Measuring Ranges The measuring range is the proportion in which measured gases normally occur during measurements. The ranges are divided into: % LEL At the Lower Explosion Limit (LEL), a concentrated proportion of air and combustible gas is reached in which the gas mixture is capable of producing an explosion. The smallest gas concentration which, when mixed with air, causes an explosive gas mixture is 100% LEL. Too much fuel 100% combustible gas - 0% air Explosion danger 100% UEL 100% LEL Too little fuel 0% combustible gas - 100% air % volume The % volume measuring range is used to measure gases which only appear in large concentrations. The application is limited to the measurement of oxygen (up to 25% volume) and some toxic gases, eg. carbon dioxide (CO2). ppm Parts per million - the measured value in ppm shows how many parts out of one million constitutes the detected gas. 10,000 ppm = 1% volume. Measurements in the ppm range are used for few gas concentrations; for example, to monitor the TLV for toxic gases (TOX).