Inject Right & Protect the Water

The injection rate comes from a mass balance: required mass per minute = target concentration (mg/L) × system flow (L/min); divide by the stock-tank strength in mg/L (g/L × 1000) to get the injection rate in L/min. For example, 150 mg/L in a 600 L/min system from a 100 g/L stock needs 150 × 600 ÷ 100,000 = 0.9 L/min, which is 54 L/h. The tool does this live as you type.

It depends on the chemical and the water source. ASABE EP409.1 sets a check valve plus a low-pressure drain and vacuum relief as the minimum for chemical injection on a dedicated line; injecting into a potable well or a public supply, or injecting acid or pesticide, raises the requirement to a reduced-pressure-zone (RPZ) assembly or an air gap. The calculator looks up your chemical × source combination and names the required device.

Without it, a pump shutdown, pipe break or pressure drop can back-siphon concentrated fertilizer, acid or pesticide out of the irrigation line and into the water source — the classic way wells and public supplies get contaminated. A correctly installed and tested backflow device is what stops that reverse flow. That is why the tool treats an inadequate backflow device as an outright unsafe verdict.

A venturi (Mazzei-type) injector narrows the flow to speed it up, which drops the pressure at the throat below the chemical-line pressure and draws the chemical in. It only works if there is a real pressure drop across it — as a rule of thumb the downstream pressure must be no more than about 70% of the upstream pressure (a 30%+ loss). If the downstream:upstream ratio is too high the venturi loses suction and injects nothing.

Because the downstream pressure is too close to the upstream pressure, so the throat cannot drop below the chemical line and no suction develops. The fix is to create more differential: open a downstream valve, fit the venturi across a pressure-sustaining loop, or add a small booster pump on the chemical line. The calculator flags this when the downstream:upstream ratio exceeds 0.7.

An RPZ is a high-hazard backflow preventer with two independent check valves and a relief valve between them that dumps to atmosphere if either check leaks, breaking any reverse-flow path. It is the device required when a fertigation or chemigation line connects to a drinking-water source, or when injecting corrosive acids or pesticides. It must be tested by a certified tester, typically annually.

Only in the lowest-hazard case — and usually not even then. A spring-loaded check valve alone does not meet the ASABE EP409.1 chemigation set (which also needs a low-pressure drain and vacuum relief) and is never sufficient for a potable-water connection, acid, or pesticide. The tool will return an unsafe verdict if a single check valve is selected where the rules require more.

A venturi holds roughly 5–2,000 L/h but needs the pressure differential; a positive-displacement (piston or diaphragm) injector meters 1–4,000 L/h accurately regardless of line pressure; a suction or by-pass tank covers about 10–1,500 L/h but its concentration drifts as the tank dilutes. If the required rate falls outside your injector's accurate band the tool flags it so you can size a different injector.

Multiply grams per litre by 1,000. A 100 g/L stock solution is 100,000 mg/L. The injection rate is then the nutrient mass needed per minute divided by that figure. Keeping units consistent (mg/L for both the target and the stock) is what makes the dilution ratio come out right.

No. The device requirements here follow ASABE EP409.1 and widely used extension chemigation-safety guidance, but many states, provinces and water utilities have their own legally binding rules, permits and inspection requirements. Use the tool to size the injection and understand the protection class, then confirm the exact device and permit with your local regulator before injecting.

Unsafe means the backflow protection is inadequate for your chemical and water source — the contamination-critical failure, so do not inject. Caution means the backflow device is acceptable but a hydraulic issue exists: either the venturi has lost suction (downstream:upstream ratio over 0.7) or the required injection rate falls outside your injector's accurate band. Caution items are tuneable; the unsafe item must be fixed by upgrading the device.

Through the same injection port you can, but never pre-mixed in one tank if they react, and the backflow protection must satisfy the more hazardous of the two — acid raises the requirement to an RPZ on most sources. Run the calculator once per chemical to confirm the device class, and inject them in separate passes or through separate metered lines to control the concentration of each.