In recent years, pressure canners for use in the home have been extensively redesigned. Pressure canners before the 1970s were a heavy-walled kettle with a clamp or turn on the lid. They often had a dial gauge, a vent port, and a safety fuse. Today, pressure canners are lightweight and thin-walled with a turn-on lid. They come with a jar rack, gasket, dial or weighted gauge, an automatic vent/cover lock, a vent port with a counterweight or weighted gauge and a safety fuse.
The pressure doesn't destroy microorganisms, but the high temperature used for an adequate period of time will destroy microorganisms. The success is based on the temperature of pure steam, free of air, at sea level. At sea level, pressure canners operated at a pressure of 10.5 pounds will reach an internal temperature of 240 degrees Fahrenheit. There are two main reasons a pressure canner won't obtain proper temperature:
To vent a pressure canner, you need to leave the vent port uncovered on newer models and manually open on older models. Once you notice steam escaping, set a timer for ten minutes. After this ten minutes of venting, close the petcock or place the counterweight or weight gauge over the vent port to begin pressurizing the canner.
If you have a weight-gauge model, they will exhaust tiny amounts of air and steam whenever the gauge rocks or jiggles during processing. This allows for precise control of pressure and doesn't require watching during processing or checking. You will know the canner is working and maintaining pressure by the sound of the weight rocking. The disadvantage of these models is that they can't precisely correct for higher altitudes.
Before each use of your pressure canner, you want to check dial gauges for accuracy. If a gauge reads high, it will result in under-processed and unsafe food. Low readings will result in overprocessing. If a gauge reads up to two pounds higher or lower, then adjustments can be made. Gauges that differ more than two pounds should be replaced.
You should handle canner lid gaskets carefully and clean them based on the manufacturer's directions. Gaskets that are nicked or dry rotted can allow steam to leak during pressurization. Between uses, you want to make sure to keep the gaskets clean. Some older models require a light coat of vegetable oil on the gaskets once a year. Newer model canners have pre-lubricated gaskets that don't need oiling. If you aren't sure about your model, then check the instructions.
Lastly, lid safety fuses are a thin metal insert or rubber plug designed to relieve excessive pressure from the canner. When cleaning the lids don't pick or scratch at the fuses if the unit has Underwriter's Laboratory (UL) approval to ensure safety. You can often get replacement gauges or other parts from stores that carry canning equipment or directly from the manufacturer.
Here is a YouTube Video on how Pressure Canner works