Leaks measurement: ppm & mbar*l/s (of He)...?

[NGiLuzzu]

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.
Many thanks in advance, 'NGL

 

[hacksaw]

It is based on the ideal gas law P1V1 =PoVo
where and Po is your std. pressure(1013.25 mbar) and Vo the molar std volume (22.4 L).

In another form P1V1=n1RT where V1 is in L/s and n1 is the # moles/s

so mbar l/s is another way of specifying the millimolse per sec that are leaking into the system.

at really low pressure (UHV) you are literally counting molecules (6.0235x10^23 molecules per mole)involved in the leak...etc.

 

[NGiLuzzu]

Dear "Hacksaw",
thanks for your help. As per your hint, if temperature is constant, the "mbar*l/s" reading is a measure of the number of moles (or molecules) of Helium detected in the unit of time (1 s). Then, this is proportional to the helium concentration (ppm) in the intaken flow of the instrument, if the intaken flow rate itself is constant.
This is true for VOC analyzers in EPA method 21 (see

http://www.epa.gov/ttn/emc/promgate/m-21.pdf,

point 6.4: "The instrument shall be equippen with an electrically driven pump to ensure that a sample is provided to the detector at a constant flow rate...&quot for instance; is it true for helium mass spectrometers too?

 

[hacksaw]

The units are very common in UHV systems and with Helium Leak detectors (mass spec.)

 

[NGiLuzzu]

I found in a Company Specification, and precisely in Shell MESC SPE 77/312 (Revision: 26-11-2002), page 16, par. 6.1, note 7:
"... For leakage rates conversion factors see EN 1779 annex B
(1ppm He = 2.5 * 10-8 cm³ /s = 2.5 * 10-7 Pa*m3/s) ..."

This equivalence has been added in this latest revision of the document: if it's true, it means that
1 ppm = 25 mbar*l/s, or 1 mbar*l/s = 0.04 ppm of He.

Unluckily, EN 1779 (1999)standard does not explane how this could be derived. Is anybody able to do that?

Thanks in advance, 'NGL

 

[hacksaw]

pv=nRT (n= # moles)

so mbar*l/s is a measure of the number of moles per second...

 

[NGiLuzzu]

I see, mbar*l/s measure a "pV-throughput" (see EN 1779, ed.1999, page 4, para.5) or a number of moles per second... but if you want to relate this with a concentration (ppm), you need at least to have a fixed volume, don't you?
Then, which is the constant volume in the case above? And... why? (I feel like there is some hypothesis I'm missing...)
My Spectrometer Manufacturer (VARIAN), for example, says that the volumic flow rate through the sniffing probe is not always constant.

 

[hacksaw]

traditionally sniffers are tuned to He, others (mass specs) may actually detect all of the components up to a mw cutoff and re-normalize the result assuming that all of the species have be counted.

the latter will give you a ppm result. in that case the flow need not be constant.

leakage is mass per unit time (i.e. relates to number of moles). you can express ppm on a fixed volume or a on a flow basis

 

[25362]

Hacksaw is absolutely right. Since ppm is a dimensionless quantity, the apparatus must have a way of measuring or inferring the "total" amount of mbar*l sampled in one second.

 

[NGiLuzzu]

See also Thread408-48808.
Thanks and regards, 'NGL

 

[NGiLuzzu]

OK: in other words, for example, let's say that I can count, on the road near our factory, 25 red cars a day passing by. If this is equivalent to say that red cars represent 25% of total daily amount of vehicles passing by, it is only because I KNOW, somehow, that this total amount is 100 vehicles a day.

Now, let's put helium moles (or molecules) instead of red cars, seconds instead of days, ppm instead of % and total intaken volumetric flow rate instead of total daily amount of vehicles: I think the reasoning is still valid.

Our manufacturer (

http://www.varianinc.com/)

confirms that the leak detector (VARIAN 979-70) we use in "sniffer mode" CAN NOT measure the total intaken flow rate and therefore CAN NOT give readings in ppm.

Regarding the above mentioned Shell statement and the relevant "=" signs, I think they are NOT CORRECT (by the physic and dimensional point of view!)! As a matter of fact, it CAN'T be a real equivalence, conversion or equation: it may be an "equipollence", true under particular conditions only. The problem is that such conditions are not specified.

What do you think about it?

 

[25362]

To anegri, pls refer to my question on thread408-48808.
Thanks.