Spa design by Aquatic Technology
This is the first in a series of home automation howtos. More coming soon!
Spa design by Aquatic Technology
If you have ever had the pleasure of maintaining a spa, you know that in such a small body of water it can be very difficult to keep the chemicals stable. This is especially true in the summer time, when the water is warmer and sunlight is continuously depleting the chlorine. A few manufacturers now offer electronic systems which continuously monitor the ORP and pH of the water, and automatically feed acid or chlorine when they go below a programmable setpoint.
You are probably familiar with pH already, but what is ORP? It's a way of measuring how effective the sanitizer in your pool water is. ORP stands for "oxidation reduction potential", where potential literally refers to a voltage that can be detected in the water using a special electrode. The higher the sanitizing activity, the higher the ORP.
ORP is different from "free chlorine" or "PPM", which is what you measure with a standard colorimetric test kit or test strips. Think of it like cruise control, where PPM is the gas pedal, and ORP is your speed. ORP is a better metric for use in automated chemical feeding, because it reflects the actual activity level of the chlorine, which can swing widely with changes in bather load, pH, and weather.
I installed one of these controllers myself and have been very happy with no longer having to test the water every couple of days and manually add chemicals. The water is just always perfectly clean and clear, and there is no chemical smell at all. Mostly this is due to the concentrations always being in the right range, but there is another interesting advantage to automated control which is that you can get by without using cyanuric acid (aka stabilizer). Essentially, the closed-loop dosing means you don't need it, and not using it increases the efficacy of the chlorine so that you can maintain a given sanitizing level at a lower "free chlorine" concentration. There is a surprising amount of controversy over the subject that I don't really care to get into, but suffice to say it works great for me and I use less than $3/mo of sodium hypochlorite even in the summer time. You can also buy cyanuric acid by itself if you feel the need.
Installation
All of my spa equipment lives in this three-sided structure, which keeps it out of the weather and also provides some space to hang my garden tools. I built this over the top of a concrete pad which was originally put in a long time ago to hold the home's propane tank. When the spa went in, the tank was moved elsewhere. The pad is actually set on a steep hillside which is why the bottom part of it is a retaining wall.
The white cabinet in the middle is a simple fiberboard/melamine unit from Home Depot. Before assembling it, I cut the side panels narrower so that it was just deep enough to house the CAT2000. It turned out to be just the right size. The GFCI outlet just to the right of it is connected to the filter pump circuit, so the controller is only powered during filter cycles.
Here is the pump and filter equipment. This was all done by the spa builder, and he spray painted everything black which I thought was a real nice touch. The reason there are so many valves is because this is all below water level. The valves need to be closed before servicing anything in order to prevent the spa from draining. The horizontal black pipe along the top is the return for the filter pump. The little grey fittings with clear tubes going into them are where the chemicals are injected.
Here is the CAT 2000 controller. Water comes in through the blue tube on the left, and goes out through the one on the right. The one in the middle is a bleed valve. The wheelie thing is a flow sensor which ensures that the controller does not attempt to inject chemicals unless the filter pump is running. The blue Cat5 cable is an RS232 output which I monitor from my computer.
Here's a close-up of the sensor tips, installed in the "flow cell". The glass bulb on the left is for pH and the metallic one on the right is for ORP. These sensors like to stay wet at all times, so the flow cell is designed to prevent air bubbles from accumulating.
Here's one of the Stenner pumps. These use the same principle as an IV drug dosing pump. A set of rollers compress a rubber tube creating negative pressure on one side to suck the chemicals out of the tank, and positive pressure on the other side that is sufficient to inject the chemicals against the outgoing pressure of the filter pump.
Stenner makes a variety of pumps with different speed ratings (in gallons per day). Also for any given pump model, you can exchange the rubber tubes to effect different pumping rates. One thing I got wrong is that even with the slowest tube installed, my pumps are too fast for the size of the spa, so I have to dilute my chemicals a lot in order to not overshoot the setpoints. One feature I wish the CAT2000 had is the ability to configure a much shorter pulse width modulation to the pumps in order to slow them down. It does have a proportional feed mode but it doesn't go to a low enough duty cycle to be useful for me.
The pumps fit just perfectly in these electrical boxes, with enough room for a few extra turns of tubing so that they can easily be removed for service. The peristaltic tubes are subject to cracking as they wear out, so it is important to install the pumps in a way that they can drain safely in that event. They should be mounted with the pump head facing down.
These are the chemical holding tanks - muriatic acid on the left, and sodium hypochlorite on the right. I dilute both solutions by approximately 4:1. Besides the overshoot issue I mentioned, diluting also increases the shelf life of the sodium hypochlorite and generally makes the chemicals safer to have around. I haven't been keeping track of the exact time between refilling these, but it is more than a month in the summer and more than two months in the winter.
By monitoring the controller's RS232 output from my computer, I get a text message automatically when the ph/ORP goes out of range. So I don't even need to check the tanks, I just wait for the text message and then refill them. Acid is consumed in proportion to the amount of chlorine that is fed - I think it's roughly 1:3.
(More info to come about the RS232 port)
The output side of the Stenners just go to these fittings called injectors. To install these you drill a hole in the PVC and then tap them with a 1/4" NTP tap. This was very easy to do but I would recommend practicing on a piece of scrap PVC before you do the real thing.
These fittings for the "bypass loop" are installed in the same manner. These are those blue hoses that go through the CAT 2000 flow cell. One goes on each side of the heater. The impedance of the heater creates a pressure differential between its input and its output ports, and this is what causes water to flow through the bypass loop.
Notice that the taps were made through the PVC slip fittings, not directly into the pipe. This ensures a thick wall allowing for a very mechanically strong fit.
And that's all there is to it!
Credits: thank you to Paolo for the installation tips!
