INVESTIGATIONS ON THE LIMITS OF UNCERTAINTY OF GAS-DENSITY MEASUREMENTS WITH VIBRATING TUBE DENSIMETERS

The aim of this work was to examine the scope and limitations of gas d ensity measurements with vibrating tube densimeters, especially their limits of measurement uncertainty. For this purpose, equipment was des igned with glass and stainless steel vibrating tubes. The stability an d sensitivity of the equipment were tested by measuring empty tubes (v acuum); the reproducibility of gas density measurements was examined w ith argon and methane. A calibration equation was derived from measure ments with evacuated tubes, liquid water, argon and methane by means o f the Wagner equation optimization method. With the gases helium, nitr ogen, carbon dioxide, butane, tetrafluoromethane and sulphur hexafluor ide the uncertainty of gas density measurements was tested. The measur ements were performed in the range 30-90 degrees C in the pressure ran ge 0-2.7 MPa. The uncertainty of gas density measurements with vibrati ng tubes is in the range of hundredths of kg m(-3); the sensitivity of the equipment to density changes is higher by at least one order of m agnitude. The primary reason for the great difference between the sens itivity and the uncertainty of measurements is the inadequate pressure stability of the vibrating tubes. In order to perform gas density mea surements of higher quality (for example for scientific purposes) it w ould be necessary to increase the stability of the vibrating tubes at changing pressure loads by modifying their construction and/or materia ls.