The instrument housing is made of aluminium alloy and consists of a single unit mounted on the lower part of the experiment platform. The rectangular box carries six attachment lugs on its base. The electronics system (ACPE) has its own structure attached at six points on one side of the instrument's main body, in which the mechanical and pneumatic subsystem is located. The figure shows the ACP unit with its electronic box, sampling tube and sealing cover.
ACP's maximum dimensions are 220x200x206 (H) mm. In addition to the inlet and exhaust tubes, and to the GCMS product transfer tube, there is an extension accommodating movement of the filter mechanism's rack. ACP's total mass is 6.7 kg (including 5% margin), shared between 4.5 kg for the mechanical box and 2.2 kg for the electronics box.
The aerosols sampling inlet tube extends downwards to penetrate the Probe's fore dome. To provide efficient evacuation of Titan's atmospheric gas after sampling and filtering, the exhaust tube exits upwards, passing through the experiment platform and the top platform. By siting ACP close to GCMS, the inlet tube is close to the Probe's axis and PTL is as short as possible.
A very strict procedure for limiting chemical contamination was followed during instrument fabrication. The objective values specified for the internal circuit of the gas transfer subsystem are 10 ppb for CO, CO2 and the organic compounds expected during aerosol chemical analysis and 100 ppb for H20. The internal surfaces of all the transfer tubes have been passivated ('silanised'). A cleanliness plan was followed first at the level of each equipment element and then at the instrument level. After assembly, the whole ACP was baked under vacuum for several days at 120°C. Also, organic materials were not used in the GV and filter mechanisms. Their bearings, in particular, are not lubricated.
In order to maintain the high cleanliness level during the instrument's ground storage and its first part of the cruise, ACP's internal gas circuit was filled to 2.5 bar with pure nitrogen. The internal circuit is hermetically closed off, by three sealing devices, at the level of the sampling and exhaust tubes and at the interface between the ACP housing and the PTL. The instrument overall helium leak rate of 10-6 mbar.litre.s-1 is sufficient to hold the gas circuit pressurised until few months after launch.
The two apertures leading to the atmosphere are sealed by specially-developed devices : the sealing cover (SC) for the sampling tube and the one-shot P2 valve for the exhaust tube. SC is screwed to the bottom of the sampling tube (ST); the tightness requirement is satisfied by a stainless steel O-ring gasket. SC's cap will be spring-ejected towards Titan at 11 m/s; the requirement was for 8.6 m/s in order to avoid back-collision with Huygens. The cap and spring, totalling 80 g, are held in position on SC's body by tin-silver solder. Ejection will be within 2 min of switching on the 53 W heater (SCH), as the alloy melts at about 150°C. Correct ejection is checked by a Hall effect sensor (HSC). The P2 concept is identical : melting a brazing allows a spring to eject the sealing cover, freeing the outlet gas exhaust.
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