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This part of ISO 18213 specifies a procedure for making accurate determinations of the densities of process liquids from in-tank measurements of the liquid content. This procedure is applicable to tanks equipped with pneumatic systems for determining the liquid content that have two or more bubbler probes of differing lengths. It is necessary that the probes be fixed relative to each other and to the tank in which they are installed. The methods presented in this part of ISO 18213 yield acceptable results only for clear (i.e. free of suspended solids) liquids that are both homogeneous in concentration and at thermal equilibrium. The accuracy of the method is limited by -- the accuracy of the density determinations for the calibration liquid, and -- the number and accuracy of the liquid height determinations used in the calculations. With state-of-the-art measurement technology and careful technique, and with water as a calibration liquid, it is possible to determine the density of homogeneous clear liquids with relative accuracy on the order of 2 x 10-4 to 3 x 10-4, or approximately 0,2 kg/m3 or 0,3 kg/m3.