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Water Pasteurization Indicator

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The WAPI on right shows that pasteurization temperatures have been reached since the wax has melted and moved to the bottom of the tube. Solar Cookers International sells the WAPI online here.

A Water Pasteurization Indicator (WAPI) is a simple thermometer that indicates when water has reached pasteurization temperature and is safe to drink. It was invented by Fred Barrett and Dale Andreatta.

Pasteurization destroys all microorganisms that cause diseases from drinking contaminated water and milk. A small polycarbonate tube contains a wax that melts when water or milk is heated enough to be pasteurized (65˚C/149˚F). This saves much fuel by eliminating the need to boil water or milk to ensure that the pasteurization temperature has been reached. See also Instructions for building WAPIs.

WAPI diagram

Since water pasteurizes at temperatures well below the boiling point of water, WAPIs save time when solar pasteurizing, and save fuel when using traditional fuels.

Articles in the media

Buying WAPIs

New designs explored

New WAPI designs 1

End plugs are hammered in, eliminating the melting step

The first prototype, from current WAPI makers North Star Devices, is similar in design to current WAPIs in that it is comprised of a wax-filled polycarbonate tube strung with a fishing line and held low in the water with a metal washer. The current WAPI, however, requires that the ends of the tube be melted and pinched - which can be a difficult and slow process - while the new design does not require any heating. Instead, tight-fitting plugs, pre-drilled for fishing line, are hammered in to each end of the tube, sealing in the wax.

New WAPI designs 2

The assembled WAPI, strung with fishing line and washers

The second prototype was designed by a group of students at Ohio State University (USA) using wax-filled glass capsules from Solar Solutions, makers of the AquaPak solar pasteurizer.

Under the direction of Prof. Dennis Guenther, these students - Arif Mohsin, Zaidi Ahmad Latiffi, Khairil Azim Khairuddin, William Hunter, Shimpei Narita, and Muhammad Yusoff Che-Lah - developed a unique spring and rod system to hold the capsule upright in the water and keep it near the bottom of the pot or jar. The middle seven coils of stainless steel spring, with an inside diameter slightly smaller than the outside diameter of the tube, firmly holds the capsule in place. The three coils on each end of the spring are bent perpendicular to the capsule in such a way that a stainless steel rod can be slid through them with a tight fit. The capsule can then be slid up or down the rod as needed. [For further information see Final report on possible improvements to the WAPI.]

Rotary Youth Leadership Awards youth Southern CA 2009

Rotary Youth Leadership Awards youth making WAPIs in Southern California for use in projects around the world.

Information from Robert Nepper on the construction of the new Cool WAPI

Regarding the new Cool WAPI, we call it “cool” since it requires no heat for assembly when compared to the earlier version WAPI. We have re-designed the WAPI in response to customers who wanted it capable of being assembled in the country of use. We offer it in the completely assembled version and in the totally unassembled version, complete with all parts.

Yes, unskilled persons can assemble the new cool WAPI with a minimum of tools and instructions. All that is really needed is a common claw hammer and a heavy, flat rock, a cement block or an anvil. It must be solid and heavy or you will find it too difficult to hammer in the plugs.

To prevent injured fingers, however, we offer a free simple holder set with initial purchases of 100 pieces or more. The holders also prevent the plugs from being driven in too far (obscuring the holes needed for the fish line).

We sell the completed version for $1.25 each and the unassembled version for $.95 each (in lots of 100 or more). There is quite a bit of manual labor in making the plugs and the wax slug so we cannot offer volume discounts at this time. If the volume gets higher, we will make them with more automation, hopefully to be able to reduce the price and offer volume discounts as well. For more information, contact Robert Nepper.

See Instructions for Assembling the Cool WAPI.

WAPIs with stainless steel wire

The Monterey Peninsula Rotary Club now has WAPI kits available in which stainless steel wire replaces the monofilament fishing line; the new WAPI is good for all applications including open flame fires, which had the tendency to melt the traditional fishing line. The kit contans all the components and instructions to build 200 WAPIs. These are reusable and virtually indestructable. The kit costs $85 (about $.42 each) plus shipping. If you are interested in a fun project that teaches about the perils of unclean water and then shows folks how to make a difference, contact us at This is a great fellowship project for Rotary Clubs, Rotary Interact Clubs, Church youth groups, etc.

A jig for making WAPIs

WAPI jig

A jig is shown for crimping WAPIs. It works quite well and is simple to use. One jaw of the clamp is secured to the plywood so you just turn the one handle to crimp the end. The little wood holder can be moved so when you are crimping it, you can keep it centered on the clamp.

The SPADE WAPI for bottles

Spade WAPI


Another technique that has developed from seeking a simple solution to water pasteurization is the SODIS approach. Basically it involves leaving sealed transparent bottles of clear, but untreated, water in direct sunshine from two to four hours. The time will depend on the surfaces below the bottles, and the intensity of the sunshine. Often the bottles are placed on a flat corrugated metal roof. When the water reaches 65°C(150°F) it is suitable for drinking. As mentioned earlier, a need developed to find a simple device, a WAPI, to indicate when the water was safe. Several designs use a melting wax method. A recent version, called the SPADE WAPI, is designed to be fitted directly to the cap on a water bottle. After drilling a 1/4" hole through the cap. The slender clear tube, with wax at one end, is submerged into the bottle. Reaching a safe temperature, the wax runs to the bottom of the tube. A compact approach to providing water pasteurization using existing bottles.

Audio and video


Disposible soda straw variation

Soda straw WAPI

Straw Pasteurizer Indicator - (see above) A disposable pasteurizer indicator invented by Eric Marlow that can be made by taking a plastic soda straw and folding it back on itself and applying hot glue. More information...

See also

External links

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