BetaCAP precision diluters: four strengths

BetaCap precision diluters have four peculiar characteristics that have made them the flagship of our business for over 15 years.

GREAT INITIAL ACCURACY

Dilution is a ratio of flows which, to produce accurate dilutions, must be carefully controlled. The typical problem with diluters is that in high dilutions the flow of the gas to be diluted and the flow of the diluent gas are very different from each other: measuring the smallest flow with the same meter used for the highest flow leads to a very low resolution and therefore to inaccurate dilutions.

In the 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 even better than the accuracy of flow measurement on the single capillary.

FUNCTIONAL SCHEMES: BetaCAP60-3G and BetaCAP30

RELIABILITY

The capillaries are ready to provide the best performance immediately as the diluter is turned on, and continue to do so for years. The response of the capillaries could only be modified by dust or condensate, which however are not found in pure or mixed gas cylinders. By diluting strong acids or bases, the formation of salts could also be possible, but just a little care in operation will assure avoiding this event.

The differential pressures applied to the capillaries are controlled in a closed loop, using 3 or 4 very linear and stable pressure sensors as a reference. Calibrating the response of the different pressure sensors (1 for the output and two or three for the inputs) can be performed in a simple and fast way (and therefore can be performed often) by applying one pressure reference (atmospheric press.) simultaneously to all the sensors for zero and later another pressure reference to all the sensors for the span. Differential errors are prevented by this calibration mode and the common mode deviation, depending on the accuracy of the applied reference pressure, has small effects on the dilution, affecting in the same way both the span gas and dilution gas.

An important advantage of the closed loop electronic control is the immunity by flow variations effects. In fact in mechanical pressure control devices the outlet pressure always decreases when the flow increases and, to worsen the problem, the flow of the gas to be diluted changes in the opposite direction with respect to the diluting gas flow.

Last but not least, the electronic pressure control allows using calculated set-points that where used to implement many useful and original functions, including the viscosity compensation and the “Continuous dilution” mode: no longer just step-by-step dilutions with capillaries.

EFFECTS RELATED TO THE NATURE OF GAS

All dynamic dilution technologies are influenced by the nature of the mixed gases. Generally the physical parameters that influence the method used are more than one.

Using the capillaries (laminar gas flow) the interfering parameter is just the viscosity and his effect is quantified by the Poiseuille’s law.

The viscosities of pure gases are readily available in the literature. The calculation of the viscosity of mixtures has been studied by many scientists for fluid dynamics and medical (Blood circulation) purposes. There are many empirical formulas available, some more complex and more accurate and others simpler and less accurate.

Unlike other K-factors, the calculation and compensation of viscosity are not influenced by the design of the diluter. All BetaCAP precision diluters automatically compensate for viscosities.

Some (CAP30) require the user to enter viscosities (using a spreadsheet with automatic resolution, available in this website). Others (CAP60) calculate and apply the same formulas themselves.

CONSTRUCTIVE FORMS

The internal active modules in all BetaCAP precision diluters are made up of a pair of solid shells (blocks made of fluorinated plastic or AISI 316L steel).

They hold the glass capillaries with sealing o-rings embedded between the joining surfaces. The control valves, solenoid valves and pressure sensors are fixed in special slots or seats on the external surfaces.

The whole pneumatic circuit (about some hundreds equivalent fittings) is made by crossing the holes in the two blocks. Fittings and tubing are used only for I / O connections to the panel.

The joints obtained from crossed holes cannot lose and leak, even after years of work. This construction is isothermal, compact, extremely robust, and therefore particularly suitable for the production of portable instruments.