Ice water is not the same as a proper ice bath. If you put ice in a glass of water its about 40F.
A chemist friend wrote this for me:
The water/ice must be pure, as in distilled.
Use slush (crushed ice) with the water level slightly below the ice. When determining melting point remember that water is densest at 4C so the ice/water mixture must be continuously well stirred or the water at the bottom will be 4C and the rest of it somewhere between 0C and 4C (magnetic stirrer works best). A steady well-stirred temperature is 0.0C.
The good people at Thermoworks have a video.http://www.thermoworks.com/blog/2010...oper-ice-bath/
Boiling point depends on pressure. 212F is for absolute barometric pressure of 29.92" of mercury at sea level. It decreases 1.8F per thousand feet elevation. For example water boils at 211F at standard pressure at 555 feet.
Most thermometer errors for homebrewers are caused by electronic thermometers that got wet. If your calibration shifts by more than a degree, be very wary.
I recommend everybody get a glass analog thermometer to use as a reference. I got mine at the LHBS for eight bucks.