What is the difference between LEL and PPM gas detection on tankers?

LEL (Lower Explosive Limit) and PPM (parts per million) are two different ways of measuring gas concentration, and both matter on tankers — but for completely different reasons. LEL tells you how close a gas mixture is to catching fire or exploding, while PPM tells you how much of a specific gas is present in the air, which determines whether it is safe to breathe. On tankers, you need both: LEL detection protects the ship from ignition risk during cargo operations, and PPM detection protects your crew from toxic exposure or oxygen displacement. The sections below break down exactly when each measurement applies, which gases require which reading, and whether a single detector can do both jobs.

When should a tanker use LEL detection instead of PPM?

A tanker should use LEL detection when the primary risk is fire or explosion, not toxicity. During cargo loading, discharge, tank cleaning, or any operation where flammable vapours may accumulate, LEL readings tell you how close the atmosphere is to the ignition threshold. If the LEL reading climbs, the risk of a spark triggering a fire or explosion rises with it.

LEL detection is the standard measurement for gas detection on tankers during cargo operations involving hydrocarbons such as crude oil, fuel oil, or chemical products. Regulations require the atmosphere in cargo tanks, pump rooms, and adjacent spaces to remain well below the explosive range before any hot work, tank entry, or ignition source is introduced.

PPM detection becomes the priority when people are entering a space. Even if a gas concentration is far too low to pose an explosion risk, it can still be high enough to harm or kill crew members. This is why both measurement types are needed on board, often from the same detector.

What does LEL actually measure on a gas detector?

LEL stands for Lower Explosive Limit, and a gas detector expressing LEL shows what percentage of the way a gas mixture is toward its ignition threshold. A reading of 0% LEL means no detectable flammable gas is present. A reading of 100% LEL means the gas concentration has reached the exact point at which it can ignite with a spark or flame.

Every flammable gas has its own LEL value expressed as a percentage of volume in air. Methane, for example, has an LEL of around 5% by volume. A gas detector set to measure LEL does not show you the raw volume percentage of methane in the air. Instead, it translates that concentration into a 0-to-100 scale relative to the ignition threshold. So a reading of 50% LEL for methane means the actual methane concentration is roughly 2.5% by volume — halfway to the point of ignition.

This makes LEL a practical, universal scale that works across different gases and mixtures, which is why it is widely used during cargo operations and tank inspections on tankers.

What does PPM measure and why does it matter for crew safety?

PPM stands for parts per million and expresses the absolute concentration of a specific gas in the air. A reading of 1 PPM means one molecule of that gas for every one million molecules of air. Unlike LEL, PPM does not relate to fire risk — it relates directly to the health impact on the person breathing that air.

PPM matters for crew safety because toxic gases such as hydrogen sulphide (H₂S) and carbon monoxide (CO) can reach dangerous or fatal concentrations at levels far too low to register meaningfully on an LEL scale. H₂S, for example, becomes immediately dangerous to life at concentrations around 100 PPM — a concentration so small that it would appear as essentially zero on an LEL readout.

Carbon dioxide (CO₂) is another example where PPM resolution is important. Under IMO Resolution MSC.581(110), which entered into force in December 2025, CO₂ levels in enclosed spaces must remain below 5,000 PPM (0.5% by volume) before entry is permitted. Standard LEL sensors lack the resolution to detect CO₂ at this level accurately. This is why dedicated CO₂ sensors using Non-Dispersive Infrared (NDIR) technology are now the benchmark for compliant gas detection on board ships.

What’s the difference between LEL and PPM readings for the same gas?

The key difference between LEL and PPM readings for the same gas is what they tell you: LEL describes explosion risk as a relative percentage of the ignition threshold, while PPM describes the actual concentration of that gas in the air. Both can refer to the same physical gas, but they answer entirely different safety questions.

To make this concrete, consider hydrogen sulphide (H₂S). Its LEL is approximately 4% by volume in air. A gas detector showing 10% LEL for H₂S means the actual H₂S concentration is around 0.4% by volume, or roughly 4,000 PPM. That concentration is not just an explosion risk — it is immediately fatal to breathe. An LEL reading alone would not communicate that danger clearly, because 10% LEL sounds relatively low.

Conversely, a CO₂ reading of 5,000 PPM (the regulatory limit for enclosed space entry) is only 0.5% by volume — nowhere near CO₂’s flammability range, which is so high it is rarely a practical concern. For CO₂, LEL is essentially irrelevant; PPM is everything.

This is why modern gas detectors on tankers display both units simultaneously, giving crew the full picture of both explosion risk and health risk in one reading.

Which gases on tankers require LEL detection, PPM, or both?

Different gases on tankers require different measurement approaches depending on whether the primary risk is explosion, toxicity, or both. The table below gives a practical overview:

Hydrocarbon vapours (methane, propane, butane, crude oil vapour): LEL detection is the primary requirement. These gases pose a significant explosion risk at concentrations well below toxic levels. LEL monitoring is standard during cargo operations, tank cleaning, and hot work preparation.
Hydrogen sulphide (H₂S): Both LEL and PPM are needed. H₂S is toxic at very low concentrations (below 100 PPM) and also flammable. PPM detection is important for crew safety during tank entry; LEL monitoring applies during operations where vapour accumulation is possible.
Carbon monoxide (CO): PPM detection is the standard. CO is toxic at low concentrations, and its LEL is so high that explosion risk is rarely the primary concern in normal tanker operations. PPM monitoring protects crew in enclosed and adjacent spaces.
Carbon dioxide (CO₂): PPM detection with high resolution is required. CO₂ is not flammable, so LEL is irrelevant. However, it can displace oxygen and become dangerous at concentrations above 5,000 PPM. NDIR sensors are now required for compliant CO₂ monitoring under current IMO recommendations.
Oxygen (O₂): Measured as a percentage by volume, not LEL or PPM. O₂ monitoring is a separate but equally important parameter — entry into enclosed spaces is only permitted when O₂ is at or above 20.9% by volume.

Understanding which measurement applies to which gas helps you select the right detector configuration and ensures your crew has accurate, actionable readings in every situation. You can read more about calibration and maintenance services to keep those sensors performing accurately over time.

Can one gas detector display both LEL and PPM simultaneously?

Yes, modern multi-gas detectors can display both LEL and PPM readings at the same time, and on tankers this is now effectively the standard. A compliant detector under current IMO recommendations must measure at least five gases simultaneously, which by definition requires both LEL-based sensors for flammable gases and PPM-resolution sensors for toxic gases and CO₂.

A typical compliant configuration for tanker use includes:

O₂ — measured as percentage by volume
Flammable gases — measured as percentage of LEL
CO — measured in PPM
CO₂ — measured in PPM using NDIR technology
H₂S — measured in PPM

All of these readings appear on the detector’s display simultaneously, giving the user a complete atmospheric picture before and during enclosed space entry. The key point is that the sensors behind each reading use different technologies: catalytic bead or infrared sensors for LEL, electrochemical sensors for CO and H₂S, and NDIR sensors for CO₂. A detector that shows only LEL readings cannot substitute for PPM-resolution toxic gas monitoring, and vice versa.

When assessing whether your existing detectors are still compliant, check whether they include a dedicated CO₂ sensor with PPM resolution. Many older four-gas units do not, and supplementing them with a standalone CO₂ detector is a practical way to close that gap without replacing your entire fleet of instruments.

How Lavastica helps with gas detection on tankers

We work with fleet engineers and technical superintendents every day who need reliable, compliant gas detection equipment — fast. Whether you are replacing outdated four-gas detectors, sourcing a standalone CO₂ unit to meet current IMO requirements, or looking for a complete multi-gas solution that works with your existing onboard setup, we can help. Here is what we offer:

Multi-gas detectors with simultaneous LEL and PPM readings, including NDIR CO₂ sensors for compliant enclosed space entry
Standalone CO₂ detectors to supplement existing four-gas units without replacing your full fleet
Compatibility advice to make sure new equipment integrates with your current gas detection systems and panels
Calibration and repair services to keep your sensors accurate and inspection-ready
Fast worldwide delivery from our Rotterdam warehouse, so you are not waiting for equipment while your vessel is in port

Get in touch with us directly and we will find the right solution for your fleet. Learn more about who we are or contact our team for fast, practical advice.

Phone: +31 (0) 10 265 5070Email: [email protected]