When forced through such a large resistance, the tiny signal will generate about 10 microvolts, which a high-impedance operational amplifier, such as the reliable AD795JN, can magnify. Unfortunately, other circuit components do not perform as well, so to achieve the necessary level of precision you will have to take a few precautions.
The circuit board you use to mount the electronic components must be kept clean of surface contamination; otherwise excess current could flow about. Cleanse the board with alcohol and thereafter wash your hands thoroughly with soap before touching it.
Also, tiny currents can pass directly through the smidgen of circuit board that separates the operational amplifier's pin 4 (connected to the power supply) from pin 3 (the input signal). Because these currents can swamp a delicate signal, bend up pin 3 to keep it from touching the circuit board. You will need to connect the sensor wire from the 10-gigaohm resistor directly to this pin. Finally, to prevent any stray electric fields from forming between the aluminum housing of the device and the tube, connect the two electrically.
For your ion counter to work properly, you must ground the brass sensor plate through the 10-gigaohm resistor. Solder four pieces of uninsulated bus wire (20 gauge or thicker), one to each of the four corners of the plate. Then drill four oversize holes in the top surface of the tube so you can pass the wires through them. Secure three of the wires well above the conductive surface of the tube by encasing them inside a mound of glue from a hot glue gun [see illustration above]. The glue is a surprisingly good electrical insulator, but if any of the adhesive comes in contact with the metal on the inside surface of the tube, enough current will flow to destroy your measurements. So be careful to keep it well away from the holes. Next, solder one end of the 10-gigaohm resistor to the top surface of the tube and the other end to the remaining fourth wire. Also solder the signal wire to that junction and secure it onto the tube's top surface with hot glue, as shown in the illustration. Last, attach the other end of the signal wire to pin 3 of the operational amplifier.
When you are ready to use your device, first block the opening so that no air gets through and adjust the potentiometer in the circuit until your voltmeter reads zero. Then turn the fan on and let indoor air be drawn into the instrument. Your voltmeter should indicate about 0.2 millivolt, which corresponds to roughly 200 ions per cubic centimeter, and the reading should jump if you hold a flame near the inlet. Your device should detect about 66 percent of the ions present. (To learn how to measure the detection efficiency and to calibrate your instrument precisely, check out the Society for Amateur Scientists's Web site.)
With this device, you can observe how the ion count changes during the day, throughout the course of the year and during big storms. And because radon gas increases the number of ions in an enclosed space, you can use this detector as a presumptive test for this dangerous element. As always, please share your findings on the society's Web page.
As a service to the amateur community, the Society for Amateur Scientists is making the electronic components (but not the mechanical ones) for this project available until September 2000 for $35. For more information about this and other projects from this column, check out the Society for Amateur Scientists's Web page. You may write the society at 4735 Clairemont Square PMB 179, San Diego, CA 92117, or call 619-239-8807.
This article was originally published with the title Counting Atmospheric Ions.