From "The Complete Guide to Home Canning"
To determine
the elevation at your location click on the
US Geological Survey elevation search tool
http://geonames.usgs.gov
Enter city name, state, and county
Underprocessing can occur if you use process times intended
for canning at sea level. If you live at
altitudes of 1,000 feet or higher, you must adjust your process times according to the tables in the USDA Guide.
Why? As your elevation above sea level increases, water boils at a lower temperature (see table). Lower boiling temperatures are less effective for killing bacteria. Increasing the process time or canner pressure compensates for lower boiling temperatures. Therefore, when you use the guides, select the proper processing time or canner pressure for the altitude where you live.
| Pounds
of Pressure |
Boiling
point of water (F) |
Boiling
point of water (C) |
Process
time |
Process
Time Weighted Gauge (min) |
| Sea
Level |
212.0 |
100.0 |
10 |
10 |
1,000
ft |
210.2 |
99.01 |
10
1/2 |
15 |
2,000
ft |
208.5 |
98.03 |
11
|
15 |
3,000
ft |
206.7
|
97.05 |
11
1/2 |
15 |
4,000
ft |
204.9
|
96.08 |
12 |
15 |
5,000
ft |
203.2 |
95.12 |
12
1/2 |
15 |
6,000
ft |
201.5
|
94.16 |
13 |
15 |
7,000
ft |
199.8
|
93.21 |
13
1/2 |
15 |
8,000
ft |
198.1
|
92.26 |
14 |
15 |
9,000
ft |
196.4
|
91.32 |
14
1/2 |
15 |
10,000
ft |
194.7
|
90.39 |
15 |
15 |
- Actual boiling points are based on barometric pressure which is a function of altitude and changes with the weather conditions each day.
- To determine your real-time barometric pressure and the exact boiling point of water in your location visit http://www.primogrill.com/boiling.htm
To compute the air
pressure (p) for each altitude, the formula used is
p = poe-ay
where
p = pressure in Pascals (Pa)
po = pressure at sea level = 1.013x105 Pa
a = 1.16x10-4 m-1
y = altitude (m)
(see Halliday & Resnick, Physics pp. 374-375, John Wiley & Sons,
Inc. (1978))
This formula assumes temperature and acceleration of gravity are constant
over the range of altitudes. T = 20 °C, and g = 9.8 m/s2.
