For the first time in Ukraine a primary measurement standard of gas volume and flow rate units in a pressure range of 1 MPa to 5 MPa has been created. The standard is based on realisation of the units by PVTt principle and units dissemination using critical Venturi nozzles. Gas passing through the nozzle during its calibration fills an evacuated collection tank located downstream the nozzle, and the respective time interval is measured. Mass flow rate is calculated by multiplying the volume of the collection tank by the gas density change and dividing by the tank filling time. Density values are determined by the equation of state and the measured initial and final pressure and temperature values. The structural and functional scheme of the measurement standard is shown in Figure 3.
The standard consists of two units. The first unit is intended to realise, keep and transfer the units of gas volume and flow rate. The general view of the block is shown in Figure 4. The unit contains high pressure collection tank, precision water thermostat, vacuum pump, piston gas pressure gauge, flow switch, fine filter, a set of seven Venturi nozzles with diameters from 0.1 mm to 3.0 mm. The second unit is designed to form a stationary gas flow and provide a critical gas flow rate (at the speed of sound) at the nozzle throat. In the case of an elementary reversible adiabatic (isoentropic) process at a critical velocity, the mass flow rate of gas passing through the nozzle is maximally possible. The unit consists of equipment for reducing the pressure from 150 bar to 70 bar and from 70 bar to 10 bar, the heat exchanger and the instruments measuring the pressure and temperature of the gas flow. The created measurement standard will assure metrological traceability of the measurements of gas volume and volume flow rate at high pressures for the flow rate up to
100 m3/h and calibration of the primary reference nozzles with a throat diameter from 0.1 mm to 3 mm.
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