Canning - How To Vent The Canner
To vent a canner, leave the vent port uncovered on newer models or manually open petcocks on some older models. Heating the filled canner with the lid locked into place boils water and generates steam that escapes through the petcock or vent port. When steam first escapes, set a timer for 10 minutes. After venting 10 minutes, close the petcock or place the counterweight or weighted gauge over the vent port to pressurize the canner. Air trapped in a canner lowers the temperature obtained at 5,10, or 15 pounds of pressure and results in under processing. To be safe, all types of pressure canners must be vented 10 minutes before they are pressurized.