A fertigator is a device which mixes a liquid fertilizer and water in the proper ratio. I did not make up the word, but I did design this monstrosity. This was designed primarily for hydroponics, but could easily be adapted for traditional gardening. This fertigator is designed to work with a variety of flowrates and different water pressures. I designed it to work at a rate of up to 5 gallons per minute. The operating theory is simple enough – water flows in, spins a paddlewheel sensor which measures the flow rate and a microcontroller proportionally controls the speed of 4 pumps in relation to the flow of incoming water. The design however proved to be very challenging.
The pumps must be controlled extremely precisely, nearly to the milliliter in order to maintain the proper ratio with fertilizer concentrates. The pumps must also be highly chemically resistant in order to stand up to harsh chemicals such as phosphoric acid, nitrates, and phosphates. The backpressure must also be regulated so the metering pumps will pump a consistent volume. Many of the fertilizer concentrates are extremely reactive in there concentrated form so I installed a static mixer so the chemicals would be diluted quickly before they have a chance to react.
The pumps are BL5-1 pumps made by Hanna Instruments that I modified extensively. The pumps are positive displacement solenoid pumps. What this means is a big electromagnet pulls a plunger which pushes a PTFE (special plastic) diaphragm. There are one-way valves (check valves) so that when the plunger moves one way liquid is drawn in and when it pushes the other way liquid is expelled. Each stroke moves approximately 1mL of liquid depending on several factors (liquid viscosity, back pressure, supply pressure, etc.)
The original pump circuitry was pretty simple. It took in 120 volts AC, used diodes to rectify it into 170 volts DC and then switched it on and off at a varying rate to pulse the solenoid piston back and forth. By varying the frequency to the solenoid you can control the flow. This is very similar to Pulse Width Modulation used for motor control but at a much lower frequency. For those that are wondering 120 volts AC turns into 170 volts DC when it is rectified because AC is typically measured RMS (Root Mean Squared, or the average voltage of the waveform). The DC voltage is the same as the AC voltage if it were measured peak to peak.
In order to connect the pumps to the Arduino microcontroller I removed all of the driver circuitry and hooked a 10 amp full wave rectifier directly up to the solenoid. These are significantly oversized for the application. I then hooked the solenoid up to a solid-state relay board which was connected to the Arduino. The relay board is fused at 2 amps per channel for those that are curious.
The Hanna pump’s had other issues that needed to be fixed. The check valve springs are made out of a very special chemical resistant plastic and glass beads are used to make the seal. The problem with these materials is they don’t create a very effective seal at all. To remedy this problem I ended up using injection quills with a built in check valve. I also replaced all of the O rings and had to use special adapters to convert the odd metric sized tubing from the pumps to 3/8” for the injection quills. I plan to add a list of the parts I used if anyone else wants to build this.
The microcontroller is a standard generic Arduino UNO clone. I hooked the flow sensor up to pin 2. This is important because this is one of the only pins that can do hardware interrupts. Interrupts are necessary for reading the high frequency signal. I will upload the source code later. My programming isn’t very good because I was just learning C++ at the time. If I have the ambition I’m sure I could write better code now. It does work however as long as it is not left on continuously for over 2 months. If it were left on continuously for over 2 months the timing buffer will overflow. This could be fixed, or you could just not leave it plugged in constantly.
There is also a remote control that I use for priming the pumps. I got this on eBay for a few dollars from China. It is a pretty simple 315 MHz, 4 button remote control. It conveniently operates at 5vdc and was even pin compatible with the Arduino.
I will add more info about this later. Let me know if you are interested in this project.