In brief
Before entering a confined space, testing is done in the order oxygen first, then flammable gas, then toxic gases, from outside and at different heights; a single reading at the manhole opening is misleading. A test is a snapshot, so continuous monitoring with a personal detector is needed throughout the work. The detector must be bump tested before every use and calibrated according to the manufacturer's instructions, and the results must be written on the entry permit.
What does confined space gas testing show, and what does it not show?
In confined spaces such as tanks, silos, reactors, wells and manholes, most fatal hazards are invisible and odourless. That is why the most critical line on the entry permit is confined space gas testing. However, a test done incorrectly creates a false sense of security: a clean reading at the manhole opening does not show heavy vapour that has collected at the bottom of the tank; a test done in the morning does not show gas brought in by a valve opened in the afternoon.
This article focuses on the test itself: which gas is measured in which order, the test points, monitoring during work and the reliability of the detector. For the definition of a confined space, all the conditions of the entry permit and the duties of the attendant, see our article on working in confined spaces, and for the general structure of the permit system, our article on the permit-to-work system.
The 4-gas detector: sensors and their limits
The most common device on site is the 4-gas detector, which measures oxygen, flammable gas, hydrogen sulphide (H₂S) and carbon monoxide (CO). Each sensor has its own weak points:
| Sensor | What does it measure? | Reading unit | Point to note |
|---|---|---|---|
| Oxygen (O₂) | The proportion of oxygen in the air | % by volume | A drop shows that another gas is displacing oxygen |
| Flammable gas (LEL) | How close flammable gases and vapours are to the explosive limit | % of the lower explosive limit | The catalytic sensor needs enough oxygen to read correctly; it is set to its calibration gas |
| Hydrogen sulphide (H₂S) | H₂S concentration | ppm | High concentrations deaden the sense of smell |
| Carbon monoxide (CO) | CO concentration | ppm | Colourless and odourless; comes from engines and combustion sources |
Do not forget two limits. First, the detector measures only the gases it has a sensor for. If there is another residue in the tank, such as solvent, ammonia or chlorine, a suitable sensor or a different device is needed for these gases; the safety data sheet shows what the residue is. Second, the LEL sensor reads relative to the gas the device was calibrated with; for a different vapour, the displayed value may deviate from the real value. Set out in writing before testing which gases are to be expected.
Testing order: why oxygen first?
Testing is always done in the same order, and the results are assessed in this order:
- Oxygen. In normal air, the proportion of oxygen is about 20.9%. In international practice, for example in the confined space standard of the US occupational safety administration OSHA, below 19.5% by volume is considered oxygen-deficient and above 23.5% an oxygen-enriched atmosphere that accelerates combustion. Oxygen is measured first because, if oxygen is low, the reading of the flammable gas sensor may also be unreliable. Even a small drop in oxygen is a sign that another gas displacing oxygen is present.
- Flammable gas (LEL). Flammable gases and vapours are read as a percentage of the lower explosive limit. 10% of the lower explosive limit is a common threshold at which entry is stopped. If hot work is to be carried out, the accepted value may be set lower in the workplace procedure.
- Toxic gases. H₂S, CO and other expected toxic gases are measured.
For H₂S measurement, the occupational exposure limit values in Annex 1 of the Regulation on Health and Safety Measures in Work with Chemical Agents provide guidance. There is no value for CO in this table; set the detector's CO alarm threshold according to the manufacturer's recommendation and the workplace procedure.
Test points: top, middle, bottom and far corners
Gases are not evenly distributed in a confined space. Methane, which is lighter than air, collects at the top, while H₂S and many solvent vapours, which are heavier than air, collect at the bottom; CO has a density close to air and can be found at any level. Testing is therefore done not from a single point but in layers:
- Start from outside. Testing is done with the hose or probe of a pumped detector without entering. Going inside to test defeats the purpose of testing.
- Three heights: Test separately at the top, middle and bottom levels. In vertical tanks, increase the distance between these points according to the height of the tank.
- Far corners and dead volumes: Gas pockets can form in compartmented tanks, behind baffles and in ducts with bends.
- Waiting time: In pumped devices, it takes time for gas to travel along the hose to the sensor. Observe the waiting time the manufacturer gives for the hose length; do not move to the next point before the reading has stabilised.
- Take account of the effect of ventilation: A clean result taken while mechanical ventilation is running does not show what will happen when the ventilation stops. To see the real state of the atmosphere, take the first test before ventilation starts, repeat it with the ventilation running just before entry, and treat ventilation failure as an emergency scenario.
Continuous monitoring during work
The entry test shows the situation at that moment. During work, the atmosphere can change for the following reasons: gas is released as sludge or sediment is disturbed, welding and cutting produce fumes and CO, a connected line leaks, ventilation stops. Therefore:
- Each person working inside, or at least the team, carries a personal detector, worn close to the breathing zone.
- Testing is repeated before re-entry after a break, a shift change or a change of task.
- When the detector alarms, those inside leave without waiting; no one re-enters until the cause of the alarm is understood.
- The attendant monitors the test values and communication with those inside from outside.
If welding or cutting is to be done in a confined space, a hot work permit is issued in addition to the entry permit, and flammable gas testing continues throughout the job. A detector to be used in areas classified as explosive atmospheres must also be suitable for that zone; for zone classification, see our article on the explosion protection document.
Gas detector calibration and bump testing
Sensors lose sensitivity over time; they can be poisoned or stop working altogether. Only a test shows that a device that appears to be working from the outside really detects gas. There are two separate procedures:
- Bump test: Test gas of a known concentration is applied to the sensors to confirm that the alarm works. It is done before every use or every day according to the manufacturer's instructions. A device that fails the test is not used.
- Calibration: The readings of the sensors are adjusted with certified calibration gas. Its frequency is set by the manufacturer's instructions; if the device has been exposed to a high concentration, has been dropped or has failed a bump test, it is done without waiting.
In addition, pay attention to the following:
- Zeroing in clean air: The device is switched on and zeroed in clean air, not near the confined space or an exhaust.
- Validity of test gas: Check the expiry date of test and calibration gas cylinders.
- Sensor life: Write the replacement dates of sensors in the device record.
- Records: Keep a logbook or digital record for each device recording the dates and results of bump tests and calibrations and the person who carried them out.
How are test results written on the entry permit?
The testing line on the entry permit should be detailed enough for anyone looking at it later to answer the question "was this test reliable?":
| Field on the permit | Example |
|---|---|
| Device and serial no. | 4-gas detector, with serial number |
| Last bump test | Same day, at the start of the morning shift |
| Last calibration | Date and the organisation or person who carried it out |
| Test time and tester | Time and full name, signature |
| Test points | Top, middle, bottom; manhole and opposite corner |
| Values | O₂ %, LEL %, H₂S ppm, CO ppm and other expected gases |
| Repeat tests | With times and values |
In chemical and plastics plants, entry into reactors, mixing tanks and storage tanks is a frequent task; for risks specific to these plants, see our chemicals and plastics sector page. In our OHS specialist service, we draw up the confined space inventory, write the expected gases and test points for each space into the entry procedure, and plan practical training for the employees who will carry out testing. You can use our OHS calendar tool to track training and inspection dates.
Frequently Asked Questions
What should the oxygen level be in a confined space?
In normal air, oxygen is about 20.9%. In international practice, for example in OSHA's confined space standard, below 19.5% is considered oxygen-deficient and above 23.5% an oxygen-enriched atmosphere. Even a small drop from the normal value may indicate the presence of another gas and the cause should be investigated.
Who can carry out gas testing?
A trained employee who knows how to use the detector, the testing order and how to interpret the results. The name of the person testing and the test values are written on the entry permit.
How often is a gas detector calibrated?
The calibration frequency is set by the manufacturer's instructions; it is done without waiting when the device has been exposed to a high concentration or has failed a bump test. A bump test with test gas is recommended before every use.
If the test is clean, can work be done without a detector?
This is not recommended. The entry test shows only the situation at that moment; during work, disturbing sediment, a leak or the ventilation stopping can change the atmosphere. Continuous monitoring with a personal detector should be carried out.
A certified specialist matched to your hazard class: site inspections, approved logbook, training plan, annual plan and report follow-up.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.