Your injector draw rate helps determine the final dilution hitting the truck. If you don’t know what the injector is pulling, you’re missing part of the mixing calculation. A measured baseline also gives you something to compare against when performance changes.
What are you measuring?
There are two useful measurements: how much liquid leaves the pickup container during a timed test, and how much combined liquid comes out of the wand over the same time. For our calculator, draw rate is expressed as water to RTU. A 10:1 draw rate means ten parts pressure-washer water to one part RTU drawn from the tank.
Keep that separate from the tank’s water-to-concentrate ratio. They describe different stages. Our dilution guide explains how those stages produce the final dilution.
Establish a repeatable test
Use a clean, marked container of water for the pickup test. Record the starting volume, run chemical draw for a measured interval, and record the remaining volume. The difference is the pickup volume for that interval. Use enough water to keep the pickup submerged throughout the test.
Keep the injector, hose length, nozzle, and chemical-selection settings consistent with normal chemical application. Measure combined outlet flow with a flow meter or a manufacturer-approved collection procedure that preserves that setup. Follow your equipment’s operating and testing instructions.
A clean-water test establishes a baseline. The viscosity of the chemical mix can affect actual pickup, so use the result alongside your product and equipment guidance. Record Step 1 and Step 2 separately if they use separate pickup paths.
Calculate water to RTU from the measurements
When you measure the combined outlet volume, it already includes the liquid drawn from the pickup container. Subtract the pickup volume from that combined volume to find the pressure-washer water volume. Divide the water volume by the pickup volume to get the water-to-RTU ratio.
For example, suppose the same one-minute test produces 5 gallons at the outlet and draws 0.5 gallon from the pickup container. The water portion is 4.5 gallons. Divide 4.5 by 0.5 to get a draw rate of 9:1 water to RTU.
This calculation uses measured combined outlet flow. The pressure washer’s rated GPM is not a substitute for that measurement. If nothing is being drawn, troubleshoot the pickup problem before trying to calculate a working ratio.
Use the result to calculate your RTU
Open the Injector Dilution Calculator, choose your measured draw rate or enter a custom rate, and enter the product-appropriate final dilution you’re targeting. It calculates the RTU ratio and the water and concentrate needed for your chosen batch size.
The calculator uses water-to-RTU for injector draw and water-to-concentrate for final dilution. Check what a manufacturer’s stated ratio means before entering it; a ratio based on total mixed output is different from a water-only ratio.
When should you recheck it?
Recheck after changing the injector, nozzle, hose arrangement, or pickup components, and whenever chemical use or wash results change unexpectedly. Compare against your recorded baseline before changing the RTU mix.
Watch our chemical-mixing fundamentals video. If your measurements don’t make sense, send us the volumes, test time, and setup details so we can help you work through them.
