Abstract: An instrument for measuring opacity of oil mist for lubrication purposes is set forth. In the preferred and illustrated embodiment, the apparatus is preferably installed in a pipe cross, opposing legs of the cross conduct oil mist past the opposite two legs supporting an opacity measuring system. Utilizing a bridge voltage supply to power an amplifier forming a reference voltage for reference diodes, a first photo cell is provided with a reference light from LED created light directed to the reference. A second LED directs light through the oil mist. The quantity of light is measured by a second photo cell reflecting the light absorbence of the sample, and the output of the two measuring devices is determined in a differential amplifier, driving a meter movement. The direct current signal is at a safe current range for use of the transmitter in a hazardous environment and is inversely proportional to the percent transmission of the signal.