Leak flow -
Volume leak flow Vs Mass flow leak Vs Standard flow leak
Unfortunately, great confusion exists when measuring leaks.
Leak flow can be described as:
- volume leak flow
or
- mass leak flow.
Volume leak flow is the
rate of volume change over time.
The most common way to describe leak flow, it is measured in volume units overtime, such as cubic centimeters per minute or cubic centimeters per hour.
Mass leak flow is the rate of mass change over time. It is measured in mass units over time, such as grams per minute, milligrams per minute or micrograms per minute.
The most common way to describe leak flow, it is measured in volume units overtime, such as cubic centimeters per minute or cubic centimeters per hour.
Mass leak flow is the rate of mass change over time. It is measured in mass units over time, such as grams per minute, milligrams per minute or micrograms per minute.
The correlation between volume
and mass leak flow is given in the equation:
m = Q× ρ
In this equation, ‘m’ is mass flow, ‘Q’ is volume flow, and ‘ ρ’ is density,
measured in milligrams per cubic centimetre.
Unfortunately, there is a third way to describe a gas leak flow, and it often causes a lot of confusion. Known as "standard" flow, it is measured in standard cubic centimeters per minute (sccm) or standard cubic centimeters per second.
Unfortunately, there is a third way to describe a gas leak flow, and it often causes a lot of confusion. Known as "standard" flow, it is measured in standard cubic centimeters per minute (sccm) or standard cubic centimeters per second.
Standard
flow is defined as actual volumetric flow
corrected to "standard conditions." Because more than one
standard condition exists, we will define a common standard
condition as a dry, pure gas at a specific gas density, a pressure of
14.695 psia and a temperature of 529.45 R.
To calculate a standard gas flow for a specific gas, we can use this equation, which is derived from the two earlier equations:
To calculate a standard gas flow for a specific gas, we can use this equation, which is derived from the two earlier equations:
Qstd = Q x {(P x T std)/(P std x T)}
In this equation, Qstd is standard flow,
Q is
actual volume flow;
P is actual absolute gas
pressure;
T is actual absolute temperature;
P std
is
standard absolute gas pressure; and
T std
is standard absolute temperature.
Why is standard leak flow so confusing? First, it is gas- dependent. For example, 1 sccm of helium has approximately seven times lower mass flow than 1 sccm of nitrogen! Second, many engineers get confused between standard cubic centimetre per minute and cubic centimetre per minute, and they get more confused converting from one unit to the other. Standard flow can be considered "semi-mass flow."
Because standard flow units are not a recognized measurement unit in the SI system and because they are confusing, it is highly recommended to avoid these units of measurement whenever possible.
Why is standard leak flow so confusing? First, it is gas- dependent. For example, 1 sccm of helium has approximately seven times lower mass flow than 1 sccm of nitrogen! Second, many engineers get confused between standard cubic centimetre per minute and cubic centimetre per minute, and they get more confused converting from one unit to the other. Standard flow can be considered "semi-mass flow."
Because standard flow units are not a recognized measurement unit in the SI system and because they are confusing, it is highly recommended to avoid these units of measurement whenever possible.
Collected from ADVANCED LEAK
TEST METHODS- Mr.HEMI SAGI, Director ATC, Inc.