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O-Ring Leak Rate Equation and Calculator
Leak rate of a gas through an O-ring seal can be approximated when the permeability of the gas through the particular o-ring elastomer is known for the temperature at which the o-ring seal must operate. The following equation is used to determine the approximation:
L = 0.7 F D P Q (1-S)2
L = Approximate leak rate of the seal, std. cc/sec.
F = Permeability rate of the gas through the o-ring elastomer at the expected operating temperature, (std. cc cm/cm2 sec bar) x 10-8
D = Inside diameter of the O-ring, inches.
P = Pressure differential across the o-ring seal, lb/in2
Q = Factor based on the percent squeeze and whether the O-ring is lubricated or dry.
S = Percent squeeze on the O-ring cross section expressed as a decimal. (i.e., for a 20% squeeze, S = .20)
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This equation provides a rough order of magnitude and approximation because permeability varies between compounds in the same o-ring polymer, and because the assumptions on which it is based are not all exact.
These assumptions are:
1. The cross section of a squeezed O-ring is rectangular.
2. The cross section area of a squeezed O-ring is the same as its area in the free condition.
3. The permeability rate of a gas through an O-ring is proportional to the pressure differential across the seal.
NOTE: For convenience, the formula contains mixed units. It was set up this way because in the United States O-ring diameters are usually given in inches, and pressures in pounds per square inch while permeability figures are usually shown in metric units. The 0.7 factor/constant resolves these inconsistencies.
For helium leak rate, a variation of ±50% of the predicted value should be anticipated to allow for limitations in the accuracy of test equipment and available standards, and for variations between samples.