DALEMANS D-TEX3F+, Instruction Manual

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5
SAFETY INSTRUCTIONS
Explosive limit
The ratio between “percentage of the Lower Explosive Limit” (% LEL) and “percentage by volume”
(% v/v) varies from one gas to another. Standard IEC 60079-20-1 setting out the method for
determining explosive limits gives the following examples:
Gas Formula 100% LEL
Methane CH
4
4.4 % v/v
Butane C
4
H
10
1.4 % v/v
Propane C
3
H
8
1.7 % v/v
Alarm levels
In an explosive gas detection system, the alarm thresholds must be fixed relative to the Lower
Explosive Limit (LEL) of the gas detector. The alarm thresholds for explosive gases are typically
20% LEL for the first alarm level (A1) and 40% LEL for the second level (A2).
The choice of alarm levels must consider the possible effects of local climatic conditions.
Temperature The LEL of a gas is affected by temperature. When the temperature increases,
the LEL decreases and explosion risk increases.
Pressure The relationship between pressure and LEL is very complex. In general, if the
pressure increases, the LEL decreases and thus the explosion risk increases.
Humidity Fluctuations in humidity rates generally found in industrial environments do not
significantly affect the LEL of a gas.
Climatic conditions Effect on the LEL of a gas Effect on the explosion risk
Temperature
  
Pressure
  
Humidity

- -
Never set an alarm level over 60% LEL