Urine Pretreat Concentration Sensor Based on Optical Absorption

Abstract

Urine is processed on the International Space Station using pretreat, a highly acidic compound. The dilution ratio of this compound in water is important; too little will inhibit the recovery of water from the urine, and too much is both corrosive and wasteful. Recently, a feasibility study concluded that a sensor based on optical absorption should meet the system requirements to monitor the concentration of pretreat in water before mixing with urine. This paper presents a design for that sensor, based on looking through the walls of a semitransparent perfluoroalkoxy tube carrying the diluted pretreat. The result is an accurate, completely nonintrusive, rapid-response pretreat concentration sensor. A theoretical model of the sensor is developed, yielding an analytic relationship between optical measurements and concentration that can be used to compensate for the nonlinear output of the optical sensor. Experiments demonstrate that the assembled device has adequate sensitivity and response time to monitor flowing diluted pretreat such that the total dose administered per use can be monitored.