##
Conversion from weight leak rate to volume leak rate

In vacuum technology, the leak rate in general is described as volume leak rate (mbar.l/s). But there are industries which define the leak rates in loss of weight per year (g/a). The following example shows the conversion from loss of weight to volume leak rate (for constant pressure and temperature). Change to a different type of gas is also considered.

In an example from the refrigerator industry, the calculation for molecular flow leak rate becomes clear. Let us assume, that a refigerator manufacturer wants to check his products for a leakrate equal to or less than 0.01 g loss of the gas (R-12) per year. The correspondig volume leak rate is calculated via the mol:.

The relative mass of an R-12 refrigerant molecule is 121.

121 g of R-12 corresponds to 22.414 litre of gas at 1013 mbar und 00C, so that 0.01 gm R-12 = 1.85.10-3 bar.l that is 1.85 mbar.l R-12 per year.

1 year has 3.1536 . 107 seconds, then the R-12 volume leak rate is:

But this is not quite correct, because the mol is specified for the conditions of 0°C and 1.013 bar. The test takes place at room temperature, say 20°C. So we have to correct this result according to Charles´ Law (the pressure stays constant at both tests):

At molecular flow, when pressure and temperature stay constant, the corresponding helium leak rate is calculated as follows:

We remember: The dependency of the leak rate at molecular flow in relation to the type of gas, is inversely proportional to the square root of the relative mass of molecules.

This example is rarely used in practice. It was only chosen as a case of molecular flow. It shows that the helium leak rate in this case is factor 5.7 times higher than the R-12 leakrate. In practice, the required leak rates for refrigerators are in the range of 1 to 5 gm per year. These leakrates are already in the range of transition or laminar flow.