Originally the hydraulic manipulators were covered by a helmet-like retractable see-through visor, but these were removed in a major overhaul in 1994.In field testing, Mir-1 descended to 6,170 m and Mir-2 descended to 6,120 m. The units are designed for pressure at 6,000 metre depth, and have been tested to 125% of that pressure.The units' life-support systems have 246 man-hour capacity, or 3.42 days for a three-person crew.The distance to the seabed can also be accurately measured when nearing touchdown. The units contain imaging sonar units of 250 metre range, so nearby objects can be visualized and their distance measured. VHF radio is used to maintain communication with the surface.Air pressure inside the cabin remains at a constant one atmosphere: the air is recycled in a manner similar to that used on board spacecraft, with lithium hydroxide scrubbers removing accumulated carbon dioxide.Water can be forced out of these tanks as required by using compressed air. Longitudinal trim is controlled using two spherical water ballast tanks, fore and aft.The aft hydraulic propulsor is rated at 9 kW and 2 side propulsors are rated at 2.5 kW each. Electric motors drive hydraulic pumps to actuate hydraulic manipulators and three propulsors. Power is provided by NiCad batteries of 100 kWh capacity.Three viewports are provided (viewport material is 18 cm thick): the forward-facing port is 20 cm diameter the two side-facing ports are 12 cm diameter each. The personnel sphere's walls are 5 cm thick, and the inside diameter of the working area is 2.1 m. The Mir submersibles are 7.8 m long, 3.6 m wide, 3.0 m high, and weigh 18,600 kg (maximum payload is 290 kg).Unlike other deep submergence vehicles that use iron ballast to reach the ocean floor, the buoyancy and depth is adjusted by ballast tanks. Additional buoyancy is provided by 8 cubic metres (280 cu ft) of syntactic foam. The resulting construction is close to the density of water, thus making it easier to move in different depths. Two hemispheres were made by casting and machining, and then bolted together, thus avoiding welded joints. This alloy contains about 30% cobalt and smaller amounts of nickel, chrome and titanium. On Mir, the personnel sphere is made of a maraging steel alloy that has ten percent better strength/weight ratio than titanium. Traditionally, the personnel sphere of a deep sea submersible is manufactured of titanium plates that are welded together. The Mir submersibles can dive to a maximum depth of 6,000 metres (19,685 ft). Their military counterparts are the Konsul-class submersibles. The two Mir units are operated by the Russian Academy of Sciences. The carrier and command centre of both Mir submersibles is the R/V Akademik Mstislav Keldysh. They might also be used to assist in submarine rescue operations, although they do not have the capacity to take anybody aboard when underwater. The vessels are designed to be used for scientific research. 3.6 2008–2010 Expedition to Lake Baikal.