Effects due to the gas nature
Excluding gas flow measuring techniques based on pure volume /time, that are not suitable for producing dynamic gas diluters, all the other techniques are sensitive in different ways, to the chemical composition of gases and mistures used in the dilution process.
The effects due to chemical composition of gases and mixtures involved in dilution can lead to deviations much higher than those due to dimensional or external factors. To correct chemical influences, two ways are open : the experimental way, consisting in evaluating the response to the specific mixture and the theoretical way, using the “gas correction Factors GCF. The second one, requiring less work, is the widely most practiced. Unfortunately the declared deviations are in the order of ±5%,may be more. The experimental way leads to much more accurate results.
A tool allowing the experimental test of the chemical influence can be made using a reference test material supply including a test gas bottle filled with the reference material, pressure reducer and needle valve to manually control the flow ; this unit is followed by the flow meter to be tested (if this is also controller, the control valve must be forced full open) and the flow meter is followed by the reference flow meter using the pure volumetric technique. The result of this meter must be compensated (environmental pressure and temperature) and compared to the result of the flow meter to be tested.
The test should be repeated with different flows in the useful flows range getting a series of GFC.
It’s difficult to find complete information about the effect of chemical composition on flow meters and diluters, but some scientific paper reports bad figures.
Naturally the effects of chimica composition on the flow response is strictly connected to the flow measuring opr flow controlling technique : let’s evaluate some of the more used techniques.
a) Sonic orifices : the formula indicating the mass flow of a mixture crossing a sonic orifice is of the type:
where:
We see that the flow through the orifice is influenced by some physical effect of chemical parameter, some in evident mode (critical specific heat ratio and density) but some more (probably the gas compressibility) can influence the discharge coefficient. Suppliers provide some value for pure gases, but in case of mixtures probably there is not alternative to experimental tests.
b) Thermal Mass Flow Controllers : the GCF (Gas Correction Factor) is widely used to indicate the response ratio between using Nitrogen and using different gases or mixtures. This correction factor is mainly influenced by specific heat and thermal conductivity, but also changes as a function of the sensor construction form.
Mainly for some chemical component , it seams more required to define different GFC for different flows : it can be this is due to movements in the turbulence map in the flow measuring cell
c) Capillaries. As a function of sizes ratio (D/L) and of the applied differential pressure the induced flow can be laminar transitional or turbulent operating in laminar conditions the flow follows the Poiseuille formulas, which have some difference for application to liquids or gases. For gas diluters application the right formula is indicating the flow at capillary output : output of capillaries is the point where all capillaries are joined. In case of gases, the output flow (Vol/time) is bigger than the input due to the lower pressure.
Viscosity is the sole physical property depending on the chemical composition of the gas. All capillaries and tubes in the world are sensitive to viscosity same way, than since a couple of hundreds years a lot of scientist did produce tables, formulas and studies. Viscosity is the sole properties having effects in capillaries flow .
Note : The flow Q2 is calculated in volume units at capillary output. An Excel spreadsheet to calculate mixtures viscosities according to the Reichemberg equations can be downloaded from this web site. Gas dividers BetaCAP30 and BetaCAP30X100 require input of the values by the user, but BetaCAP60-3G and BetaCAP60X100 calculate this value using the composition of TG0, TG1 e TG2 .
