TIP OF THE MONTH 04

QUESTION: How come the rather odd unit mbar l/s is used to quantify leakage rates? Is there any explanation for this?

ANSWER: When a tap drips, the rate of water leakage can be stated quite simply, e.g. in l/h (litres per hour). But since gases are compressible, the volume depends on pressure, which is given here in mbar (1/1000 bar). 1 mbar l/s means that under normal ambient conditions there is a loss of one cubic centimetre of gas per second.

BACKGROUND: "Rate" expresses something as a "per time unit". So, the birth rate tells us how many children are born per year. The repayment rate for a new car tells us how much money has to be paid out per month. Accordingly, a leakage rate tells us how much gas is flowing out of a leak per unit of time.

With liquids, a statement of the volume is sufficient to describe the amount of liquid, but with gases the compression factor has to be taken into account. Let us take the example of a single litre of gas which is held in a container under a pressure of 10 bars. After it has flowed through a leak in the wall of the container to the outside world, then, under normal atmospheric conditions (1 bar absolute pressure), it will take up a volume of 10 litres.

The ideal gas law states that the product of pressure and volume stays the same when the gas goes from one state (internal, 10 bar · 1 l = 10 bar.l) to the other (external, 1 bar · 10 l = 10 bar.l). If this is considered in terms of time, then this becomes a "rate" and the unit used in Europe is mbar.l/s. The legal unit, compiled from SI units, would be e.g. Pa m³/s.

Another leakage rate which can be thought of is the rate of 1x 10^-4 mbar l/s, generally considered as "watertight", which corresponds to a volume of gas equal to the size of a fine grain of salt per second.

A good way of picturing a small leakage rate is to think of it as the time which it would take for a given volume of gas to escape from a known object. For example a leakage rate of as little as 1x 10^-11 mbar l/s can be measured with Adixen helium leak detectors. This corresponds to a disposable cigarette lighter taking 50 million years to lose all of its gas.