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PGBA, a 0.08m2 / 27 liter spaceflight plant chamber payload employs two temperature-controlled liquid coolant loops to control the temperature and humidity of the sealed plant chamber independently. Cabin-air cooled thermoelectric heat pumps control the temperature of the water-alcohol coolant fluid in each loop, which is circulated by small, low-power, magnetically-coupled positive displacement gear pumps, designed to meet NASA safety requirements. Pulse-width-modulated DC current control circuits, controlled by two PI software controllers, maintain temperature and humidity accurately. The coolant loops feature bellows-based expansion vessels to accommodate thermal expansion and pressure fluctuations. Pressure sensors monitor the proper function and performance of the system. Pressure decay tests and unique fill procedures should ensure leak and air bubble-free operation.