The parts are soldered in place on the perforated board as shown here. This goes very quickly, since the long leads from the resistor can be used to make contact with the leads from the battery holder. In our example, we use a Waldom connector to take the signal from the board, but there is no reason the voltmeter leads cannot be connected directly to the resistor leads.
Connect the voltmeter across the resistor and set the voltage range to 0-1 or 0-3 volts. Disable any autoranging or auto-shutoff features if you can.
The battery is inserted by removing the strip of tape to expose the air holes (usually four). As quickly as possible, slip the battery into the holder with the holes up. Look at the voltmeter. You should see a rapidly-rising voltage, topping out over 1.0 V and then slowly falling to a reasonably stable value around 0.8 V. Record these numbers.
Leave the battery for about an hour. The voltage will decline somewhat, due to the accumulation of reaction products in the cell, but the voltage will soon become relatively constant.
Put the sensor into the plastic Zip-Lok bag, being careful not to puncture the bag. Cut open the outer bag of the hand warmer pack, being careful not to penetrate the porous inner bag. Knead the chemical packet to start the reaction. Immediately, put the hand-warmer in the bag with the sensor. Partly inflate the bag by spreading your hand inside (do not use your breath), and seal up the bag around the voltmeter wires. Use transparent tape to hold the bag shut and to seal around the wires so no air can get in or out.
Over a period of hours, the hand-warmer will consume the oxygen in the bag. The sensor signal will fall, quickly at first, then more gradually. The reason for this is that as the oxygen is consumed,the reaction in the hand-warmer also slows down. Many hours are needed to consume all the oxygen.
If time is of the essence, you might use several hand warmers at one time.
The dramatic part of the experiment comes next. When the signal from the sensor has reached a suitably low level in 4 to 16 hours, open the bag and take the sensor out. The voltage will return rapidly to a high level, generally within 60-90 seconds. It may even overshoot the original voltage, then fall back to a signal close to the original pre-experiment voltage.
It is important to remember that the output of an amperometric sensor, such as this one, is measured as current, rather than voltage. Between the voltage measurement and the resistor value, the current can be calculated by Ohm's Law:
Current in amps = voltage / resistance in ohms = V / R
For the hand-warmer experiment, our results are shown below.
A single Zn/air cell can be used for about two days worth of experiments. It can then be discarded ( do not attempt to recharge these cells). Zn/air cells do not contain ingredients that are harmful to the environment.
There are many experiments that can be performed with this simple sensor system, although it may be necessary to collect some more apparatus to carry them out. The hardware store is a good place to start.
1. Calibration curve. If you can get pure nitrogen or oxygen, you can make accurate dilutions of oxygen in air or nitrogen in air to attain different O2 concentrations. Doing this, you can draw up a curve that can be used in other experiments. Look at our apnote on dilutions
2. Exhaled air. Collect your exhaled air in a plastic bag and measure the oxygen content. Note: Excess carbon dioxide causes more rapid deterioration of the sensor, by dissolving in the alkaline electrolyte. Avoid long or repeated exposures unless you have lots of sensors.
3. Measure oxygen consumed and produced by a plant.
4. Make pure oxygen made with drugstore 3% hydrogen peroxide and a few crystals of copper sulfate.
5. Measure the effect of temperature on the cell response. Temperature can be used to determine if the sensor is operating under pure diffusion control, where the sensor response is proportional to the square root of the Kelvin temperature. If diffusion control is not operating, the temperature curve will be more complex, and probably steeper.