A method and apparatus for sampling and measuring a physical property of a product in a container (11). The container (11) has an aperture (11a) in its bottom surface, to which a sampling tube (13) is affixed. A piston (14) moves back and forth in the sampling tube (13), thereby withdrawing and replacing samples of product via the aperture (11a). While a sample is in the sampling tube (13), a sensor (15) senses a measured value of the physical property of interest. The piston (14) has a calibration layer (14a) that is made from a material having a property that is similar to that of the product with respect to the property of interest. For calibration, the piston (14) is positioned so that the sensor (15) senses a measured value that can be compared to a reference value to determine if the sensor output is properly calibrated.
James D. King