Downstream injector calculator

A downstream injector pulls chemical into the water stream after the pump, which is why it is safe for the machine and why nobody quite knows what strength is leaving the gun. This works it out in both directions: what your bucket produces at the wall, and what to mix to hit a target.

Downstream injector draw

Bucket strength% SHInjector draw ratioTarget at the surface% SH

What reaches the wall from your bucket

1.14% SH

To hit 1.0% at the surface, mix the bucket to 11.0% SH

Draw ratio is not a fixed number: hose length after the injector, nozzle size and the soap tip all move it. Measure yours once with a marked bucket and a stopwatch, then trust that figure.

A downstream injector only draws with the black soap tip fitted. Put a 25-degree tip on and the pressure downstream rises, the venturi stops pulling, and you are rinsing with plain water while believing you are applying chemical.

Photograph-style brass downstream injector on a concrete slab, a black hose screwed into each end and a clear chemical tube pushed onto its side barb — A brass injector plumbed in line: hose into the inlet, hose out of the outlet, chemical tube on the barb.

PSI & nozzle quick reference

0 degRed: Spot jet. Etches most surfaces.4000 PSI
15 degYellow: Stripping bare concrete.4000 PSI
25 degGreen: General concrete and walkways.3500 PSI
40 degWhite: Siding, wood, vehicles.1500 PSI
~65 degBlack: Detergent draw, low pressure.500 PSI

What moves the draw ratio

Injector ratios are quoted as a range because the ratio is not a property of the injector alone. Every one of these changes it:

Hose length after the injector. More hose, more back-pressure, weaker draw. This is the big one — 200 feet of hose can halve it.
Nozzle orifice. The soap tip has to be big enough to drop the downstream pressure. A worn tip draws differently to a new one.
Chemical viscosity and temperature. A thick surfactant blend pulls more slowly than thin hypochlorite.
Injector wear. The check ball and spring wear. An injector that drew 1:10 last season may draw 1:16 now.

Measure yours once

Fill a marked bucket, run the gun with the soap tip for sixty seconds into a barrel, and see how much left the bucket against how much water passed. That number is worth more than any published range, and it takes five minutes.

When the soft wash stops working

In order of likelihood: the wrong tip is fitted, the strainer in the bucket is blocked, the injector's check ball is stuck, the hose run got longer than the injector can pull through, or the hypochlorite in the drum has aged past useful strength. Work down that list before you buy anything.

Downstream injector calculator

What moves the draw ratio

When the soft wash stops working

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