Building a Sprayer that Mixes Chemicals Automatically

If I told you that you could avoid mixing any chemicals, limit exposure, and avoid spills, you would probably say it’s too good to be true.

When you spray anything, mixing the chemicals in the tank is likely one of your biggest pain points. It doesn’t matter whether you’re spraying insecticides, herbicides, or even a cleaning chemical; the same problems exist. Mixing accurately, avoiding spills, and limiting exposure to potentially harmful products.

For these reasons, I set out to create a spot sprayer that would handle the mixing for me and (almost) completely eliminate the need for me to measure, mix, or handle chemicals. Furthermore, cleaning the tank would be a breeze because there would only be water in the sprayer tank.

How I Built & Tested The Sprayer

My strategy was to modify my existing sprayer so that it carries only fresh water in the main tank. Instead of premixing chemicals, the sprayer pulls clean water from the freshwater tank while simultaneously drawing concentrated herbicide from a separate chemical container.

The two liquids mix inside the sprayer plumbing before being discharged through the spray wand or boom.

The basic plumbing wasn’t especially difficult to build. The bigger challenge was figuring out how to accurately meter the chemical so the correct amount would be mixed with the water.

I got this idea after seeing the Chapin Mix-On-Exit Sprayer. The way I designed this sprayer operates on a similar principle but operates a little differently.

Diagram of How the Sprayer Mixes Chemicals After Leaving the Tank

My first design used a small needle valve to restrict chemical flow, allowing the injection rate to be adjusted. I also experimented with a TeeJet QuickJet body and metering orifice disc, which offers a more predictable flow rate because each orifice size has a known output at a given pressure.

To prevent contamination of the freshwater tank, I installed a check valve on the pump inlet so mixed solution cannot flow backward into the tank. During testing, I also realized the pressure regulator couldn’t return bypass flow back to the freshwater tank like a traditional sprayer.

Instead, I rerouted the bypass directly back to the inlet side of the pump, allowing the pump to regulate pressure without sending diluted chemical back into the clean water supply.

Testing The Sprayer

After assembling the system, I tested it on a small patch of weeds. By the following day, the weeds were clearly dying, proving the concept worked and that the sprayer was successfully mixing chemical with the water.

The biggest remaining challenge is accurately controlling the injection ratio. While the system is effective for spot spraying, I don’t yet have a reliable way to know the exact application rate needed for broadcast spraying.

That’s the next step in the project, and I’ll continue experimenting with different metering methods, injectors, and proportioning devices to improve consistency.

Auto Mixing Sprayer Parts List

The major parts are shown in the diagram. Some small fittings or adapters may not be listed here. For more details, take a look at my full DIY sprayer diagram.

Design Changes

The bottom line is that the sprayer will work for spot spraying where your application rate doesn’t matter as much. For broadcast/boom spraying, it will be very difficult to know if you are applying the correct rate.

I plan to make adjustments to improve the design. I mentioned earlier that I was inspired by the Mix-On-Exit sprayer, which mixes chemical outside the tank, but all the chemical still runs through the pump. Ideally, I want to build a system that mixes everything after the pump so I don’t have chemical wear on the internal components.

In the future, I plan to experiment with downstream chemical injectors commonly used in pressure washers and car wash systems. These tools might make the sprayer cost a bit more, but if they can eliminate the issues of my current design, they may be well worth it.