A method of calibrating a measurement system operable to measure a spatial position of a meniscus of a liquid included within a container and therefrom generating a corresponding liquid content signal indicative of a proportion of the container filled with the liquid, said method including steps of: (a) from a first time instance when said container is in a first state including a first quantity of said liquid, progressively filling during a calibration phase said container with the liquid at a substantially constant volumetric rate of liquid delivery whilst periodically simultaneously recording corresponding generated values of a meniscus position indicative signal generated by said measurement system; (b) at a second time instance after said first time instance during said calibration phase, terminating progressive filling of said container with said liquid when said position indicative signal corresponds to the container being in a second state including a second quantity of said liquid; (c) determining the position of the meniscus at a lower and upper position over which calibration is desired and a measure of the capacity of the tank between the aforementioned lower and upper positions of the meniscus; and (d) subsequently, in normal use after said calibration phase, applying signal processing to said position indicative signal provided from the measurement system to generate said corresponding liquid content signal indicative of said proportion of the container filled with the liquid, said signal processing accounting for said container during said calibration phase having been filled at the substantially constant volumetric rate of liquid delivery to the container, wherein said substantially constant volumetric rate is not measured. (a) from a first time instance when said container is in a first state including a first quantity of said liquid, progressively filling during a calibration phase said container with the liquid at a substantially constant volumetric rate of liquid delivery whilst periodically simultaneously recording corresponding generated values of a meniscus position indicative signal generated by said measurement system; (b) at a second time instance after said first time instance during said calibration phase, terminating progressive filling of said container with said liquid when said position indicative signal corresponds to the container being in a second state including a second quantity of said liquid; (c) determining the position of the meniscus at a lower and upper position over which calibration is desired and a measure of the capacity of the tank between the aforementioned lower and upper positions of the meniscus; and (d) subsequently, in normal use after said calibration phase, applying signal processing to said position indicative signal provided from the measurement system to generate said corresponding liquid content signal indicative of said proportion of the container filled with the liquid, said signal processing accounting for said container during said calibration phase having been filled at the substantially constant volumetric rate of liquid delivery to the container, wherein said substantially constant volumetric rate is not measured.