NIST uses pressure, volume, temperature, and time (PVTt) standards to calibrate gas flow meters. PVTt flow standards generate selected, steady gas flows through a meter under test and measure the time t it takes to fill a collection volume V starting from an initial, evacuated state and ending at a final state with a pressure on the order of 1 bar. The initial and final densities of the collected gas are calculated from the gas’ measured pressure P and temperature T. These densities, together with the known collection volumes, determine the mass of the gas collected during the timed interval. NIST has three collection tanks: 34 L, 677 L, and 26 m3. The 34 L and 677 L tanks are submerged in a water bath that has 1 mK stability and uniformity. The uncertainty of the temperature of the collected gas is low enough to achieve a flow uncertainty of less than 0.025 % (with a 95 % confidence level) for flows up to 2000 L/min. NIST calibrates meters for larger flows with low uncertainties by calibrating several meters, one at a time, and then using the calibrated meters in parallel. For example, NIST’s natural gas calibration service calibrates gas flow meters up to 4 x 107 L/min. (This step-up of flow by a factor of 20,000 in flow is conducted at CEESI’s site that handles large flows of natural gas in Garner, Iowa.) The PVTt standards are also used to make dynamic (or rate of rise) measurements of gas flows as low as 0.1 cm3/min. The volumes of the 34 L and 677 L tanks are known within 0.01 %; they have been used to measure the volume of other pressure vessels via the gas-expansion method. The PVTt standards are research tools used to study, for example, gas species effects on laminar flow meters and thermal boundary layers in critical flow venturis.