Environmental Laboratory

  • A new approach and highly innovative solution to reliable and accurate gas dilution

A new approach and highly innovative solution to reliable and accurate gas dilution

May 27 2021

Dilution is effectively all about flow ratio: to ensure maximum accuracy, selecting nozzles or calculating the linearity correcting polynomial coefficients, flows must be measured. The most common problem with diluters is that in high ratios the flow of the gas to be diluted and the flow of the diluting gas are very different to each other. Therefore, measuring the smallest flow with the same meter used for the highest flow results in very low resolution and therefore inaccurate dilutions. Using different meters creates different chain tracing errors which is even worse.

With Be.T.A. Strumentazione’s BetaCAP series of precision diluters, different flows are obtained by grouping in parallel a different number of equal capillaries. The capillaries are measured one by one with high resolution and the flow of the groups is not measured but calculated by adding the individual capillary flows. The relative accuracy of the sum is considerably better than the accuracy of flow measurement on the single capillary. 

To provide an electronic pressure control across capillaries Be.T.A. Strumentazione use pressure meters. In comparison to flow meters they have a lot of advantages in term of stability, linearity and reliability.       However, the main advantage of electronic pressure control comes from knowing and using the physics of capillaries setting calculated set-points to adjust pressure, differential pressure and viscosity relationships.  Traditionally, when capillaries or critical nozzles are used the diluter operates "step by step”, individual capillaries or nozzles combinations give individual dilution ratios.

MFCs diluters are based upon linear flow control, dilutions are linearly changing in a continuous dilution range.

The BetaCAP product uses discrete components (capillaries) to produce a continuous dilution range.   Capillary combinations give the dilution steps, and those steps are moved by changing the pressures applied to capillaries complying with their physical laws. 

To calculate the viscosity of a gas mixture many empirical formulas are available in which the physical parameters of the components must be placed: one of them is the component viscosity, that we may find in literature tables (each fluid chemical component has a viscosity)

This calculation is one of the more widely used, because it solves many problems both in fluid dynamic and in medicine (blood circulation) and it therefore enables accurate results.

Other technologies (MFCs) are influenced by the calorific power of the fluid, as well as by the heat transfer coefficient and other factors such as the design of the flow measuring sensor.

The BetaCAP is built by manifold, using two solid blocks and constructing all the pneumatic circuit by crossing holes within the solid.  This construction creates a very compact and highly rugged unit that is isothermal, with low leakage levels and minimal dead volumes.

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