Tim,
some pipings can not be pressure tested with water. Examples:
1. Instrument quality air. Instruments are not pneumatic any more, but instrument quality air, with dewpoint that may be as low as minus 40 degrees, is still used for many purposes.
2. Dry chlorine.
3. Carbon dioxide.
etc.
In those cases, and I ran into many of them back in my days of erector engineer, you should use a dry gas (i.e., with no moisture) as in N5UZT's case.
Giovanni S. Crisi