Agriculture & Hydroponics

A hydroponic growing system typically contains three water tanks: clean fresh water, nutrient enriched and pH controlled water, and wastewater. The nutrient enriched and pH controlled water comprises the growth medium for the plants. The pH level and nutrient composition in this water reservoir is controlled using valves and pumps to add nutrients, carbon dioxide, or fresh water. Draining wastewater from the nutrient enriched water reservoir helps to maintain consistent water levels, while adjusting nutrient concentration and pH.

To set up a hydroponic monitoring system, conductivity sensors and pH sensors should be installed in the water reservoir that will be used to supply the plants’ growth medium. Based on the measurement data from the sensors, pH controllers and conductivity controllers signal for the opening and closing of valves or operation of pumps. For example, if nutrient concentration in the water reservoir became too high, the conductivity controller could turn on the fresh water pump to dilute the nutrients. Conversely, if nutrient concentration became too low, the conductivity controller could turn on the nutrient pumps. Similarly, if pH became too high, the pH controller could open the solenoid valve, allowing carbon dioxide to flow into the water reservoir. Carbon dioxide reacts with water to form carbonic acid, which lowers the pH of the solution.

• High nutrient concentration > high conductivity reading > add fresh water
• Low nutrient concentration > low conductivity reading > add nutrients
• Overly alkaline solution > high pH reading > add carbon dioxide

Hydroponics is considered a relatively light duty application for pH and electrical conductivity sensors. This makes sense, because typically if a plant can survive in the environment, then so can a sensor. For this reason, hydroponics systems typically incorporate laboratory style pH and EC sensors, which have the benefit of being low cost and easy to use.

Laboratory style pH sensors come in both glass and plastic bodies; plastic bodies are favored for hydroponics applications, since they are more durable. Glass bodies are used when chemicals in solution are likely to react with the polymer body material, but this is not typically an issue in hydroponics.

Laboratory pH sensors are designed with junctions that separate the sensor internals from the liquid being measured. Most hydroponics measuring systems can use single junction pH sensors with good results. Double junction pH sensors offer added protection against contamination of the sensor reference system. This protects the sensor from “reference poisoning,” which may extend the sensor’s lifetime in solutions that contain chemicals that react with silver.