I’ve been using Sub-Irrigated Planters (SIPs) using 27 gallon storage totes in my greenhouse, office, and basement for a couple years now. They solved the biggest problem I had in the first few years of growing in greenhouses, which was the need to constantly water and if you missed a day, everything wilted, especially if it was hot out! With a several gallon reservoir, I can go days to weeks without watering. However, it still is a bit of an art to know when to water them and how full they are when you are filling them up. In the greenhouse, I’m not as worried since overflow drains away, but the ones in my basement and office I usually err on the side of caution to prevent a big watery mess. This means the SIPs really aren’t operating at their 100% capacity of saving me from watering.
So I wanted something that could measure water level so I knew when the reservoir was running low and when it was about to overflow as I watered it. I found several versions online, such as the IKEA self watering pot or this cool dowel + fishing bobber solution. I didn’t have any bobbers or dowels around (and currently hard to get with the COVID-19 restrictions), but that got me thinking about foam insulation. I had some 1/2″ x 4′ x 8′ XPX foam insulation in the basement and I cut off about 1/2″ x 1/2″ x 2′ strip and then fed it down into the pipe and lo and behold it floated! I pushed it down to the bottom, marked the empty level with red and then filled it up until it overflowed and marked that level with green. It seems to be working pretty well so far!
My two biggest worries are (1) sun deterioration though the UV protection in the polycarb should help with that and (2) absorption of water into the foam, which will throw off my gauge. I might build a bobber + dowel version as well for comparison and see how they do!