I have used an EZ-Flow with drip irrigation to pots for many years. Cant say anything about accuracy or how well it it adjusts. I just notice that the Miracle Grow disappears over time...

Its basically a sealed container with water/fertilizer and 2, 1/4" pipes which connect inline to your hose/pipe via a short (4") pipe section - so some amount of your water flow is going through the injector and coming back into the main flow. Its designed to only be under full pressure when active, so needs to be switched off upstream.

Thank you for your reply.

The thing I had heard from others about this kind of injector (EZ-Flow and Hoss) is that they are sensitive to water pressure and the fertilizer they give out gets more and more dilute over time. I really do not know much about that though. I am growing orchids so they do not need much fertilizer and can get burned rather easily. I would ideally water with 1/5th strength 20/20/20 every time I water

Instead of using drip irrigation, I would likely be watering with a garden hose. I just want something that puts the fertilizer into the water near the splitter so I can use whatever hose head I want without having a cumbersome Chapin mixer that gets empty after a few gallons.

Yes I would agree with "sensitive to water pressure and the fertilizer they give out gets more and more dilute over time"... In my case I have output from a fixed pressure bell valve and time my watering cycle so that no other circuits are in use. The only way to control the concentration would be to use the adjustment control in its lowest setting and increase it every few days/weeks...

Look at a fertilizer injector called Dosatron. There are many different models - check them out on Amazon. I have not used them myself - too expensive for my taste. Commercial nurseries use them. It fits your requirements. It is inline injector with a tube that is dipped into a tank of fertilizer. It is supposed to be very precise. Uses no power.

I use a cheap fertilizer injector from Miracle grow. https://www.miraclegro.com/en-us/products/plant-food/miracle-gro-liquafeed-universal-feeder-w-liquafeed-all-purpose-plant-food

The fertilizer is held in a bottle. It delivers at a fixed rate. It is pretty accurate too. Does not fluctuate with water pressure as long it is above 30psi, I think. Does not depend on the length of the hose or water flow rate either. But the bottle is pretty small and they want you to keep buying a new bottle. But you can refill it yourself. The cap is a bit finicky. But I modified the bottle so that I can fill it without removing the cap. Basically I drilled a hole in the back and inserted a pvc tube. Since it is a fixed ratio (1:325), the only way to change dosage is to control the strength of the solution in the bottle. Another advantage is that I can use my own fertilizer. The biggest advantage is that you can put two of those in series.I use one for regular fertilizer and the other to inject Protekt or vinegar as needed.