When analyzing gases there are seven basic types of measurements that can be made. Percent LEL or lower explosive limit or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Another measurement is percent volume, which measures the amount of a specific gas within a sample. Trace measurement is usually given in units of ppm or ppb. Leakage and consumption rates can also be measured, as can gas density and signature or spectra, which is the spectral signature of the gases present.