Contributed by: Ranger2000
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.
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.
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.
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:
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
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
I have found this technique works well
and can be used instead of a float valve. Besides the
materials, the only thing it requires is to ensure the
sealed bottle is always higher than the water level in
your reservoir. If you need more capacity than one
bottle will hold, I have done this with up to seven
sealed bottles that feed into each other. This gives me
fourteen liters of storage. You can extend it to as many
bottles as you need for whatever capacity you