Sensors

There are many different pH sensors on the market, but the operating principle remains the same. A semipermeable membrane lets ions (the dissolved nutrients in the hydroponic solution) pass into a chamber, which holds an electrode. This electrode is connected to a similar one being placed in a second reference chamber with a specific acidic level, creating a so-called galvanic cell. Due to the different potential of the solutions, a small voltage is measurable, which then has to be translated into the level of acidity or alkalinity, respectively.

Hence, when implementing such a sensor in your system, you will have to convert the sensor's output voltage into the level of acidity (including temperature coefficient when aiming for improved accuracy). Additionally, as ions will alter the electrodes surface, recalibration will be necessary once in a while.

Different than pH sensors, there are not that many electric conductivity (EC) sensors on the market and the prices are a little higher. Nevertheless, the working principle is easily explained: two symmetrical electrodes are hold into the solution in question and some power put onto them. The electrodes react like a conductor, creating a electromagnetic field between them, which is affected by the dissolved nutrients of the hydroponic solution in between: the more ions, the higher the conductivity of the nutrient solution and hence the higher the voltage transmitted between the electrodes.

Hence, when implementing such a sensor in your system, you will have to convert the sensor's output voltage into the specific conductivity value (including temperature coefficient when aiming for improved accuracy). Additionally, as ions will alter the electrodes surface, recalibration will be necessary once in a while.

Different than pH sensors, there are only a very few selective ion sensors as standalone product on the market and the prices are very steep towards more sophisticated sensors. Also keep in mind, that these types of sensors only make sense, if you are testing for many macro or even micro nutrients and hence are able to add each nutrient separately. Nevertheless, there are two working principles for selective ion sensors: optical and electro-chemical. The latter is more likely to get as standalone product and hence the only one interesting for the purpose of a small hydroponic system at home. It works very similar to the one of a pH sensor: Either, a semipermeable membrane lets only the specifically dissolved ion pass into a chamber, which holds an electrode or this electrode only binds the specific ion in question, causing to sense exclusively the specific desired ion.

The rest is similar to the pH sensor: this electrode is  connected to a similar one being placed in a reference chamber with a specific solution of the ion in question.  Due to the different potential of the solutions, a small voltage is measurable, which then has to be translated into the concentration of the ion you are testing for.

Hence, when implementing such a sensor into your system, you will have to convert the sensor's output voltage into the specific ion concentration (including temperature coefficient when aiming for improved accuracy). Additionally, as the ions will alter the electrodes surface, recalibration will be necessary once in a while.

There are many analog and digital sensors on the market suitable for hydroponic systems measuring air and water temperatures, air humidity and/or atmospheric pressure, respectively. And there are also many different working principles. Nonetheless, at least the most of them use metals changing their electrical resistance or volume with changes in temperature, humidity or pressure, which in return alters the power put through. The data is then either send directly back to the controller or converted into a digital signal and then send back.

There are many light sensors for different wavelengths on the market. While they may use different technologies to capture and process the analog signal, the latter is always obtained through the photoelectric effect: Light particles called photons hit the metals' surface and cause through their provided energy to expel some electrons, now called photoelectrons. These provide small powers, which may be measured and converted into light intensity or some radiation scale.

There are different pressure sensors for many divers applications on the market. In a hydroponic system, you could measure the air pressure to experiment the plants' behavior to storms, etc., or you could measure the cellular pressure of leaf cells to control the plants' health. Both intentions probably use different working principles. For the atmospheric pressure, the piezoelectricity is often used. It appears at a conductor, which has one plate fixed and the other one movable according to the outside pressure. With being pushed towards the other plate, the piezoelectricity is conducted.