a turbidimeter includes a housing having a cavity with an inlet through which the fluid enters the bottom of the cavity , and an outlet through which the fluid exits the top of the cavity . a removable baffle assembly is located within the cavity between the inlet and outlet . the baffle assembly is formed by three vertical plates spaced from each other and extending across substantially the entire cross sectional area of the cavity . the first plate defines a first passage near the top of the cavity through which all the fluid entering the cavity must flow . the second and third walls define a second passage near the top of the cavity through which gas bubbles entrained in the fluid travels to the outlet . a third passage is defined between the first wall and the outlet and a mechanism is provided for measuring the turbidity of the fluid flowing through the third passage . a calibration device formed by a block of glass ceramic material is insertable in the third passage to simulate a known turbidity fluid .