This FAQ entry offers a way to make a device that can maintain the water level in your hydro reservoir or humidity cup. It is gravity powered and made from stuff you can get at your local home maintenance store. There are no moving parts other than water. I have built this and it does work. Sorry, I do not have a digicam to submit pictures.
I have been using this for the last 3 months to keep a constant level in my humidity water within the grow room. It works like a champ, keeping the level to within 1/16 inch of where I set it at all times. I used to fill the cup every day, now I just replace the water bottle when it is almost empty. I found that I was using about 2 liters per week during the winter while in veg mode (150W HPS). Now that spring is here and ambient humidity has risen, I have entered the flowering stage and water consumption has slowed to about 2 liters every 3 weeks.
For those doing hydroponics, this is an alternative method to maintain your reservoir level if you go on vacation. Past methods I have seen involve float switches or valves. This technique may remove some of that complexity for people who don't have easy access to those items.
The technique is scalable. I have used it with 2 liter soda bottles, but you can easily use a 5 gallon carboy for holding the extra water. The only key requirements are that the bottle holding extra water can be totally sealed against any atmospheric air leaking in around the edges and it must have walls that are stiff enough to withstand a bit of negative pressure. The pressure comes from the column of water and is equal to about 1/2 psi for every 14 inches of height difference between the bottle and the reservoir top. A 2 liter soda bottle can handle about 2 feet of difference (1 psi). A carboy has thicker walls and can be mounted much higher. Sorry, plastic bags of water won't work.
Here is a diagram that explains how to build the device.
(click to unlarge)
To seal the tubing as it enters and exits the soda bottle, I use a material called Goop. This is a plastic paste that hardens after solvents evaporate from it. You can get it at Home Depot for $5 per tube. One tube will do dozens of tubing closures. There are many other alternatives to this, so pick the one that is right for you.
Most of the tubing is 1/4 inch black vinyl tubing found in the drip irrigation section of your home improvement store. It is about $5 for 100 feet. The larger diameter tubing can be found in the same store in the plumbing department.
I had trouble with getting the large 1/2 inch diameter tubing to mate with the small stuff until I figured out that you can buy a few inches of intermediate tubing to join the largest size to the smallest size. I found that 1/4" OD (outside diameter) will fit snuggly into 3/8" OD, 3/8" OD will fit into 1/2" OD, and 1/2" OD will fit into 5/8" OD. I recommend using about 2 inches for each of the intermediate tubing sizes. If possible, they should overlap each other to add rigid strength to the entire tubing. Use water or spit to lubricate each piece of intermediate tubing when connecting and then push them on tight! This will be your biggest source of air leak problems if they do not seal against each other. If you have still have trouble getting all the tubing to have an air tight seal, apply a thin layer of Goop or other sealant to the tubing ends before mating them.
You will need at least 6 inches of vertical height for the 5/8" OD vinyl tubing, more is better. This size is important because it has 1/2" ID (inside diameter). 1/2" ID is the smallest size tubing I could find that would reliably let surface tension break when the bottom of the air hose is exposed to the atmosphere.
When working with some of the larger diameter vinyl tubing sizes, you may find that it wants to curl or flatten. This is because the tubing is shipped to the store in rolls and was curled and flattened during shipping. To fix this, dip the tubing in very hot water. I boil water on the stove and then put it into a shallow bread pan to let me work. When the tubing is heated in this manner, it will relax and become very pliable. Remove the tubing from the water and lay it down straight while cooling. Use a small weight (scrap wood, shoe, hammer) to hold it in place for about 15 minutes. Most or all of the curl and flatness should be gone. If you have trouble inserting intermediate tubing inside the next piece, dip the pieces in hot water and then fit them together. The tubing is much easier to work with when it has been warmed. You can reheat the tubing as many times as you need to in order for it to behave.
Make sure you pay attention to where the ends of the tubing are inside the sealed bottle and reservoir. The water line must extend to the bottom of the sealed bottle and be under water in the reservoir, the air line must be short enough inside the sealed bottle to be above the water in it. Failure to do so will result in a system that won't work.
To use the system, stick the water line into the reservoir you wish to maintain. Attach the air line to the side of the reservoir so that the bottom is right at the water level you wish to maintain. I have found that keeping the air line at the right level is the most difficult part of operating this. I ended up putting a piece of soft copper 1/4" tubing on the end of my water line and then using small diameter wire to attach the air line to the copper tubing. The copper has enough weight and stiffness to sit in my humidity cup without moving around. You will have to determine the best method for your setup. If you come up with good ideas, please post them in this thread for others to use.
To begin using the system, fill the bottle with water and then seal it. With 2 liter soda bottles, I just screw the lid on tight and the tubing sticks out of the top of the lid. Now, you need to prime the tubing with water. You can use one of two methods:
If you find water keeps dribbling out past the cutoff point, you have an air leak into the sealed bottle or the bottle is too high above the reservoir. To find air leaks, check to make sure the lid on the bottle is screwed on tight. Make sure you used enough sealant where each tube enters the bottle. If you have different sized tubing fitted together to change sizes for the open end of the air line, check to make sure you aren't getting air leaks through the gaps where tubing fits. Air leaks can be a pain to find and I have had most trouble with it coming in through the different sized tubing I use to adapt the 1/4 inch air line to the larger diameter at the open end.
You will know if the bottle is too high above the reservoir because the sides of the bottle will start to collapse. This happens if using soft sided soda bottles like I have. To fix it, you must reduce the height difference between the sealed bottle and the top of the reservoir.
If air gets into the sealed bottle or the air line is not properly secured and gets exposed to the atmosphere, the entire contents of the sealed bottle will leak out. You should be prepared for this by making sure you have reservoir large enough to hold the entire contents of the sealed bottle or use a catch basin (reservoir in a bucket) around the outside of your reservoir. I left a system to eat dinner and accidentally let the air line flip out of the reservoir. The entire contents of the sealed bottle where all over the floor when I got back.
How it works
This system is really a siphon. Like all siphons, it is powered by gravity. The sealed bottle and air line are the secret to keeping the siphon from spilling all its water at once. Air must enter the sealed bottle to displace the water as it leaves. The only place air can come from is the air line. When the reservoir level reaches a point where the air line entrance is submerged, no more air can enter the sealed bottle. If you use clear tubing for the air line, you will actually see the water climb up inside it until the water level reaches the same height as the water level in the sealed bottle.
When the reservoir level drops, an air gap opens up between the reservoir surface and the air line. The air line is full of water, but the large diameter tubing prevents the water surface tension from holding the water in place. The water that was sucked up into the air line runs back into the reservoir and air is again allowed inside the sealed bottle. When the air line entrance is again submerged, the cycle is complete.
If you use a reservoir with a large surface area, be prepared for the filling cycle to suck relatively large amounts of water from the sealed bottle. This is because the amount of water required to bring the reservoir back up to level is relatively large compared to what the sealed bottle contains. In this case, you need to use more than one sealed bottle or a larger bottle.
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