In order to estimate the performance of the Anton Paar densimeter and to find coefficients A and B, four fluids were used: distilled water, Standard Reference Material 211d (Toluene), Standard Reference Material 2214 (Isooctane) and air. Certificates for SRM 221d and SRM 2214 can be found via: https://srmors.nist.gov/pricerpt.cfm. Measurements were performed at 20°C. Water density was calculated using the Patterson and Morris polynomial [10]. Density of the air was calculated using the temperature, atmospheric pressure, relative humidity and corresponding equation [11]. Relative humidity of the air inside the vibrating tube was re-calculated for the tube temperature assuming that the partial pressure of the water vapor is the same for the room and tube. First, the water vapor pressure was calculated for the room temperature and after this value was divided by the saturated water pressure at the tube temperature to get the relative humidity of the air in the tube.