In Illinois we are above sea level by about 1,000 feet, so we will need to process at 11 pounds per square inch by dial gauge (PSIG) to get the internal temperature of the canner up to 240 F. If we find that at 11 pounds pressure the maximum temperature thermometer reads 238 F, our dial gauge is reading high by 1 pound--we are actually processing at 10 pounds pressure. To get the temperature up to 240 F, we need 1 more pound of pressure, so we adjust our processing pressure up to 12 pounds (11 lb present + 1 lb needed = 12.
Take the temperature inside your canner at several pressures, compare the temperatures with the temperatures in the table above to see how close to accurate your canner is. If you have questions about testing a dial gauge, call your local Cooperative Extension Service office.
A gauge which is inaccurate by 1 pound, high or low, should be replaced. For every 1 pound of pressure the gauge is "off", it will be off by 2-3 F. If the maximum temperature thermometer reads 2 F high or low (at the corresponding pressure), replace the gauge.
