Water Boiling Test "How-to"
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There are a number of different approaches to carrying out the Water Boiling Test. These include : - The Water Boiling Test based on VITA 1985 - the 'International Standards' - described below.
- The Indian standard WBT procedures developed by the Bureau of Indian Standards, for measuring the efficiency of the cookstoves.
- The Chinese standard WBT procedures developed by the State Standards Organization of the People's Republic of China - "Testing Method for the Heat Properties of Civil Firewood Stoves".
- The Comparative Water Boiling Test is an adaptation of the test method and procedure of the international standard Water Boiling Test (WBT), modified to account for the real customs and habits of cooking in Cambodia. While designed for Cambodia, the test, developed by Jean-François Rozis, can be adapted for use in other countries.
The VITA WBT - 1985
Equipment required - Pot with lid
- A balance accurate to 10 grams with a recommended capacity of 5 kg
- Locally available wood, charcoal or other fuels, air dried preferably 2 to 3 cm diameter
- Water, within 2°C of ambient temperature
- Equipment for removing and weighing hot coals
- Scale (accuracy and range?)
- Mercury or digital thermometer for measuring temperatures up to 105°C
- Stopwatch (timer)
- Forms for recording data and calculations (idea: prepare one and attach for people to download - perhaps an Excel spreadsheet?)
Optional extras - Device to measure/estimate the moisture content of fuel. It is not necessary to measure moisture content of charcoal under normal circumstances
Method Upon heating, biofuels are converted into a mixture of volatiles and carbonaceous char which burn with entirely different characteristics - see here for a more detailed description.
Within a stove, during burning, the proportions of wood, pyrolising solid, and char are unknown. Efficiency can only be determined in the Water Boiling Test by separating the wood and char and by measuring the proportions of each, and hence calculating the energy released. To do this without disturbing the fire is impossible.
The second problem involved in measuring the efficiency of biofuel stoves is caused by the stove. Since all the fuel is inside the stove it is usually required that the fuel be removed from the firebox, measured and replaced. Combined with the added problem of separating the unburned wood and the char this presents major practical problems.
A common solution is to place the entire stove on a scale (Baldwin 1987:83). Any wood in the firebox is removed, the stove is weighed with the charcoal, and then the wood returned.
Either way, this means operating the stove in a way that does not entirely reflect rural practice.
The following procedure may be used (needing extensive editing and reality check): - Weigh a quantity of wood - more than will be needed for the test.
- Weigh the pot
- Fill the pot to 2/3 of capacity with room temperature water.
- Weigh the pot and water (to calculate how much water) at the start
- Record the temperature of the water
- Start the fire at high power to bring the water in the main pot (if there are more than one) to a boil.
- Note the time and temperature
- Remove all wood from the stove, knock off any charcoal and weigh it together with the unused wood, record the water temperature from each pot, weigh each pot, including water and lid, and return charcoal, burning wood, and pots to the stove to begin the "low-power" phase of the test.
- Continue the test at a low power, so that the temperature of the water stays within 2 °C of boiling.
- Continue for 30 minutes using the least amount of wood possible.
- Weigh, and record separately the charcoal and the wood
- Weigh and record the remaining water in each pot.
Calculations The equivalent mass of fuel burned, on an energy basis (with reference to the unburned wood), can then be calculated in the following manner:
The fire power (the ratio of the energy content of the fuel consumed during a test or phase to the duration of the test or phase) is calculated as follows:
Criticism of the Water Boiling Test According to the Eindhoven Woodburning Stove Group (as described on http://www.cookstove.net/others/fuel-economy.html ), the Water Boiling Test has the following weaknesses.
- Recovery of the charcoal is not sharply defined and is therefore operator dependent.
- The charcoal weights are small and measurement of such small weights is always subject to large errors.
- Estimation of the combustion value of charcoal, produced in a wood fire is at best a tricky task.
- A lot of things must be done in as short a time as possible between the high-power and the low-power phase, so the risk of making mistakes or misreadings is pretty high.
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Contributors
User:Grant Ballard-Tremeer 25 September 2003
User:Erin Boyd 27 March 2006
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