housing 12 includes alignment pins 23 for aligning top 14 with housing 12 within housing 12 is a compartment or cavity 11 which includes solid side walls a floor with an inlet opening 11a and outlet opening 1b and an open top the cavity 11 is illustrated as being annular in shape although the cavity may be any other desired shape interiorly of annular cavity 11 is an open cavity 15 cavity 15 is the location of the light source 30 lens 31 and detector 32 below cavity 15 is cavity 17 which is a specially shaped chamber which serves several functions it provides a vertical path for the fluid to reach cavity 15 plus it is shaped to allow the light reflected from the light source to dissipate in the water before impacting a wall and reflecting back into the detector cavity 17 is tapered in the vertical direction to cause any stray light to reflect in an angular direction thus taking a longer path before the stray light reaches the detector the longer the path the more the light energy is dissipated positioned within cavity 11 is a bubble elimination device 20 which comprises a plurality of compartments or stages where flowing liquid e g water is forced upwardly through a narrow passage and then downwardly to the next stage where the process is repeated there may be any desired number of these stages e g 2 to 20 or more depending upon the amount of bubbles in the liquid that are desired to be eliminated a cover 16 rests on top of the cavity 11 the underside of cover 16 shown in fig8 includes a groove or channel 16a which extends continuously along the length of the cover and is open to the.