A volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions. Gases are compressible and change volume when placed under pressure, are heated or are cooled. In engineering contexts, the volumetric flow rate is usually given the symbol Q. Energy flow rate is usually derived from mass or volumetric flow rate by the use of a flow computer. The energy flow rate is the volumetric flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. When gases or liquids are transferred for their energy content, as in the sale of natural gas, the flow rate may also be expressed in terms of energy flow, such as gigajoule per hour or BTU per day. This is not the case for gases, the densities of which depend greatly upon pressure, temperature and to a lesser extent, composition. The density of a liquid is almost independent of conditions. These measurements are related by the material's density. For very large flows, tracer methods may be used to deduce the flow rate from the change in concentration of a dye or radioisotope.īoth gas and liquid flow can be measured in physical quantities of kind volumetric flow rate or mass flow rates, with respective SI units such as cubic meters per second or kilograms per second, respectively. Flow may be measured by measuring the velocity of fluid over a known area.
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