Evaluation of sensor technologies for automated control of nutrient solutions in life support systems using higher plants

The complete closure of life support systems will require the development of low-mass, hydroponic nutrient delivery systems. At present, there is a technology vacuum associated with the reliable application of various sensors to the in-line management of re-circulating hydroponic solutions. We have investigated the performance of a common electrical conductivity (EC)/pH based control system in a sealed environment chamber and quantified its impacts on nutrient concentrations in a recirculating hydroponic solution. This paper also discusses bench test evaluations of Ion Selective Electrode (ISE) technology and the potential limitations of these sensors in closed systems. Limitations included significant drift, biofilm accumulation and temperature dependencies. A discussion of the potential application of more advanced sensors, such as Ion Selective Field Effect Transistors (ISFETs) is also included. All evaluations were made with reference to the constraints imposed by design and operational criteria for night testable life-support systems incorporating higher plants.