Vacuum chamber testing at large facilities can require hundreds of instruments, necessitating even more feedthroughs. The number of instruments and sensors that can be fed into a vacuum chamber is limited by the number of feedthrough ports dedicated to instrumentation. Thus high-pin-count, mil-spec-style feedthroughs have been developed, but these are all custom-made and also expensive to make and replace. The high-pin-count feedthroughs also make it much harder to troubleshoot individual wires in case of a problem. By using a multiplexer within the vacuum chamber, the number of wires required per instrument can be reduced to much less than one. The multiplication of wires from within a vacuum chamber allows a drastic increase in sensor and instrumentation channel count, while using the same number of sensor ports and feedthroughs within an existing vacuum system.

A 25-thermocouple Multiplexer in an environmental chamber.
By placing the multiplexer within the vacuum chamber, multiple sensors could be wired to the multiplexer. The only wires required to go through the vacuum chamber wall are the power, the control signal, and the single sensor output. If the multiplexer has enough channels, the number of wires required to go through the vacuum chamber wall (and thus the number of feedthrough pins) can be cut by several times. If the reading and changing speed is much quicker than the frequency of data recording, the multiplexer can be changed in between data recordings to still capture the data from all the sensors attached to it. The multiplexer tested survived the conditions that are encountered in the vacuum chamber, such as extreme temperatures and high vacuum.

The use of multiplexers can reduce the number of feedthroughs and ports required for thermal vacuum testing and, with ever expanding channel counts per multiplexer unit, may allow older vacuum chambers to remain relevant as the demand for sensors during testing is ever expanding.

This work was done by Andrew Kelly and Wesley Johnson of Kennedy Space Center. KSC-13863

NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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