In order to perform Fugitive Emissions tests, we use a VARIAN sniffer probe helium leak detector, which gives readings in mbar*l/s. Our usual European reference specification (Shell MESC SPE 77/312) also gives limits in the same measurement unit.
   First of all, is anybody able to explain the physical meaning of "mbar*l/s"? Is that a flow rate or a concentration?
   Then, how is it possible to relate this with "ppm"? As many leak detectors manufacturers have instruments with both scales, we think there must be a simple relation between the two units.

The Varian 979 cannot measure the flow rate of the gas coming in and thus cannot count the helium passing through and simply state the concentration reading or "PPM"

However a smaller test device also From varian, the Helitest, can.  The Helitest allways has the same fow rate of gas being aspired so measuring the quantity of He atoms makes that the instrument can detect concentrations "PPM".

mbar l /sec or I prefer to have the reading in atm cc /sec means that at one atmosphere pressure there are X cc of helium atoms passing per second through the machine.

THERE IS NO CONVERSION POSSIBLE BETWEEN mbar l / sec, atm cc / sec and PPM/filter.  One is a flow reading and the other is a concentration reading.  There are tricks to establish a link but they are empiric and have no scientific basis.

The Varian 979 has a callibrated leak source internal.

At start the unit wil self callibrate.  in fact what it does is it lets the callibrated leak pass through the unit. ionises the helium atoms and separates them off with a magnetic field.  the helium atoms fall on a capturing electrode and create a small current.  The machine then establishes that this current is equal to the flow rate of the callibrated leak.

When in sniffing mode the machine will draw in helium atoms and see how many atoms pass through the unit by measuring the detection electrode amperage.

Allow me to continue later Lunch break is over.

When the unit self calibrates all of the leakage from the calibrated leak source passes through the instrument.

When sniffing helium only a part of the leakage that is coming from the leakage source passes through the instrument.

When one calibrates the instrument with a calibrated leak source of say 1.7E-8 atm.cc./sec.  we are sure that all the helium leaked passed through the machine.

Now it depends on the method of measurement whether or not we will be measuring all the leakage.

Method TA Luft; good old German Grundlig und Punktlich.
The leak source is completely encapsulated and all leaking gasses are passed through the leak detector.  This is great, all leak gass is detected and we can determine how much helium has escaped into the atmosphere.

Method 21: EPA Uncle Sams way; a sniffer with a determined opening cross section is placed at a location 5 mm from the leak source and parts of any whiffs of Helium that are leaking out pass through the leak detector.  We can only guess at which percentage of the leakage is captured.

Method TA luft may be good and correct but as many times is the cases with our German friends it is impractical.  Who would like to go out and make 5000 adaptors to be able to sniff all the leakage in a chemical plant.

Method 21 is inaccurate but practical.  the idea I guess is that if you standardise on the way and the instruments with which to measure you will be able to pick up trends.  And also it is easy to equip a helium detector with a correct sniffer and a distance spacer and to go out measure 5000 valves in a plant.

One more thing: Helium intake by a varian leak detector is influenced by pressure and temperature.  the detector compensates for temperature changes since the last self callibration.  I still need to get from Varian whether the detector compensates for pressure changes also.

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