MASS FLOW Versus Volumetric Flow measurement

Posted by Richard O'Leary on June 09, 2003 at 23:04:05:

In Reply to: Re: Gas Flow Controllers (N2/Ar/He) posted by Gordon England on February 26, 2003 at 02:27:32:

: : What are the general specifications (Accuracy, etc.) required?

: : I am looking to provide solutions to applications in which reliability and automation is required.

: : p.s. hope this isn't deemed as 'blatant advertising'!
: : :-)

: Hi Richard

: I see no advert here! I'm sure most people would not find a little self or company promotion objectionable as long as it is relavent and useful. I think most people know when that grey line is crossed.

: Re: gas flow controllers which I assume is for plasma spraying processes and I assume you work for a company supplying/manufacturing mass flow meters. Generally, the tube and float type mechanical type flow meters were expected to work within about + or - 5% of normal working flows. Most modern plasma spray equipment now use mass flow meters, which I would hope work well within this degree of accuracy.

: Plasma gas flow is a critical parameter and has a complex relationship with plasma arc voltage and anode and cathode geometry. Some rely on fixing gas flows and monitor the resultant plasma arc voltage (system I prefer) and others prefer adjusting the gas flow to achieve a specific voltage.

: Which ever way you set the parameters it is important that the plasma gases are delivered with consistent accuracy. I'm tempted to babble on further with this complex subject, but I will stop here.

: Richard, perhaps you could give us some pro's and con,s on mass flow meters vs mechanical tube and float flow meters.

: Regards Gordon

Gordon,

Thanks for your polite and concise response.
I'm afraid I lost my 'favourites' link and have only just now found this page again!

MASS FLOW Versus Volumetric Flow measurement?
I am answering in general terms of course, with reference to your suggestion.

VOLUMETRIC (by volume, e.g. Litres per Minute) flow of GAS is dependent on the consistency of both process and ambient conditions.
This is because the VOLUME of a gas changes according to Pressure and Temperature.

A Volumetric meter, such as a mechanical 'float in tube' type (Often referred to as 'VA' = Variable Area meters) are widely used quite successfully where either
a) Accuracy is not a primary consideration
or
b) Pressures and temperature(s) are always the same (in theory!)
or
c) A flow computer (or pre-prepared 'correction factor' table) is available to provide a calculation to correct for Pressure/Temperature changes.

Advantages/Plus Points:
a) Relative low cost
b) 'At a glance' local indication of flow rate.

DISADVANTAGES OF MECHANICAL 'Tube & Float' Meters:
a) Basic accuracy is limited by the design, typically an error of +/- 5% of Full scale can be expected.
b) 'Turndown' is limited (lowest achievable measurement as a percentage of the full scale flow range), typically 10:1.
c) Accuracy/Repeatability of measurement is affected by temperature and pressure changes.
d) Limited installation options. Vertical alignment of inlet/outlet is almost always necessary.

MASS Flow (Molecules 'counted')
The most widely used method to measure Mass Flow of gases is based on 'Thermal' priciples.
Every gas/gas mixture has a pre-determined 'Heat Factor', i.e. the rate at which heat is lost.
The Thermal Mass Flow meter takes advantage of this to measure the flow without being affected by process pressure and temperature changes.
This is possible because it is effectively molecules of gas are being measured rather than a volume.
In practise, most users will have their Mass Flow instrument calibrated and arranged to give a reading in Volumetric units.

Advantages/Plus points:
a) Highly Accurate/Repeatable Measurements. Typically an error of +/- 1% (or less) of Full scale range can be expected.
b) Superior Turn Down, typically 50:1
c) Generally unaffected by process temperature/pressure changes.
d) Reliable, having no moving parts.
e) Virtually attitude insensitive - can be mounted in either Horizontal or Vertical pipe/tube.
f) Available with controlling valve for automated flow regulation/control (i.e. a Mass Flow Controller).

Disadvantages of Thermal Mass Flow Meters
(versus 'Tube & Float' type flow meter):
a) Requires an electrical power supply and control signal/readout electronics or computer.
b) Initial cost, although this is not a issue when a Mass Flow meter is NECESSARY for the required accuracy or remote control capability!
c) Not well known enough!?

Perhaps surprisingly, the European market leaders in Mass Flow Measurement and Control have their UK base close to Cambridge.

Bronkhorst EL-Flow series Mass Flow Meters for Gases

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