atmospheric test results, the controls in place and the emergency arrangements before entry is authorised.
Can I hire confined space gas detection?
Yes. Calibrated 4-gas monitors with pumps can be hired from £45 a week.