Owemon,

So you should calibrate your thermometer every day. If you don't have boiling pure water handy, then calibrate it with the sap as soon as it starts to boil. Whatever number that says, add 7F-7.5F. +7.1F is 66 brix, +7.5F is 67 brix. So, if your thermometer says boiling is 220F, then 227.5 is 67 brix. IOWs, even if your thermo is horribly wrong, it can still work fine.

Cheers,
Russ