The interesting design decision in Neometrix's HST 300U Hydraulic Refilling Trolley isn't the pressure rating, it's the drive method: twin 50-litre nitrogen bottles at 220 bar pressurize the dual-chamber reservoir directly, rather than an electric pump moving the fluid. On an airfield apron with fuel vapor, dust, and inconsistent power access, that removes an ignition risk and a power dependency without giving up flow rate -- the system still delivers up to 300 bar and 100 L/min per circuit.
Getting fluid genuinely clean before it reaches an aircraft's hydraulic system means solving two separate contamination problems, not one. Bypass and mainline 3 µm absolute filtration handles particulates and varnish, holding ISO 4406 Class 18/16/13 cleanliness. Dissolved and entrained air is a different problem entirely -- a gas mixed into the liquid that filtration doesn't touch -- so an engine-driven vacuum pump extracts air and moisture down to under 200 mbar absolute before transfer, preventing the cavitation or hydraulic lock that entrained air causes once it compresses and expands under operating pressure.
Two independent circuits can be serviced at once from twin 70-litre SS304L chambers, which matters directly for depot-level maintenance where primary flight controls and utility hydraulics often need attention in the same window. Redundant relief valves and burst discs on both the nitrogen and hydraulic circuits, over-temperature shutdown, and an emergency-stop circuit protect equipment and operator throughout.
Full writeup: https://neometrixgroup.com/products/hydraulic-refilling-trolley
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