Why does my fertiliser injector not work
 

Hi. I have a small shadehouse with an overhead watering system. Just a 12mm mainline directly from a garden tap, a couple of short branch lines, and a few spinner style spray heads. I wanted to be able to inject some fertiliser into the line so I bought one of these

1/2 Garden Irrigation Device Venturi Fertilizer Injector Switch Water Tube KitZ5 193571793492 | eBay

It doesn’t work. No Venturi effect is created and no fertiliser is drawn into the main line. I tried different water flow rates and other variations and alternated directions but without result. Can anyone suggest what I might be doing wrong ? ?.

Cheers
Arron

For a Venturi to work, there must be a significant pressure drop across it. I suspect the combination of 12mm hose length, branch lines and spinner nozzles create too much back-pressure.

Try placing it as close as possible to the tap if you haven't already.

I believe placing it as far from the tap would be better.

For a venturi to work, the PTap must be significantly greater than the PHose+Feed Lines+Nozzles. If moved to the far end of the hose, because of the flow rate limitation of the feeder lines and nozzles, the PTap and PHose are probably close to the same, so PTap+Hose >> PFeed Lines+Nozzles.

In other words, the flow through your main line is too low to generate enough Venturi effect. The device probably has a minimum flow stated somewhere in the documentation. If you had a larger diameter supply tubing with more emitters it might work.

There are injectors designed to handle low flow rates. Look at Chemilizer.

Ok, thanks for the responses.

Tomorrow I will disconnect some of the devices on the output end to reduce the back pressure, see if it works, and then go from there.

If you can get it to work, the mixing ratio might not be as specified, so you’d do well to “calibrate” it:

1. Mix up a liter of water with food dye as your feed tank.
2. Use a larger container (20L bucket, for example) as your “catching vessel”. Having two will help a lot, so you can dump one while the other continues filling.
3. Drop the feed tube in the colored water and the delivery end(s) of your irrigation lines into the bucket, then turn on the water.
4. Keep track of how many buckets you fill, including fractions, until the dyed feed is used up.

As long as the mechanics of the system (hoses, branches, nozzles, etc.) and the water pressure and flow stay the same, that ratio ought to hold fairly steady going forward.

Another thing I had to do to increase flow to create the venturi effect was using a Dramm Rain wand attachment so water flow wasn't restricted by at the business end of the hose. It makes for easy watering too, as I find the rain style attachment delivers the most water without moving around media or creating a mess.