This part of the UNE 53-172 standard defines particular conditions for determining the viscosity number and the limit viscosity number of polyethylenes and polypropylenes in diluted solutions at 135 °C. This standard is applicable to low, medium and high density polyethylenes, and to a wide series of polypropylenes, including pure ?sotactic polymers, block and atactic polymers; It is not suitable for high molecular mass fractions. The viscosity of polymer solutions can be affected by the presence of additives in the sample. The values obtained for the viscosity number are unreliable when determined by this method if the sample contains fillers or another similar additive.