The temperature test, and your altitude changes it
Updated
The published table
The physics is simple enough to carry in your head. Water boils when its vapor pressure matches the air pressing down on it, and the higher you are the less air there is. A sugar solution boils above water by an amount that depends on how concentrated it has become, so the real test is a fixed number of degrees above where water boils for you. 220F is only what that works out to at the coast. The table is the same arithmetic, already done.
| Altitude | Target temperature |
|---|---|
| Sea level | 220F |
| 1,000 ft | 218F |
| 2,000 ft | 216F |
| 3,000 ft | 214F |
| 4,000 ft | 212F |
| 5,000 ft | 211F |
| 6,000 ft | 209F |
| 7,000 ft | 207F |
| 8,000 ft | 205F |
The steps
Look your elevation up once and write it on the wall above the stove. It is the number most likely to be wrong in a recipe you did not write, and the one nobody thinks to check, because a recipe saying 220F reads like a fact rather than like a measurement somebody took somewhere else.
You need
- A jelly or candy thermometer that reads above 220F
- Your elevation, which is worth looking up once and writing down
The bulb position is the step people get wrong, and it is wrong in a way that reads as success. A bulb touching the base is reading the pan, which is hotter than the jam, so it reaches the target early and you take a batch off the heat before it has set.
- Clip the thermometer to the pan so the bulb is in the liquid and not touching the base.
- Bring the mixture to a full rolling boil.
- Read the target for your altitude off the table above.
- Take the pan off the heat when it reaches that number.
Checking the thermometer rather than trusting it
Do this once at the start of the season and once more if the thermometer is ever dropped. It takes the time one pan of water takes to boil, and it is the only way to know whether the number you are about to trust all evening is the temperature of your jam or an artefact of a bent stem.
Boil plain water and read it. It should read the boiling point for your altitude, and whatever it reads instead is the offset to apply all evening. An uncalibrated thermometer gives a confident, precise, wrong number, which is worse than not having one.