A pressure canner is controlled by a dial gauge, which reads pressure but drifts with age and must be checked against a reference, a weighted regulator, which holds pressure at a value set by its mass and the area of its vent opening, and a safety valve, which vents steam if pressure climbs too high. The first two decide whether food reaches sterilising temperature and the third prevents rupture, so a canner with an unverified gauge, a replacement weight of unknown mass or a blocked relief device must not be used for low-acid food.
Part of Canning & Pressure Preservation
The pressure controls of a home canner are a dial gauge, a weight-and-vent regulator and an overpressure relief device, each reading or limiting the steam pressure above the surrounding air.
Three Devices, Three Different Jobs
The dial gauge is a measuring instrument. Inside it a curved, flattened metal tube straightens slightly as pressure rises and moves a needle (Pressure Gauge describes the Bourdon tube). It reads pressure above the surrounding air, called gauge pressure. Its weakness is drift: the tube fatigues, the linkage wears, and a dropped or overpressured gauge can read wrongly while the needle still moves smoothly. The cook can only manage what the gauge shows, so a gauge that reads low causes the cook to run the canner hotter than needed, and one that reads high causes under-processing. Under-processing is the dangerous direction: the dial says the canner has reached the schedule’s pressure when it has not, and the steam is cooler than the schedule assumed.
The weighted regulator (a “jiggler” or a weight that sits on the vent pipe) is not a measuring instrument. It is a pressure limiter. Steam pushes up on the weight over the area of the vent opening, and when that force exceeds the weight, the weight lifts and releases a little steam. The pressure at which it rocks is fixed by the weight’s mass and the area of the opening. It does not drift the way a dial does, but it fails if the weight is the wrong one, if the vent opening is partly blocked or worn, or if the seat is damaged.
The safety valve or overpressure plug is the last defence. It must open only if the pressure exceeds what the vessel is meant to handle, and it must not be the thing you rely on to set the process pressure. The same logic applies to boilers (Safety Valve).
Worked Example: Why A Weight Is Specific To Its Vent
Pressure is force divided by area. A weight of mass m sitting on a vent with bore area A lifts when the steam pressure above ambient reaches
P = m × g / A
as a worked illustration only. Take a 50 g weight on a 3 mm diameter bore:
- Radius is 1.5 mm, so A = π × 1.5² = 7.07 mm² = 7.07 × 10⁻⁶ m².
- Force is 0.050 kg × 9.81 m/s² = 0.49 N (newtons, not kilograms).
- P = 0.49 N ÷ 7.07 × 10⁻⁶ m² = about 69,400 Pa, or about 69 kPa, which is about 10 PSI (1 PSI = 6.895 kPa).
The lesson is not that you should build a weight to this recipe; it is that both mass and bore matter. A weight swapped from another canner, a bore worn larger or smaller, or a weight set on a different vent, gives a different pressure. Proportionally, a weight 10% too heavy gives roughly 10% more pressure. Calibrate any replacement against a known-good gauge before it touches food.
Why Altitude Matters To Both
Both a dial gauge and a weight act on pressure above the surrounding air. At altitude the surrounding air is thinner, so a gauge reading the same number means a lower total steam pressure and a cooler canner. That is the reason the parent article needs an altitude correction in the gauge setting, and Altitude adjustments covers why the correction exists and where to find its values.
Method: Keeping The Controls Honest
Step 1: Test a dial gauge against a reference gauge at several pressures, annually and after any knock. Guidance from tested-canning sources (NCHFP; recalled) retires a gauge reading more than 2 PSI off.
Step 2: Keep each weight with its own canner and mark them. Never improvise a substitute.
Step 3: Clear the vent pipe before every use. Hold the lid up to the light and look through the bore.
Step 4: Check the safety valve or plug before every season. It must move or be intact; replace a hard, cracked rubber plug.
Step 5: Watch the pressure during the process. A weight should rock gently at the schedule’s pressure, a dial should hold steady; neither should drift to either side.
Common Mistakes
- Trusting a gauge that “looks right”. The needle sits at zero when cold, but that says nothing about accuracy at pressure.
- Using a gauge reading without correcting for altitude. See above.
- Replacing a lost weight with a washer or a bolt.
- Letting the safety valve substitute for the regulator. If the safety device is working, something else has failed.
Safety
A blocked vent plus a failed relief device is a rupture waiting to happen
If the vent pipe is blocked and the relief device is stuck, the only thing left is the metal of the vessel. Never operate a canner whose vent or relief device is blocked, never pin down or weight a relief valve, and never leave the canner unattended at pressure. If a safety plug blows, the pressure came from somewhere: stop, cool the canner, and find the cause before using it again.
What’s Next
- Learn next: Altitude adjustments — why the same gauge reading is not the same temperature everywhere
- Combine with: Salvaging pressure canners — the other checks a found canner needs