Water softener: Control strategy vor Co² dosing

Hello All,

New project, but need some advice.
The principle is simple, water softener (anti limestone) by injecting CO² in the drinking water.
This exist, but costs about € 1800.

So I need to inject somewhere between 0,07 - 0,10 kg CO² in 1 m³ drinking water.
Considering filling a bath, the flow is about 1500 l/h so the CO² flow will be somewhere at 150 g/h CO².

Measuring the water flow is easy and mechanically injecting the CO² also.

The right amount CO²… that’s difficult since water pressure varies between 2.5 - 1,5 bar.
The professional solution has a water pressure reducer but the pressure is sometimes already too low.
I guess I will have to operate CO² at about 4 bar.

I see two possible solutions:

  • Critical flow nozzle: constant flow, independent of pressure difference CO² - Water.
    Pulse CO² valve open close > CNG injector for car.
    Yes, but what about CO² volume after the injector, this will act as a buffer and result in non-linear behaviour, probably messing up the theoretical side.
  • Control valve for CO², but did not find unless too expensive.

Another approach is measure the CO² flow and built a control algorithm around it, instead of use a fixed and experimental approach without feedback loop.
Bubble counters is out of the question, since this wil result in about 4 ml/sec at 4 bar and can’t find a reasonable cheap solution.

Nota bene: It’s not that I need to know the exact mass of CO², this will be derived from water hardness test, but it needs to be linear to water flow and independent from water pressure swings.

Please, some suggestions to keep the CO² part as simple as possible.