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    Updated: 10 May 2020


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Explosion Hazards
In monitoring explosion danger the focus is entirely on the lower limit of the flammable range.

This lower limit is called LEL (Lower Explosive Limit).

Explosive gases are not explosive until they reach a certain mixture ratio with air that is unique for each gas.

Therefore, each explosive gas its own specific LEL level.

Detectors for explosive gases are calibrated to give a warning for a preset alarm value, well below the LEL level.

The measure range is divided into 0 – 100%.

This means 0-100% of the lower explosive limit.

The lower explosive limit for hydrogen is 4% by volume. 4% is equal to 100% LEL.

This means that at concentrations below 4% by volume there is no explosion risk.

In order to have a safety margin, Sweden has determined as an SIS standard that in order to regard a gas as dangerous the concentration of this gas / air mixture must exceed 25% of the LEL value.



Hydrogen burns/explodes in an air/gas mixture of 4.0 till 75 vol %.

We regard 10% of the LEL to be alarm level 1 (warning alarm) and 20% of the LEL to be alarm level 2 (sharp alarm) as appropriate alarm levels for explosive gases.


Hydrogen has 4% vol = 100% LEL

Early warning alarm will then be 10% x 4% = 0.4% as alarm level 1.

Sharp alarm will then be 20% x 4% = 0.8% as alarm level 2.

Early warning alarm will occur at alarm level 1, which is scaled in with 1V = 10% at 0-10Vdc control signal.

Sharp alarm will occur at alarm level 2, which is scaled in with 2V = 20% at
0-10 Vdc control signal.

These alarm levels are fed into the controller or PLC. For detectors with relay outputs, these are pre-calibrated to specified levels by default.

Usually the alarm levels 1 is activated at 10% of the LEL and the other preferably closer to LEL (type disaster alert)

You can of course set different values as alarm points.

The important thing is that the alarm levels are always activated before LEL is reached.

When the LEL concentration is exceeded, there is no danger of explosion because the oxygen content is too low for combustion to occur.


Explosion Hazard
Several different gases are used for heating in one form or another.

Examples of common flammable gases are natural gas (methane), LPG, propane and hydrogen.

All these gases have unique flammable gas / air mixtures.

Gas monitoring is intended to warn of unwanted and uncontrolled emissions of these gases, which may lead to explosion hazards.

When monitoring explosion risks concentration is entirely on the lower limit of the flammable region.

This lower limit is referred to as follows in different languages:

  • Swedish: UB (undre brännbarhetsgränsen)

  • English: LEL ( Lower Explosive Limit)

  • German: UEG (Untere Explosive Grenze)

In order to obtain a safety margin Sweden has determined that a gas/air concentration exceeding 25% of the UB value should be considered dangerous.

Consider 20% of UB as appropriate alarm point level for explosive gases.

Methane (natural gas) for example, will burn in gas / air mixtures between 5.0 - 14.0

Lower flammability limit and appropriate alert levels for a few gases.


LEL Suitable alarm level


5,0% 1,0% (= 10.500ppm)


2,1% 0,4% (= 4000ppm)

Hydrogen Gas

4,0% 0,8% ( = 8000ppm)

You can of course set other alarm levels.

Normally two alarm levels are used, one of which is activated at 5 or 10% of LEL and the other closer to the LEL level (emergency alarm).

The important thing is that the alarm points are always activated before LEL is reached.




Continuous measurement of produced gas.
Through continuous monitoring, even in sample form, we can measure that the produced gas is kept within the limits that are set.

The need may be, for example, to control hydrogen sulfide in natural gas or methane content in the production of bio- or sewage gases.

Avoiding unnecessarily long production cuts
Early leakage warning is a prerequisite for good preventive maintenance.

Early leakage detection means that repairs can be scheduled and not as an unpleasant surprise with production losses and even loss of sales.

larly Leakage detection means that repairs can be carried out planned and not come as an unpleasant surprise with production losses and even losses of sales.

Estimated cost savings
The estimated cost savings are (unfortunately) based on a possible location of an accident or disaster, which is subject to a cost estimate.

Personal injuries
Personal injuries are examples of estimated cost savings that can be difficult to calculate.

Explosion, oxygen deficiency and dangerous concentrations of toxic gases can lead to personal injuries.

In addition to serious personal consequences for all involved, the injuries cost companies a great deal of money (insurance claims, recruitment costs, etc.)

Below you will find several examples of the estimated cost savings that should be easy to calculate:

Property Damage
Uncontrolled combustion - read explosion - of flammable gases may lead to extensive property damage.

Major releases of ammonia can damage food in refrigerated warehouses.

Production Stops
A gas monitoring system which activates an alarm will reduce or eliminate production stops since the problems are detected at a very early stage.

Without a gas monotoring system, leakages are sometimes not discovered until the damage has occurred which may lead to huge costs and income losses.

A poor and unsafe working environment leads to a higher staff turnover which increases recruitment costs and staff training costs.

Insurance premiums
A high level of safety in hazardous activities can lead to better insurance conditions.

Fines and imprisonment
Ensuring that you stay within the laws and regulations undoubtedly reduces the risk of fines.

This thinking is now a reality in environmentally hazardous activities with strict regulatory requirements.

In addition to savings in the above categories a well marketed security policy, which includes gas monotoring, will create goodwill towards employees, authorities and local residents as well as increased security of supply for the company´s customers.


E-mail: lars@fjellcom.se     

Phone: +46(0)31-287202

Mobile: +46(0)706-755561



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