The AlphaLab Air Ion Counter ($390 US dollars) is a handheld meter designed to measure ion density-- the number of ions in air per cubic centimeter (ions/cc). The resistivity of air (6x1018/N in ohm cm if N is in ions/cc) and the discharge half-life of charged objects (in seconds, half-life is 1.2x105/N) can also be determined directly from the readings. It measures both positive and negative ions, and displays them separately. This instrument is a true ion density meter, based on a "Gerdien Tube" design. It contains a calibrated fan which pulls air through the meter; then the ions are detected. It compensates for winds up to +/- 15 km/hour (9 mph). Accuracy is +/- 25% of the reading within the range -10 to +50C (14 to 122F), and air speeds up to 15 km/hour. (Meter accuracy is +/- 3%, but precise ion readings are difficult due to external conditions. Some ions or ion pairs are destroyed when forced to change direction or travel through a hole. This destruction causes slightly over 20% uncertainty in the actual number of ions/cc arriving at the meter. The calibration of the meter is set to give +/- 25% accuracy over all conditions up to 15 km/hr air speed.) The display is in units of 1000 ions/cc, so that a displayed reading of "- 0.60" represents 600 +/-150 negative ions/cc. The meter has three ranges, and it measures up to 1,999,000 ions/cubic cm (essentially 2 million) with minimum resolution 10 ions/cc. All normal natural sources are well within that range, but if you are measuring the output immediately in front of an ionizer, the ion number density may be as high as 20 million ions/cubic cm. Therefore, for ionizer measurements, request the 20 million version. (It is the same price as the standard 2 million ions/cc version, but minimum resolution is 100 ions/cc. A 200 million ions/cc version is also available).