Know Your Leaks!

The five types of leaks are :

• Permeation

• Molecular Flow

• Transitional Flow

• Viscous

• Choked Flow

Let’s take a look at the characteristics of each:

Unlike the other types of leaks, permeation involves the passage of gas or liquid through a barrier wall, usually a solid. Molecules are dissolved in the material, diffuse through it, and are released at the outer surface. As with the flow of any matter there must be a driving force. The force can be a pressure or concentration difference, for example a mixture of gases for example O2 in N2 on one side of the wall and only one gas, N2, on the other, giving rise to the permeation of O2 until the concentration either side of the wall is the same. In my school science classes this was demonstrated with Visking tubing containing sugar solution being immersed in a beaker of distilled water. After a while the water in the tubing and the water in the beaker had the same concentration of sugar.

This can be considered to be the smallest form of leak through a micron hole. The mean free path of the gas is greater than the hole through which the gas passes. (The mean free path is the average distance travelled by a molecule before hitting another molecule).

Molecular flows are most usually encountered in vacuum application where the gas is expanded and the distance between molecules becomes comparable with the capillary diameter. Once any section of a leak path becomes molecular the whole path will flow at the molecular rate regardless of upstream conditions.

The transition from molecular to laminar flow is gradual. Here the mean free path of the molecules is about equal to the size of the hole and the lower end of the spectrum making the leak rate insensitive applied pressures but becoming more responsive to applied pressures at high delivery pressures. For this kind of flow to occur, the diameter of a circular hole would be in the order of 0.1 mm (100nm).

Viscous Flow occurs when the mean free path is less than the hole size, and probably constitutes the most common form of leak. In an ideal case, the leak rate is proportional to the difference of the squares of the pressures. The nature of viscous flow can be either laminar, with a Reynolds number below about 2000, or turbulent above this number, with vortices and eddies existing in the flow.

At very high flow rates, it is possible for the flow to reach the speed of sound, and it cannot exceed this. The condition is reached at a critical pressure where a further increase pressure difference will not increase the flow rate. However do not confuse flow rate in terms of gas velocity with the actual mass flow rate. Increasing the applied pressure may not increase the velocity of gas flow but the gas will increase in density with increasing in pressure, and hence an increase in atmospheric cc/min when measured as an external leak.

Seems like a lot to try and remember, right? Actually the type of leak is not that important – your leak tester only cares about the magnitude of the leak. So if you have an idea of the type of leak you may have it can help you select the right type of leak tester – more sensitive for molecular flow. To help you choose a leak tester we have a product comparison chart at http://www.uson.com/leak-testers/ an ,of course, always on hand to help you with free advice is our team of technical support experts.