A pressure canner is a kettle made from a material, usually aluminum,
that is strong enough to safely withstand the pressures used in home
canning. The lid is built so it can be locked to the base of the canner.
On one type, metal in the sealing edge is ground smooth so little or no
leakage occurs between the lid and the base. Care should be taken to
avoid damage to the sealing surfaces which could ruin the canner.
Other canners have a gasket made of a rubber-like substance that
prevents leakage of steam. The gasket should be washed in hot suds,
rinsed, and dried thoroughly after use.
All pressure canners include a safety plug or fuse. One type has a metal
fuse that melts when the temperature is too high. If the canner is used
properly the fuse should never need replacing. Another type of canner
has a rubber-like safety plug. Care should be taken to replace the plug
when the rubber gets hard. As the rubber hardens, it takes a greater
pressure for it to blow out. Some older type canners have a petcock that
serves as a vent and safety valve.
In canners that have a pressure gage, vents serve to exhaust air from
the canner. The air is exhausted by venting for 10 minutes after steam
starts escaping. All the air must be exhausted before the canner is
sealed because the steam has much more heat energy. For example, air in
a 212° F oven feels just warm while 212° steam from a teakettle will
burn you.
Be sure to read and follow the instructions with your canner.
All canners should have some type of rack in the bottom. A rack keeps
the jars from touching the bottom of the canner and breaking. It also
aids in transferring the heat more evenly within the canner by
permitting water and steam circulation.
There must be enough water in the canner to provide steam throughout
processing. Two quarts of water is usually recommended, although this
may vary depending on size of the canner and the quantity of jars.
Pressure canners have either a dial gage, a pressure control or a
combination of these. The dial pressure gage indicates the pressure and
corresponding sea level temperature. The control type canner has a
precision weight that sits on the vent pipe and jiggles to regulate
pressure. A third type is a combination gage and control.
The dial pressure gage measures steam pressure. The tube in the pressure
canner gage operates like a New Year's Eve noisemaker, which is a flat
paper tube rolled up. Blowing into the tube causes it to become more
round and unroll.
The pressure gage works the same way except not as dramatically. The
gage is made of a partially flattened metal tube. When pressure is
applied, the tube becomes more round and straightens slightly. The
needle [pointer] moves as the tube straightens. The gage is calibrated
to indicate pressure. Pressure is controlled by adjusting burner heat to
maintain the desired pressure. This type of gage should be checked
yearly or after suspected damage, such as dropping, to be sure it
functions properly.
A pressure control consists of a precision weight that rests on a
specially designed vent pipe. It automatically maintains an even
pressure and temperature inside the canner. Pressure builds inside the
canner until the upward force (steam pressure times seat area) is
greater than the downward force of the weight on the seat area. At this
point, the control weight is lifted, releasing steam and reducing
pressure, until the upward force equals the weight. The pressure inside
again increases slightly, lifts the weight, and releases the pressure.
The repeated lifting and reseating or jiggling of the control weight
indicates that the pressure is being controlled.
The burner is adjusted so the control jiggles at least several times a
minute. Excessive jiggling will deplete the supply of water in the
canner.
There are two types of pressure controls. One type is a single weight
with 3 holes which fit on the vent pipe. The diameter at the base of the
hole (seat area) is different for each of the 3 pressures--largest for 5
pounds pressure, and smallest for 15 pounds pressure.
The second type has 1 seat area and a 3-piece weight. For 5 pounds
pressure, the small center weight is used. One additional ring or weight
is added for 10 pounds pressure, and a second ring or weight is added
for 15 pounds pressure.
With care, the pressure control remains accurate throughout the canner's
lifetime. Be sure that seat areas where the weight and the vent pipe
make contact are not damaged or excessively worn; this could affect the
canner's performance.
The combination gage is not as common as the other two types. It has a
sliding piston which pushes up on a spring. As pressure inside the
canner increases, the piston is pushed up. Rings on the piston indicate
pressure. If the heat is not regulated correctly, pressure builds up to
beyond 15 pounds, at which point the weight is lifted to release the
excess pressure. It will jiggle audibly, indicating to the user that the
pressure is too high. This system serves as a gage as well as a safety
device.
The combination gage should be kept clean and dry when not in use, to
prevent corrosion. It also must be checked yearly to be sure the piston
slides easily and indicates the correct pressure.