Key values
Source: IEC 60079-20-1:2010 data table.
Explosive range and ppm values
Acetylene can ignite when mixed with air between 2.3 % and 100 % by volume. Below the LEL the mixture is too lean to ignite, above the UEL too rich. A rich mixture re-enters the range when it is diluted with air, so concentrations above the upper limit are not safe either. An upper limit of 100 % means the substance can also decompose and react without air.
The lower explosive limit in ppm and g/m³. Useful to compare a gas detector's %LEL reading with ppm values in a measurement report.
| Concentration | ppm | g/m³ (20 °C) |
|---|---|---|
| 1 LEL (100 %LEL) | 23,000 | 24.9 |
| 20 %LEL | 4,600 | 4.98 |
| 10 %LEL | 2,300 | 2.49 |
Lighter or heavier than air?
Acetylene is slightly lighter than air: relative density 0.90 (air = 1). A release tends to rise, but as its density is close to that of air it mixes readily with air movement and spreads around.
The relative density applies to the substance at room temperature. Very cold releases, for example from an evaporating liquefied gas, can behave as heavier than air for a while.
Ex equipment selection
Autoignition temperature 305 °C. The maximum surface temperature of equipment in the zone must stay below it: T2, T3, T4, T5 and T6 are suitable; T1 is not, because the limit is not below the autoignition temperature.
- T1450 °C not suitable
- T2300 °C suitable
- T3200 °C suitable
- T4135 °C suitable
- T5100 °C suitable
- T685 °C suitable
- IIA not suitable
- IIB not suitable
- IIC suitable
Gas group IIC: equipment marked IIC may be used. The equipment protection level (EPL: Ga, Gb, Gc) depends on the zone, not on the substance; the zone is decided in the explosion protection document from the release and the ventilation. Specific conditions in the certificate (X) and the ambient temperature range are checked separately.
Where is it found?
Used for oxy-acetylene welding and cutting; stored in cylinders dissolved in acetone. Also a raw material in the chemical industry.
Where these values sit in the explosion protection document
- Substance properties. The first step of the document is to record these values, with their source, for every flammable substance on site.
- Hazardous area classification. The lower explosive limit and the density are inputs to the release calculation and to the direction in which the zone extends.
- Equipment selection. The temperature class and the gas group decide whether Ex equipment in the zone is suitable.
This page does not decide zones. The type and extent of a zone are calculated from where, how often and how much is released, and from the ventilation.
Sources
Explosive limits, autoignition temperature, gas group, flash and boiling point: IEC 60079-20-1:2010 data table.
The relative density is calculated from the molar mass (M / 28.96). The ppm and g/m³ values apply at 20 °C and 1 atm (molar volume 24.055 L/mol). The temperature class follows the rule that the surface temperature must stay below the autoignition temperature.
This page is for information. For the substance on your site, the current safety data sheet (section 9) and the relevant standards apply. All substances are in the explosive limits table.
Frequently asked questions
What is the LEL of acetylene in ppm?
2.3 % by volume, that is 23,000 ppm. 10 %LEL corresponds to 2,300 ppm and 20 %LEL to 4,600 ppm.
Is acetylene heavier than air?
No, slightly lighter. Relative density 0.90 (air = 1): a release tends to rise but mixes readily with air movement.
Which Ex equipment is suitable for acetylene?
Temperature class: T2, T3, T4, T5 or T6 (T1 not suitable). Gas group: equipment marked IIC. The protection level (EPL) depends on the zone decided in the explosion protection document, not on the substance.
What is the autoignition temperature of acetylene?
305 °C. A mixture touching a surface at this temperature can ignite without a flame or spark, so the surface temperature of equipment must stay below it (at least T2).