The main purpose of solar cookers and ovens is to change sunlight into heat which is then used to cook foods.
Water too can be heated for beverages, dishwashing or personal hygiene.
They can also be used for water pasteurization.
Worldwide, about 30 million people die each year from drinking polluted water. In underdeveloped areas the pollution is usually in the form of bacteria and viruses from fecal matter.
It was once thought necessary to boil water to kill waterborne pathogens. In 1984, Dr. Robert Metcalf of the University of California, Sacramento showed that if water is held at 150ºF for at least five minutes, all pathogenic microbes would be inactivated.
To indicate when this 150ºF temperature was reached, without ready access to a thermometer, the Water Purification Indicator or WAPI was born.
The WAPI is a clear plastic tube sealed at both ends, containing a daub of specially formulated wax that melts at about 150°F (65°C). With the solid wax at the top end of the tube, the WAPI is placed in container of water that is solar heated. When the wax melts and falls to the bottom of the tube and the heat maintained for at least five minutes, it ensures that water pasteurization conditions have been reached.
To use again, the WAPI is simply turned over with the wax at the top end and the process is repeated.
Most manufacturers of solar cookers and ovens supply one with each device. Many thousand are now in use, worldwide.
North Star Devices manufactures three variations of the WAPI. The SUPERWAPI suspends the tube in the water container by means of a stainless-steel cable, the SPORTSWAPI utilises a magnet allowing the tube to “float” submerged, and the SPORTSWAPI II uses a coupling to do the same.