Btu (thermoc.)
The Btu (thermochemical) variant aligns with the thermochemical calorie and is defined as the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure of one atmosphere and is equivalent to approximately 1,054.350 Joules.
Btu (3.9°C)
The Btu (3.9°C) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at the temperature where water reaches its maximum density which occurs at approximately 3.9°C (39°F).