© 2009
Custom Sensor
Solutions, Inc.
A Cheap and Simple Gas Sensor
Demonstration for Home or Classroom

...by our teaching staff

A common zinc-air hearing-aid battery from your corner drugstore can be used as a simple oxygen sensor. In this document, we will describe an experiment to measure the oxygen consumption by a chemical hand-warmer used by hunters and hikers. Only a few inexpensive materials are needed to carry out these experiments.

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DISCLAIMER: This document describes a simple oxygen sensor. It is intended for demonstration or educational purposes only. An oxygen detector using the zinc-air battery as a sensor must not be used for applications involving the safety of human or animal life, nor should it be used in industrial processes or to determine the composition of flammable or explosive mixtures.

This demonstration assumes elementary skills in the use of tools and soldering equipment.

The World's Cheapest Oxygen Sensor

The zinc-air battery is commonly used in hearing aids, where weight and low cost are important. Most batteries carry both the oxidant and reductant in one package. The zinc-air battery carries only the reductant, as a lump of zinc metal. The oxidant is oxygen, supplied by the ambient air. The zinc lump is the cathode. The anode is a screen of nickel wire coated with a mixture of carbon and manganese oxides, which catalyze the reduction of oxygen to water. These are immersed in an alkaline electolyte.

Normally, the Zn/air battery is inert and can be stored for several months unused. A strip of tape prevents air from entering the battery. If the tape is removed, air diffuses into the cell through tiny holes, and an "open-circuit" potential of about 1.4 volts develops across the electrodes. This voltage prevents the redox reaction from proceeding. But when current is drawn from the cell, the chemical reactions will attempt to maintain the open-circuit voltage. Zinc will be consumed at the cathode:

Zn0(metal) ---> Zn++ + 2 e-1

and oxygen at the anode:

O2(gas) + 2 H2O + 4 e-1 ---> 4 OH-

In its intended application (a hearing aid), only a tiny current is drawn and the reaction proceeds very slowly. As long as some oxygen is around, the battery can function at full voltage.

On the other hand, if the battery is short-circuited, unlimited current can flow and the reaction will proceed as quickly as it can. After a while, the current stabilizes at its "short-circuit"current, which is about 20 milliamperes for a Type 675 cell. This current is determined by the rate at which oxygen can diffuse into the cell. If the oxygen around the cell increases or decreases, more or less oxygen will diffuse to the anode, and the output current will change accordingly.

In our experiment, we connect a low-value small resistor across a Zn/air cell, small enough to not limit the current, and large enough to allow us to measure a voltage drop. This is how we determine the "short-circuit" current from the cell.

Sensor circuit
Parts and Tools Needed

    * zinc-air batteries, Type 675 (Radio Shack)

    * Keystone Type 501 battery holder for 5 mm thick x 12 mm diameter cell
            (Newark Electronics P/N 95F3285)

    * resistor, 18 to 25 ohms, 1/2 watt or greater

    * small piece of perforated electronic prototyping board 1" x 1"

    * digital voltmeter, capable of measuring 10 millivolts, with clip-type electodes

    * stopwatch or timer

    * transparent 1- or 2-gallon Zip-Lock plastic bag

    * disposable chemical hand-warmers
             (Most sports and outdoor stores -- MUST be of the type
             containing iron powder as one of the ingredients)

    * soldering iron and solder

    * transparent tape

Notes on the parts list:

1.    Zn/air batteries are thicker than other types of coin cell, and it is difficult to find a battery holder which will fit one of the available battery types. The Type 675 battery and the Keystone Type 501 battery holder are a perfect match. However, the battery holder may prove difficult to track down. Ordering a $2.00 part from Newark Electronics is not a cost-effective strategy, as they have a minimum order and a shipping charge. Other supply or surplus houses may be easier to buy from.

    It would be nice to eliminate this troublesome part, but we have not found a good way. Soldering to a caustic-containing battery is not wise; it is also very likely that the solder will not stick. Taping wires to front and back is frustrating; they will invariably fall off. Conductive epoxy cement may work, but it is more expensive and troublesome than buying the battery holder.

Note: Custom Sensor Solutions will consider putting together a kit of the hard-to-get parts if there is enough expressed interest. Please email us if you would be interested in buying such a kit. We would strive to keep the price under $15.

2.    The chemical hand-warmers are a fall and winter item at K-Mart, and usually disappear from the shelves during the fish-and-golf season. On the other hand, I have had no trouble in buying them by asking the clerk to get them from the back. The first ingredient should be iron powder. The supersaturated-salt types, containing sodium acetate or sodium thiosulfate, will not work for this purpose.

3.    If you have a chart recorder or electronic datalogger, this is much preferable to the use of a voltmeter. Set the input range for 0-1 V or 0-2 V, and the speed to about 4 inch/hour.

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