Why are the steam and ice points of water better fixed points for a thermometer than the temperature of a human body?

The temperature of the human body changes with each person. The 98.6 F we quote for the "normal" temperature of a person is an average but varies from individual to individual. The steam and ice points of water, however, assuming everything is kept the same, don't vary.

The steam and ice points of water are better fixed points for a thermometer than the temperature of a human body for several reasons:

1. Consistency: The steam and ice points of water have well-defined and consistent temperatures under normal atmospheric pressure, making them reliable reference points. The steam point is set at 100 degrees Celsius (212 degrees Fahrenheit) and the ice point at 0 degrees Celsius (32 degrees Fahrenheit). In contrast, body temperature can vary between individuals and can be influenced by various factors such as health, activity level, and external environment.

2. Reproducibility: The steam and ice points can be easily reproduced in controlled laboratory conditions, ensuring accurate calibration and calibration checks of thermometers. On the other hand, precise body temperature measurement requires specific techniques, such as using accurate thermometers, proper placement, and considering factors like ambient temperature and measurement location.

3. Universal standard: The steam and ice point values of water are widely accepted as international standards. They provide a common reference for calibrating thermometers used across different scientific fields and industries, allowing for consistent and comparable measurements. The temperature of a human body, however, is subjective to individual variations and may not be suitable as a universal standard.

To measure the temperature of a human body accurately, a reliable thermometer specifically designed for medical purposes, such as a digital thermometer, is used. The thermometer should be placed under the tongue, in the armpit, or rectally for the most accurate readings. However, it is important to note that body temperature can still have slight variations depending on the measurement technique and location.