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There are a wide variety of flowmeters, such as rotameters, vortex flowmeters, and electromagnetic flowmeters, but the most commonly used in industry today is still the glass rotameter. However, flow meters are certainly chosen in order to obtain accurate values for controlling the operation of equipment and instruments. The better the quality of the flow meter, the higher the accuracy of the measurements; yet it is the correct parameters that determine which flow meter is suitable to be selected. So which parameters affect the accuracy of glass rotameter flow meters? When selecting a rotameter or glass rotameter, factors such as temperature, pressure, the medium to be measured, and the measurement range are important; any error in any of these factors can affect the measurement results. In the case where the medium being measured is a gas, temperature and pressure have the greatest impact on accuracy. Gases are easily compressible, and changes in temperature and pressure lead to changes in the density of the gas as well. When calibrating the scales, flow meter manufacturers influence the calibration based on these parameters. In fact, for gas flow meters, there are also slight errors in the measurement results obtained in different environments. The second issue is the flow range. The flow range must also be specified accurately. One should not assume that for flow meters with the same diameter, a higher flow rate is always better. For glass rotameter and metal-tube rotameters, the minimum and maximum flow rates differ by a factor of 10. Generally, a flow rate at around 60% of the specified range yields the most accurate measurements. Therefore, this flow range must also be taken into account when selecting a flow meter.