The Bureau of Mines has developed a portable flammable gas detecting system for estimating the flammable gas concentrations in a mine containing one or more combustible gases at concentrations up to 100 vol pct. With this portable detecting system, mine rescue or recovery personnel can quickly determine if they are working in a potentially explosive gas atmosphere and either leave the area or make appropriate changes in the fresh air ventilation of the mine to render the area safe. The detecting system uses a commercial, intrinsically safe, flammable gas detector with a range from 0 to 5 vol pct methane (ch4). The flammable gas detector has a catalytic heat-of-combustion sensor that can detect all flammable gases that would be encountered in a mine including methane (ch4), ethane (c2h6), carbon monoxide (co), and hydrogen (h2). To determine the mine gas concentration, the rescue worker dilutes a sample with fresh air carried within the detecting system. The concentration of the diluted sample is then measured with the permissible flammable gas detector. The actual mine gas concentration is calculated from the product of the indicated diluted gas concentration and the system dilution ratio using a pocket calculator or the nomograph on the side of the unit.