Metalcutting fluid mixing seems like a solved problem — take a Venturi injector from brands like Nimatic or Blaser, connect it to your water line, and the concentrate gets drawn in. The result looks like a mixed coolant. The problem is that what comes out of a Venturi injector at 7 bar is not the same as what comes out at 4 bar, and what your machine actually receives is not always what the injector was set to deliver. Concentration errors in metalcutting fluid are not minor inconveniences. They translate directly into tool wear, surface finish problems, corrosion, and bacterial contamination. The Venturi injector is a convenient tool. It is not a precision dosing device.
How a Venturi Injector Works and Where It Fails
A Venturi injector works by passing water through a constricted throat at high velocity, which creates a low-pressure zone that draws concentrate from a reservoir through a side port. The mixing ratio — the proportion of concentrate to water — depends on the differential pressure across the Venturi: the inlet water pressure minus the downstream back-pressure. This differential determines the suction force at the concentrate port, which in turn determines how much concentrate is drawn in per unit of water flow.
The problem is that neither inlet pressure nor back-pressure is constant in a real workshop environment. Water supply pressure varies with demand across the facility — when three machines are refilling simultaneously, pressure drops. Back-pressure varies with hose length, fluid viscosity, and the elevation of the receiving sump. The Venturi injector has no mechanism to compensate for these variations. Its mixing ratio shifts with every change in hydraulic conditions. A Venturi calibrated to deliver 8% concentrate at 6 bar supply pressure may deliver 5% at 4 bar or 11% at 7 bar, and there is no feedback or indication to the operator that the ratio has changed.
A second problem is concentrate viscosity. Many high-performance metalcutting fluid concentrates — particularly synthetic and semi-synthetic formulations — have viscosities that vary significantly with temperature. A concentrate drawn from a drum that has been sitting in a cold workshop at 20°C has measurably higher viscosity than the same concentrate at 30°C in summer. The Venturi injector treats both identically. The actual suction rate changes because the concentrate flows differently through the inlet tube, the filter, and the check valve — none of which compensate for viscosity change.
How the Dosatron Works Differently
The Dosatron is a water-powered proportional dosing device. A piston or diaphragm motor inside the Dosatron is driven by the flowing water itself — the water acts on the motor to drive it, and the motor simultaneously draws concentrate from a suction tube and injects it into the water stream. Because the same water volume drives the motor and receives the concentrate, the dosing ratio is mechanically fixed regardless of flow rate, inlet pressure, or downstream back-pressure. If 100 litres of water passes through, the motor has made exactly the number of strokes required to inject the set volume of concentrate, whether the flow happened quickly or slowly, at high or low pressure.
This is a fundamental mechanical difference from a Venturi injector. The Dosatron’s mixing ratio is set by adjusting the stroke volume of the dosing piston — expressed as a percentage of the water flow — and does not change with hydraulic conditions. A Dosatron set to 8% delivers 8% across a water flow range of approximately 2:1, which covers the realistic variation in water demand across most machine refilling operations.
Concentration Accuracy in Practice
A Venturi injector typically has a stated mixing ratio accuracy of ±20–30% under ideal conditions. Under variable workshop conditions, this widens further. A Dosatron delivers accuracy of ±5% of set ratio across its operating flow range under typical conditions. For a target concentration of 8%: the Venturi may deliver anywhere from 5.6% to 10.4% (±30%), while the Dosatron delivers 7.6% to 8.4% (±5%). The range of outcomes is dramatically different, and both undershooting and overshooting concentration have direct consequences.
Undershooting concentration — delivering 5.6% when 8% is specified — means inadequate lubrication and cooling. Tool life suffers, surface finish degrades, and if the refract reading is checked and found low, the operator adds more concentrate manually, usually without precise measurement, compounding the variability. Overshooting concentration — delivering 10.4% — means excessive foam, dermatitis risk for operators, fluid that is too rich and may actually perform worse than specified (some formulations become tackier and retain more chips at high concentration), and wasted concentrate cost.
The Dosatron Advantage for Central Systems
In workshops using a central mixing station to prepare coolant before distribution to multiple machines, the Dosatron provides an additional advantage: it automatically compensates for varying demand. When one machine is being filled, flow through the Dosatron is at one rate. When five machines are being filled simultaneously, flow increases. The Venturi injector’s ratio changes with this flow variation. The Dosatron’s ratio does not — the motor strokes faster with higher flow, but each stroke still injects the same concentrate volume per volume of water processed.
For CNC workshop operations where coolant concentration directly affects tool life and surface finish — and where the cost of incorrect concentration is measured in scrapped parts and shortened tool life rather than just wasted concentrate — this distinction between convenience mixing and precision dosing is commercially significant.
For help sizing a Dosatron for your coolant mixing application, contact Autoflo at info@autoflotechnology.com.