The present invention relates to an apparatus (100) suitable for use in substantially continuous measurement of a physical property particularly the rheology and/or viscosity of a drilling fluid in use thereof in a drilling fluid or mud flow circuit of a drilling device. The present invention also relates to an apparatus (100) suitable for use in substantially continuous measurement of a physical property particularly the rheology and/or viscosity of any non-newtonian fluid is use thereof. The apparatus (100) comprises of a sample chamber (26) having an inlet (7) and an outlet (9) and is arranged to allow a drilling fluid to flow therethrough from the inlet 7 to the outlet (9). Detector devices (33, 34, 35) such as Viscometers are provided for substantially continuous measurement of a physical property particularly the rheology and/or viscosity of a drilling fluid flowing thought the sample chamber (26) in use of the apparatus (100). The sample chamber (26) has different diameters and has different flow rates and/or shear rates and will thus have different viscosities in a non-newtonian fluid which will be measured by a separate device such as a viscometer. Supply and return conduits are provided for connection of the inlet (7) and outlet (9) respectively in use of the apparatus (100) to a drilling fluid flow for delivering at least part of the drilling fluid flow through the apparatus (100) for measurement. A pump (18) produces a controlled flow of the diverted drilling fluid though the sample chamber (26) of the apparatus (100) so that the properties of the drilling fluid, particularly the rheology and/or viscosity can be measured on a more less continuous basis. A desirable backpressure in the sample chamber is controlled by a valve (8) on the outlet (9). The measurements of the separate devices (10) such as a viscometer can be used with other instruments to create a rheological model of the mud. The electrical signal generated by the viscometers (33, 34, 35) are sent for processing to a PC (12) and a device such as a chart and/or a data recorder (13). The direction of flow through the sample chamber (38) is shown by the arrows (39).