Since blood production requires iron, you should eat iron-rich foods for two weeks before your appointment. This will help you have strong blood for donation and help you better recuperate after your donation. Iron-rich foods include spinach, whole grains, fish, poultry, beans, organ meat, eggs, and beef.
To calculate the change in temperature, subtract the initial temperature from the final temperature. Example: 84.2 degree Fahrenheit - 41 degrees Fahrenheit = 43.2 degrees Fahrenheit. The temperature from morning to afternoon changed by 43.2 degrees Fahrenheit.
The other two are both known as the Mean Temperature Method. They approximate the degree days for each day using the average temperature for the day. For HDD they subtract the average temperature from the base temperature (taking zero if the average temperature is greater than the base temperature on that day).