Choosing a Portable Gas Detector for Confined Spaces
Before anyone enters a tank, vessel or manhole, the atmosphere must be tested. This guide explains single- versus multi-gas units, the sensor types for O2, CO, H2S and combustible gases, typical alarm thresholds and the daily bump test.
View gas detectorsSingle or multi-gas - which fits the job?
The first choice is the number of gases monitored. A single-gas detector measures one parameter, such as H2S or CO, and suits a clearly defined single hazard. For confined space clearance, however, a multi-gas detector with four sensors is almost always required.
Tanks, sewers and manholes present several hazards at once: oxygen deficiency, combustible gases and toxic gases. A 4-in-1 unit monitors O2, combustible gases (LEL), CO and H2S simultaneously and alarms on any exceedance.
- Single-gas: cheap, compact, ideal as a personal alarm for one known risk.
- Multi-gas (4-gas): the standard for clearance and confined space entry.
- Pumped unit: internal or external pump for pre-entry sampling from a safe distance.
- Diffusion unit: no pump, measures at the wearer's breathing zone.
From the clearance record to the PPE - how to prepare the entry.
Read the guideWhich sensors and alarm thresholds do you need?
Each sensor measures a different hazard in its own unit. Oxygen is shown in volume percent, combustible gases as a percentage of the lower explosive limit (LEL), toxic gases in ppm. Thresholds follow the exposure limits and are usually set in two stages as pre-alarm and main alarm.
The LEL sensor is usually a catalytic pellistor and needs oxygen to work. If the O2 level drops sharply, the LEL reading can become unreliable - which is exactly why combined measurement matters. Oxygen deficiency below 19.5 % is the most common fatal hazard in confined spaces.
Why is a bump test mandatory before every use?
A bump test is a short functional check: the unit is exposed to test gas of known concentration to confirm that all sensors respond and the alarms fire. It does not verify accuracy, only basic function - and it is done before every working day.
- Bump test: brief gas exposure before each use, confirms sensor and alarm function.
- Calibration: adjusts measuring accuracy, typically every 6 months or per manufacturer.
- Test gas and docking station automate the test and the record keeping.
- Document results - a complete audit trail is required for clearance work.
Sensor life matters just as much: electrochemical CO and H2S sensors typically last 2 to 3 years, while the LEL pellistor can be poisoned by silicones or lead. After strong alarms or impacts the unit should be checked outside the normal schedule.
Frequently asked questions
Is a single-gas detector enough for clearance?
Usually not. Confined spaces carry several hazards at once, so a 4-gas unit with O2, LEL, CO and H2S is standard. A single-gas detector only makes sense as an extra personal alarm.
How often must a bump test be done?
A bump test is recommended before every use, at least daily before work starts. It confirms that sensors and alarms work, but does not replace regular calibration.
What is the difference between a bump test and calibration?
The bump test only checks that the unit responds and alarms. Calibration adjusts the measuring accuracy against a test gas and is done typically every six months or as the manufacturer specifies.
Why does the LEL sensor need oxygen?
The catalytic pellistor burns the combustible gas on a wire and measures the heat. Without enough oxygen this reaction does not run correctly, so the reading becomes unreliable in low-oxygen conditions.
Looking for the right gas detector?
We supply single- and multi-gas detectors for confined space clearance - including test gas, docking station and advice on sensors and alarm thresholds.
Standards compliant
Detectors for clearance to recognised confined space rules.
Auditable records
Bump test and calibration fully documented.
4-gas technology
O2, LEL, CO and H2S in one unit.
Expert advice
Specialists help with sensor choice and thresholds.