Angus, The HP 5065A has a fairly open path from atmosphere to the cells as far a barometric pressure is concerned. Response is rapid to a change. The mechanism is "oil-canning" of the windows on the ends of the cell. Depends on diameter of the cell, thickness of the windows, and stiffness of the particular glass used. (for a particular gas mix of course) It is VERY repeatable, and VERY linear. I have heard that one European observatory routinely tracks the barometric pressure by watching their 5065A shift. After seeing Wulf's data I can believe it! My circuit is eliminating that variable quite nicely. As others have said, temperature will be a different matter. I'm currently doing temperature tests and plan to eliminate the major causes rather than compensate. Early measurements show as-built to be better than the spec of 1.6X10-12/degree C. Cheers, Corby
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