Abstract: A method of continually monitoring the heating value of a fuel gas, and the apparatus for carrying out this method. The method includes mixing a fuel gas of known heating value with a combustion-supporting gas, putting the mixture through a combustibility meter, adjusting the mixture to 100% or some fixed portion thereof of the lower explosive limit of the combustible gas, substituting a fuel gas of unknown heating value without changing the air to gas ratio and using the percent LEL now indicated by the combustibility meter to calculate the heating value of the unknown fuel gas. The apparatus for carrying out this method includes a combustibility meter (10) having a sensing transducer (12), a source of certified known fuel gas (30), a source of unknown sample fuel gas, a mixing system (14) for mixing these gases with air in controllable ratios, and an instrument (28) for recording the heating value of the unknown fuel gas.