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How to Convert a Helium Leak Rate to an Air Leak Rate

It is useful to convert a gas leak flow into the corresponding air flow for design purposes. Engineers use standard air flow calculations to size pipelines, ducts, fans and compressors. If a gas other than air is present in the system, it must be converted into air flow. This calculation takes the specific gravity (ratio as compared to air) into account.

Instructions

- 1
  Determine the leak conditions of helium such as volumetric flow rate, temperature and pressure. For instance, assume the flow rate of 500 cubic feet per minute (cfm) at a pressure of 100 pounds per square inch (psi) and a temperature of 75 deg F. The specific gravity of helium is 0.138.
- 2
  Convert the helium volumetric flow rate into a mass flow rate. This is done by using the specific gravity converted to density. Air has a density of 0.075 pounds per cubic foot. The ratio with helium is the specific gravity multiplied by the air density, which is 0.138 x 0.075 or a helium density of 0.01 pounds per cubic foot. Multiply the density by the volumetric flow rate to determine mass flow rate. This is 500 cfm x 0.01 pounds per cubic foot or 5 pounds per minute.
- 3
  Convert the helium flow rate to an air flow rate using the formula Q = m x 13.36 / s.g. ͩ0;1/s.g. where Q is the air flow rate in cubic feet per minute, m is the mass flow of the helium and s.g. is the specific gravity of the helium. This calculates out to be 5 x 13.36 / 0.138 x ͩ0;1/0.138 or an air flow rate of 180 cfm.

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