Another Little Lesson on The Difference of Standard and Actual F

Another Little Lesson on The Difference of Standard and Actual F

When measuring flow rates of gases we have the option to measure at 'standard' or 'actual' flow rates, which can be confusing to many people. Here we'd like to try and give a simple explanation of the difference between these two units -

Standard cubic feet per minute (SCFM) is the volumetric flow rate of gas changed to "standardised" conditions of temperature and pressure, which then gives the equivalent of the mass flow of the gas. Normally the "standard" condition for pressure is defined as an absolute pressure 1.0 bar and the standard temperature is normally stated as 0 °C, 15 °C, 20 °C, or 25 °C. In some specifications, the relative humidity (e.g., 36% or 0%) is also included as standard conditions. However, it is worth noting that there is no universally accepted set of standard conditions.

The temperature change has the largest effect on standard flow measurements. In Europe standard temperature is normally 0 °C, but not always. In the USA, the standard temperature is normally 60 °F or 70 °F, but again, not always. Change in standard temperature can result in a significant volumetric variation for the same mass flow rate. For example, a mass flow rate of 1,000 kg/h of air at 1 atmosphere of absolute pressure is 455 SCFM when defined at 32 °F (0 °C) but 481 SCFM when defined at 60 °F (16 °C).

An actual cubic foot per minute (ACFM) is the volume of gas flowing in a system, independent of its temperature and pressure. The most important change between 'Standard' and 'Actual' flow rates is the pressure. To move a gas, positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of the gas, it is compressed. When a vacuum is applied to a standard cubic foot of the gas, it expands. The volume of gas after it is pressurised or rarefied is referred to as its "actual" volume.

Now you know the difference, why not take a look at our range of flow meters and try and spec yours up? If you're still a bit confused give our Sales Team a call on 0151 343 9966 and one of them will be happy to help you out!

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