Pour water into the measuring cylinder up to the one-liter mark, then transfer it to another cylinder made of an insulating material such as plastic to prevent excessive loss of heat to the surrounding environment, which affects the measurement accuracy. Take the temperature of the water with the thermometer and note it. It is likely to be at room temperature, ranging from 20 to 25 degrees Celsius.
Plug the heater into a power outlet and then immerse the heating element into the water in the cylinder, making sure that the water level is not above the marked point on the heater. Immerse the thermometer into the water.
Use a digital timer for accuracy. Simultaneously turn on the heater and start the timer, while observing the thermometer reading.
Continue heating the water till the thermometer reading is 90 degrees Celsius. Stop the timer and note the reading on it. Repeat the same procedure on water drawn from the same source as the first to make sure all the tests start off at the same temperature, but this time transferring the heater into the cold water while the heater is still hot. For each subsequent time that you repeat the test, the time taken for the water to reach the 90 degree mark will be lower than the first because the heater starts out hot.
Repeat these steps four or more times, then take all the readings with the exception of the first and add them together, then divide the total sum by the number of readings added. The quotient is the average reheat time for the immersion heater for one liter of water at room temperature.