This unit of temperature is still used customarily in the United States.
Definition: 0° is the coldest temperature achieved by using an ice and salt mixture, and 100° is set at the temperature of the human body. On this scale, the freezing point of water turned out to be about 32 °F and the boiling point about 212 °F.
1 °F equals 5/9 °C. To convert a temperature in °F to the Celsius scale, first subtract 32° and then multiply by 5/9. In the other direction, to convert a temperature in °C to the Fahrenheit scale, multiply by 9/5 and then add 32. The unit was defined by the German physicist Fahrenheit.

See also Thermal Units Per Hour.

(K) The SI unit of temperature.
Definition: One Kelvin is 1/273.16 of the difference between the triple point of water (at exactly 273.16 K) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states. Absolute zero is the temperature at which all molecular motion discontinues. 0 K is according to -273.15° Celsius and -459° Fahrenheit.

(BTU/Hr) BTU/Hr is a common measure of heat transfer rate. Capacities of furnaces and boilers are for example expressed in thermal units per hour (British unit). One BTU/Hr is the amount of heat required to heat one pound of water one degree Fahrenheit per hour.
BTU/Hr is the unit that expresses how much the ultrasound machine heats up the surrounding area.

See also Thermal Effect, Ultrasound Physics.