Now that we understand what the Lower Flammable Limit is, let's look at how these values are determined and what that means for your application.
LFL is determined empirically. This means that there are variations in the values published by different authorities at different times.
It is likely that the following test conditions account for the deviations in these published values:
- vessel size and materials
- flow rates
- observed direction of propagation
- observation method
Almost all LFL data is rounded off to the nearest tenth of a percent by volume. A significant proportion of substances have LFL values with a standard deviation from the average of 0.1% by volume or less. Some, however, have deviations at or above 0.2% by volume. In fact, a survey of LFL values from six authorities for approximately 50 common industrial solvents shows three-quarters have a standard deviation equivalent to less than 10% LFL, and one third have a higher deviation, up to about 20% LFL.
Why is this important to note? Because when comparing calculations, ventilation rates, or analyzer response factors, there will always be some variation that depends on the particular set of LFL values used.