High-frequency pulse tubes can't always be tipped

High frequency (>30 Hz) 'pulse tube' coolers are often observed to perform well even when the 'pulse tube,' or thermal buffer tube, is in an orientation other than cold-end down, counter to the intuition that a column of gas with the colder, denser region above the warmer region is not convectively stable. In a recent paper, Swift and Backhaus advance a theory to explain this phenomenon, and offer some guidelines for a 'tip-safe' design. The present work offers some tipped vs. vertical data for a number of different sized pulse tube coolers, and compares the results to the theory of Swift and Backhaus. In general it is found that the results are in qualitative agreement with their theory, but that the 'safety factor' in a pulse tube design must be several times higher than their work suggests, to ensure orientation independence.