How do you test a gas detector before tank entry?

Before entering a tank or enclosed space, you test your gas detector by performing a bump test to confirm the sensors respond to known gas concentrations, then you test the actual atmosphere inside the space before anyone steps in. Both steps are required — one checks your equipment, the other checks the space. Here is exactly how to do both correctly, and what to watch out for.

What gases should you check for before entering a tank?

Before entering any tank or enclosed space on board, you need to check for at least four gases: oxygen (O₂), flammable gases or vapours (expressed as % LEL), carbon monoxide (CO), and carbon dioxide (CO₂). A fifth toxic gas, typically hydrogen sulphide (H₂S), is also required based on a risk assessment.

The acceptable limits you need to confirm before entry are:

O₂: at or above 20.9% by volume
CO₂: below 0.5% by volume (5,000 ppm)
Flammable gases/vapours: below 1% of the Lower Explosive Limit (LEL)
Toxic gases: below 50% of the applicable Occupational Exposure Limit (OEL)

One thing worth knowing: CO₂ is now a mandatory measurement parameter under the latest IMO recommendations. Traditional 4-gas detectors measuring only O₂, LEL, CO, and H₂S are no longer sufficient. Even a tank that looks perfectly safe can contain dangerously elevated CO₂ levels, for example from iron oxidation (ordinary rusting) in a damp, confined space. A space does not need to carry hazardous cargo to become lethal.

For CO₂ specifically, standard electrochemical sensors often lack the resolution to accurately detect the 5,000 ppm regulatory limit. Non-Dispersive Infrared (NDIR) technology is now the benchmark for compliance because it provides the ppm-level resolution needed to catch dangerous CO₂ buildup before it becomes fatal. If your current detector uses a standard electrochemical CO₂ sensor or has no CO₂ sensor at all, it is worth checking whether it meets current requirements.

What is a bump test and why is it required before tank entry?

A bump test is a quick functional check where you briefly expose your gas detector to a known concentration of test gas to confirm that each sensor responds and triggers the alarm. It does not verify accuracy — it verifies that the detector is working at all. Before any confined space or tank entry, a bump test is required to confirm your equipment will actually alert you to danger.

Think of it this way: a gas detector that has been sitting in a locker for a week might have a depleted sensor, a blocked inlet, or a software fault. Without a bump test, you would not know until it fails to alarm inside the tank — which is too late. A bump test takes only a minute or two and gives you confidence that your equipment is functional before you rely on it with your life.

The bump test uses a certified calibration gas mixture that matches the gases your detector is configured to measure. You briefly expose the sensor head to the gas, and the detector should alarm within the required response time. If it does, you are good to proceed to the next step. If it does not, the detector must not be used for entry.

How do you perform a gas detector test step by step?

Testing a gas detector before tank entry involves two separate processes: the pre-use bump test on your equipment, and the atmospheric test of the space itself. Here is how to carry out both correctly.

Step 1 to 4: Bump testing your detector

Power on the detector and allow it to complete its warm-up cycle. Do not skip this — sensors need time to stabilise before they give accurate readings.
Zero the detector in fresh, clean air away from the tank opening. This sets the baseline for all subsequent readings.
Apply the test gas using a certified calibration gas cylinder and a flow regulator or bump test station. Hold the gas at the sensor inlet for the duration specified by the manufacturer.
Confirm alarm activation for each gas channel. The detector should alarm visually, audibly, and (if equipped) with vibration. All sensors must respond.

Step 5 to 7: Testing the tank atmosphere

Lower the detector into the tank on a rope or extension hose before anyone enters. Test at multiple levels — top, middle, and bottom — because gases stratify. CO₂ and H₂S are heavier than air and concentrate at the bottom; flammable vapours vary by density.
Allow readings to stabilise at each level before recording. Moving too quickly gives you inaccurate numbers.
Record all readings and confirm they fall within the acceptable limits listed above. Under current requirements, these results must be officially documented — an unrecorded test is treated by inspectors as a test that did not occur.

Entry is only permitted when all parameters are within limits. If any reading falls outside the acceptable range, ventilate the space and re-test before anyone enters. You can find more guidance on compliant detection equipment through our fire and gas detection systems overview.

What’s the difference between a bump test and full calibration?

A bump test checks that sensors respond to gas and trigger alarms. A full calibration adjusts the detector’s readings to match known reference concentrations, ensuring accuracy. Both are necessary, but they serve different purposes and are performed at different intervals.

Here is a clear comparison:

Bump test: Done before every use. Takes 1 to 2 minutes. Confirms the detector is functional and alarms are working. Does not correct sensor drift.
Full calibration: Done at regular intervals (typically every 3 to 6 months, or as required by the manufacturer and your Safety Management System). Adjusts sensor output to match certified reference gas concentrations. Requires calibration gas, time, and documentation.

A common mistake is assuming that a recent calibration means you can skip the bump test. It does not. A detector can pass calibration and then develop a fault the next day. The bump test is your last line of defence before you trust the instrument in a potentially lethal atmosphere. Our service and repair team handles both bump testing validation and full calibration for portable gas detectors.

How long before tank entry should you test the atmosphere?

You should test the tank atmosphere immediately before entry, and the results must be recorded and confirmed as acceptable before any permit is issued. Under current IMO recommendations, an entry permit is valid for a maximum of 8 hours — after that, the atmosphere must be re-tested and the permit reissued before work continues.

If the work team takes a break and ventilation to the space is interrupted, the permit is immediately void and all personnel must evacuate. Upon re-entry, a full atmospheric re-test is required. There is no grace period.

Testing too early and then waiting creates real risk. Atmospheric conditions inside a tank can change quickly — cargo off-gassing, iron oxidation, or a change in ventilation can shift gas levels within minutes. Always test as close to the moment of entry as practical, and monitor continuously once inside with a personal gas detector worn by the entrant.

What should you do if your gas detector fails the test?

If your gas detector fails the bump test or shows readings outside acceptable limits during atmospheric testing, do not enter the space. Remove the detector from service, tag it as faulty, and either switch to a verified spare unit or arrange for inspection and repair before proceeding.

Here is the correct response depending on which test fails:

Bump test failure: Do not use the detector. Check for blocked inlets, depleted sensors, or low battery. Switch to a spare and re-bump before use. Send the faulty unit for service.
Atmospheric test shows out-of-limit readings: Do not enter. Increase ventilation and re-test after sufficient gas exchange time. If readings remain out of limit, investigate the source of the hazard before proceeding.
Detector alarms during entry: All personnel must exit immediately via the planned escape route. Do not attempt an unplanned rescue — this is how fatalities multiply. Follow the ship-specific Enclosed Space Emergency Response Plan.

Never improvise a workaround when detection equipment fails. The cost of a delayed entry is always lower than the cost of an incident.

How Lavastica helps with gas detector testing and compliance

We understand that keeping your gas detection equipment compliant, calibrated, and ready for use is not always straightforward — especially when regulations change and older equipment no longer meets current requirements. At Lavastica, we help fleet engineers and technical superintendents stay ahead of that challenge.

Supply of compliant gas detectors, including 5-gas models with NDIR CO₂ sensors that meet current IMO recommendations
Standalone CO₂ detectors to supplement existing 4-in-1 units without replacing your entire fleet
Calibration gas and bump test equipment to keep your testing procedures documented and defensible
Service and repair for detectors that fail bump tests or are due for full calibration
Technical advice on which equipment is compatible with your existing onboard installations

Whether you need a quick replacement before a vessel departs or a broader review of your gas detection setup, we are ready to help. Learn more about us or contact us directly for fast, practical support.

Phone: +31 (0) 10 265 5070Email: info@lavastica.com