This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Dial gauges must be tested *every* year before canning season
[Hey! Maybe near the time of daylight saving; you're changing your
clock and checking your smoke alarm anyway.--LEB], and sometime
throughout the canning season, depending on the amount of use. This
gauge should also be tested/retested if the lid was dropped, because
a sharp jolt can cause a dial gauge to lose its calibration.
Even if you buy a brand-spanking-new dial gauge pressure canner, you
*must still* test the gauge. I've found that nearly 50% of new dial gauges
have gross errors on the minus scale (i.e. inside doesn't get as hot as the
dial gauge would lead you to believe).
[ Dial gauge are required at elevation in excess of 10,000 ft.as the weight
of a deadweight canner is insufficient to generate the pressure needed to
Weights are considered foolproof. A few folks have reported seepage from
jars when using dead-weight type canner. Jars lids must be clean and tightened
properly before processing. REDUCE the heat to the minimun required to keep
the weight rocking gently. Any more heat than this and the jars will be
over-pressurized in relation to the pressure inside the pot - seepage will
result. Opening a canner or inducing a sudden temperature drop will cause
a pressure drop - seepage will result.
Do not over-pressure ANY canner, NEVER douse a canner with cold water, and
allow the canner to cool to 0 pressure before opening the canner. There should
be no seepage - period. Seepage is a sign of an imperfect seal caused by
improper procedure or faulty equipment. --ED]