A linear liquid density gradient is fitted in a cylindrical glass tube and maintained at 23°C to prevent the gradient being disturbed by thermal convection. Temperature control circuits work in conjunction with electric heaters and a simple cooling coil to maintain accuracy. The density gradient obtained is determined by introducing a set (usually nine) of accurately calibrated marker floats whose densities cover the desired range of the column at equal density intervals. The floats provide not only the calibration reference values of the gradient, but also act as a check on the stability of the density gradient.