Consistent Venturi Proportioning

What is Venturi Proportioning?

Venturi proportioning (also known as diluting or mixing) equipment uses water pressure to "draw" chemical concentrate into the water stream, using precision metering tips to control the rate of chemical draw. This method of chemical dilution has many advantages. Most notably, the units are simple to operate, cost-effective, and contain no moving or wear parts.

Venturi Theory

In any piece of venturi style equipment, the injector or foamer body is responsible for utilizing water pressure to dilute chemical concentrate on-demand. Each body is precision machined to accommodate various operating conditions. The body is connected to a water source and the chemical pick-up tube is placed in the chemical concentrate.

When water flows through the body, it speeds up (due to the decreased volume), which creates a vacuum in the body. This vacuum is used to draw the chemical concentrate into the water stream. The chemical and water mix in the body and as they flow through the discharge hose.

Venturi Theory Applied

Most venturi systems come with 20 different precision metering tips. These metering tips are placed in the chemical pick-up tube to regulate the flow of the solution. "Larger" tips will allow chemical through at a faster rate and increase the concentration of the final solution.

The tips allow a fixed volume of chemical through at any time, regardless of the water pressure coming into the unit. However, the volume of water allowed through the body will vary based on the water pressure. Because of this, variations in the water pressure will cause variations in the final dilution ratio.

What Does This Mean?

For facilities with consistent water pressure, dilution ratios can be set by matching the metering tip to the actual water flow rate. Minor variations in water pressure will result in minor variations in the dilution ratio. A fluctuation in city water pressure of +/-5 PSI will result in a fluctuation of +/-6% in solution strength.

For facilities with widely varying water pressure or where minor dilution ratio variations are unacceptable, a water pressure regulator and pressure gauge will be required. Metering tips will need to be selected based on the flow rate at the pressure specified by the regulator. This is typically a consideration for facilities using city water pressure, which can vary greatly between 35 and 125 PSI.

  • As long as the incoming water pressure is over 40 PSI, a 40 PSI regulator will ensure consistent water pressure and therefore a consistent dilution ratio. Installation of a pressure gauge will allow the operator to ensure that the incoming water has not dropped below 40 PSI during operation and thus validate the dilution ratio.
  • With an adjustable pressure regulator it is possible to "dial in" and achieve a specific dilution ratio.  Use the Metering Tip Calculator and enter the reference flow rate for your injector.  Try entering different values for the actual water pressure until a metering tip shows the desired dilution ratio (make sure to stay within the pressure range of the water supply).  Adjust the pressure regulator to this pressure and install the appropriate metering tip.
Lafferty Equipment Manufacturing, Inc.
5614 Oak Grove Road
North Little Rock, AR 72118
TFN: 1-800-999-2820

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