We've gotten a few questions about the auto top-off system (ATO). Since the tank engineering tasks are my bailiwick, April asked me to explain the system.
The system consists of 5 basic pieces:
- RODI unit
- Sprinkler timer
- Reservoir and pump
- Water level sensors (2)
- Control system
RODI
The RODI unit is just a basic 25gpd system that we got from Foster's and Smith. I also purchased a Coralife float valve, for the reservoir, that comes with a cut-off valve (circled). The cut-off is optional but it can help reduce water waste.
The cut-off splices into the clean water output line as well as the membrane feed. When the output line is closed, pressure builds in the line and activates the cut-off, turning off water to the membrane; eliminating excess waste water from being produced.
Sprinkler timer
The timer was purchased at the local home store and is connected between our water source and the water filter. RODI systems typically need to run for a short time to produce truly pure water. Rather than refill the reservoir every day, the timer is set to allow the reservoir to almost completely empty before being refilled.
Reservoir and pump
I considered building a reservoir out of acrylic but while at Walmart, for something completely different, I ran across this Sterolite file box. It's the perfect size for under the stand and it cost just a few dollars. I drilled a large hole in the lid for the water line, installed the Coralife float valve (to avoid over filling) and stuck a Maxijet 1200 to the bottom of the box. I used standard 1/4" vinyl water line from the pump to the tank. A 1/4" line routed through the attic/wall supplies RODI water to the reservoir.
Water level sensors
The sensor assembly is mounted to a small acrylic panel that is mounted in the pump well of our BioCube HQI with a couple of nylon screws. (I waited until April was out of the house before I drilled the holes in the tank baffle. Shhhhhhh!!!!) The float switches are staggered so they trigger at different water levels.
The lower sensor does all of the heavy lifting. It triggers anytime the water drops below the desired level. The upper float switch is a safety device.
Control system
I built this unit based on a system that I used on a previous tank. It consists of a transformer, DC rectifier and a couple of relays. The float switches and relays operate on 12V dc, and switch the 110VAC to the top-off pump. The transformer and rectifier provide DC to the device without the need for a separate power adapter. The sensors and indicator lights plug into the unit using 1/8" stereo jacks.
When the lower float switch activates, water is pumped from the reservoir into the tank. Additionally, anytime the float is active a small red LED mounted in the tank stand illuminates to let us know the system is active. If the float switch, or primary relay, sticks for any reason the upper float switch gets activated to power down the pump. The light remains lit until the original sensor is cleared; i.e. a steady light that doesn't go out means we've had a fault.
One problem that plagues ATO systems is sensor chatter; or rapid cycling of the float switch. While mostly just annoying, that chatter causes undo wear to the pump and relays. I use a decidedly non-technical solution to the problem. The float switches are in the final chamber, or sump, on the tank. Rather than pump the water into the sump, I feed it into the display portion of the tank. The system takes a little time for the water level to be reflected in the sump level. The result is that each pump operation brings the water level to slightly above the float switch trigger; eliminating chatter.
Finally, I made sure there was an air gap between the top-off water hose and the tank to prevent tank water from syphoning back into the RODI reservoir.
Summary
On challenge, especially in a small tank, is keeping up with evaporation. When water levels drop, all the tank parameters fluctuate, causing stress to the inhabitants. With this system in place, evaporation is no longer a concern. Levels stay constant, animals are happy and we get to go out of town every now and then and not worry about the tank.
Cheers
I use the kent sea squirt to spot feed and it makes it easier, although mysis seem kinda big for it lol.
