Last updated: 11 December 2016

The Boil 3, Add 1 Method is a method of water disinfection proposed by Tom Sponheim that provides the fuel savings of pasteurization without the need for a thermometer to determine if pasteurization temperature has been reached. This is achieved by replacing temperature measurements with simple volume measurements that anyone can perform.
Please note: This method needs to be validated by testing in the field.
The problem
Everybody knows that water contaminated by pathogens can be made safe to drink by boiling it. Solar Cookers International cofounder Dr. Bob Metcalf, a microbiologist, confirmed in an article in APPLIED AND ENVIRONMENTAL MICROBIOLOGY (Feb. 1984) [1] that water could be made safe to drink using much lower pasteurization temperatures (65 °C or 150 °F), thus saving fuel.
Even though the fuel savings of pasteurization make it preferable to boiling, boiling is still commonly recommended because the rising bubbles are a clear indication that boiling temperatures have been reached. In order to achieve the fuel savings of the lower pasteurization temperatures however, a thermometer has been needed. The advantage of the Boil 3, Add 1 Method is that these fuel savings can be achieved without the use of a thermometer. Thus this method it can be spread as information, which is much easier to propagate than hardwarebased solutions.
Testing the worst case
A worst case test at sea level shows that adding 1 measure of water at 1 °C (33.8 °F) (the coldest temperature where water is still liquid) to 3 measures of boiling water resulted in 4 measures of water at 78 °C (172.4 °F), which is well above the temperature where water is made safe to drink instantly. Mathematics predicts that the resultant temperature should have been 75.25 °C (167.45 °F), but the residual heat in the pot may have contributed to the higher experimental temperature. Calculations show that the Boil 3, Add 1 Method will always produce safe water—65 °C (149 °F) or above—up to an elevation of 4000 mt (13123.3 ft) elevation.
Using the Boil 3, Add 1 Method
 Select a container appreciably smaller than your cooking pot to use to measure the water.
 Pour 3 containers full (3 measures) of water into the cooking pot.
 Heat the water to boiling.
 As soon as the water starts to boil, add 1 measure of cold water to the boiling water and take the pot off of the stove. (The stove can be used immediately to boil another pot of water.)
 Stir the water to mix the hot and cold water together.
 If you plan to use the measuring cup to drink the water, pour some of the heated water into this cup immediately to pasteurize it too.
 Let the water cool before drinking since it will still be hot enough to cause very bad burns.
Low altitude variation: Boil 5, Add 2
The normal "Boil 3, Add 1" method works at any altitude up to 4000 mt (13123.3 ft) (using worstcase assumptions) since it takes into account the fact that water boils at a lower temperature at higher altitudes. At sea level, however, one can mix the water in a proportion of 5:2 instead of 3:1 and arrive at a temperature of 71.7 °C (161.06 °F). This variation allows for more water to be pasteurized at sea level."
Disadvantages
 People using this method must be taught not to use the measuring cup as a drinking cup after the pot of water has cooled down.
 This method will not work unattended as does the Water Pasteurization Indicator.
Topics for research
 The above method was designed to work even if the colder water is close to freezing and the boiling temperature is only 86.4 °C (187.52 °F), as it is at an altitude of 4000 mt (13123.3 ft). Design for the worstcase scenario is necessary for this method to work 100% of the time and to be easily communicated with others. However, humans are able to determine when water is lukewarm (body temperature) within a few degrees(as yet to be tested). This lukewarm water could be added in a proportion of 1:1 with boiling water and pasteurization might still be achieved (mathematics shows that the resulting temperature at sea level would be 68.5 °C (155.3 °F). At 5000 mt (16404.2 ft) a ratio of 2:1 would result in water at 67.5 °C (153.5 °F).
 Might it be possible to combine this method with heatretention cooking in that a great amount of cold water could be added if the pot was then placed into an insulated bag that would hold the water at this lower temperature for a longer time?
External links
 Excel spreadsheet for calculating the resulting temperature when mixing quantities of water a different temperatures