Altitude makes every canning schedule less safe unless it is corrected: water boils at a lower temperature in thinner air, so a water bath needs a longer process, and a pressure canner gauge measures pressure above the surrounding air, so the same reading gives cooler steam. Find the elevation of the place where you do the canning, and take the correction for that elevation from a tested source for your type of gauge, because the correction is not a rule of thumb.
Part of Canning & Pressure Preservation
Altitude adjustment is the lengthening of water bath times and the raising of pressure canner settings that tested canning schedules specify above sea level.
Two Effects With One Cause: Thinner Air
At sea level the atmosphere presses with about 14.7 PSI. Higher up there is less air above you, so ambient pressure falls (about 12.2 PSI at 5,000 ft in the standard atmosphere; recalled). That has two consequences for canning.
Water bath. Water boils when its vapour pressure equals the surrounding pressure, so in thinner air it boils cooler: about 1.8°F lower per 1,000 ft of elevation (recalled). A jar in a “full boil” at 5,000 ft is therefore in water several degrees cooler than at sea level, and it takes longer to deliver the same lethal heat. The remedy is a longer process, taken from a tested table.
Pressure canner. A dial gauge or a weight reads pressure above the surrounding air. The steam temperature depends on the absolute pressure, which is the gauge reading plus the ambient pressure. At sea level, 10 PSI gauge plus about 14.7 PSI ambient gives about 24.7 PSI absolute, near 240°F. At 5,000 ft the same 10 PSI gauge reading adds only about 12.2 PSI ambient, about 22.2 PSI absolute, and steam-table values put the steam about 6°F cooler (an illustration of the size of the effect, not a correction to apply). For low-acid food, where heat alone defeats the spores, a canner at the wrong temperature is not a small error.
So tested schedules for pressure canning raise the gauge pressure with elevation. The numbers in published guidance differ for dial gauges and weighted gauges (as I recall them, with weighted gauges offered in fewer steps), and the parent article gives a single table of PSI by elevation band. Use the parent table only after checking it against a current tested source for your kind of gauge; see Pressure and time requirements. This article does not give corrected values, deliberately: a correction is as much a tested value as a time.
What You Need
- The elevation of your canning site, from a survey, map or reliable altimeter reading. Use the elevation where you can, not where the jars will later be eaten.
- A tested source with altitude tables for both the water bath and the pressure canner, for your type of gauge (Gauges, weights, and safety valves)
- A canning log to record the elevation and correction used with each batch
Method: Apply The Correction
Step 1: Establish the elevation and write it on the canning station’s wall.
Step 2: Pick the correct source table: water bath or pressure, dial or weighted.
Step 3: Apply the correction to every batch, every time, including the first. A water bath at altitude gets the longer time; a pressure canner gets the raised pressure setting.
Step 4: Do not combine corrections. If you correct the pressure, you do not also lengthen the time unless the tested table says so.
Step 5: Log the elevation, the gauge type, the pressure and the time.
Common Mistakes
- Ignoring altitude because “it’s only a little higher”. The effect grows with elevation, and the margin in a low-acid schedule is thin.
- Using a correction table written for the wrong gauge type.
- Taking the correction from a rule of thumb. The parent article’s water bath rule of 5 extra minutes per 3,000 ft is one such rule; check it against the tested source.
- Using the settlement’s elevation when canning at a lower valley site.
Safety
Under-processing at altitude is silent
A jar processed with the sea-level setting at altitude will look, seal and taste exactly like one processed correctly. For low-acid food the consequence is botulism. If you cannot determine the elevation or cannot find a tested correction for it, preserve the low-acid food by another method instead.
What’s Next
- Learn next: Pressure and time requirements — why a correction is a tested value and not a calculation
- Combine with: Water Bath Canning (High-Acid) — where the longer water bath times apply