Method and apparatus for detecting gas conveyed by drilling fluids

A method of detecting gas conveyed in a drilling fluid returning from a well, which method comprises the steps of:

• (a) extracting gas from said drilling fluid;
• (b) transmitting infra-red radiation through said gas;
• and (c) detecting with a detector infra-red radiation that has passed through said gas and providing an output signal representative thereof;
• characterized by the step of:
• (d) examining the intensity of a portion of the infra-red spectrum within a range of approximately 3.1 .mu.m and 3.6 .mu.m to estimate whether or not said gas comprises any light or heavy hydrocarbons, and wherein step (b) comprises driving an infra-red emitter in a cyclical manner so as to produce pulses of infra-red radiation, whereby a cyclical output voltage corresponding to said pulses is obtained from said detector the magnitude of which represents the intensity of infra-red radiation received by said detector, and wherein said driving comprises driving said infra-red emitter with a 50% duty cycle, whereby an output from said infra-red detector is substantially sinusoidal, and converting said cyclical output voltage to DC, the magnitude of which is proportional to the amount light or heavy hydrocarbon present in said gas.