A turbidity head for a turbidity sensor or probe comprising: a light source providing an outgoing light ray ; a detector capable of detecting an incoming light ray ; an isolator separating the light source from the detector ; and a first reflector in the path of either the outgoing light ray or the incoming light ray ; wherein the first reflector is positioned to reflect either the outgoing light ray or the incoming light ray to achieve a measurement angle defined between the outgoing light ray and the incoming light ray of ninety degrees plus-or-minus two and a half degrees, characterised in that either: the turbidity head comprises a second reflector , wherein the first reflector is positioned to reflect the outgoing light ray and the second reflector is positioned to reflect the incoming light ray and the central longitudinal axis (B, C) of the light source and the detector are each arranged at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head ; or whichever of the light source or the detector that does not include the first reflector has a central longitudinal axis (B, C) that is oriented at about a 5° to about a 60° angle to the central longitudinal axis (A) of the turbidity head and the other has a central longitudinal axis that is at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head . a light source providing an outgoing light ray ; a detector capable of detecting an incoming light ray ; an isolator separating the light source from the detector ; and a first reflector in the path of either the outgoing light ray or the incoming light ray ; wherein the first reflector is positioned to reflect either the outgoing light ray or the incoming light ray to achieve a measurement angle defined between the outgoing light ray and the incoming light ray of ninety degrees plus-or-minus two and a half degrees, characterised in that either: the turbidity head comprises a second reflector , wherein the first reflector is positioned to reflect the outgoing light ray and the second reflector is positioned to reflect the incoming light ray and the central longitudinal axis (B, C) of the light source and the detector are each arranged at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head ; or whichever of the light source or the detector that does not include the first reflector has a central longitudinal axis (B, C) that is oriented at about a 5° to about a 60° angle to the central longitudinal axis (A) of the turbidity head and the other has a central longitudinal axis that is at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head . the turbidity head comprises a second reflector , wherein the first reflector is positioned to reflect the outgoing light ray and the second reflector is positioned to reflect the incoming light ray and the central longitudinal axis (B, C) of the light source and the detector are each arranged at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head ; or whichever of the light source or the detector that does not include the first reflector has a central longitudinal axis (B, C) that is oriented at about a 5° to about a 60° angle to the central longitudinal axis (A) of the turbidity head and the other has a central longitudinal axis that is at an angle of about 20° or less relative to the central longitudinal axis (A) of the turbidity head . <EOS>