Alpha-ray Ionization Anemometer for Measuring Low Wind Velocity

Abstract

An anemometer utilizing α-ray ionization was investigated to measure low wind velocity below a few ten centimeters per second. It is based on the decrease of ionic current due to air flow into an anemometer probe. The anemometer consists of the measuring probe fitted with an ²⁴¹Am α-ray source, a picoammeter, and a direct current power source. A parallel plate probe was designed to eliminate the dependence of measurement on two dimensional wind direction. Component parameters of the measuring probe were experimentally optimized by wind tunnel examinations so as to have a good response for wind velocities ranging from 0 to 100 cm/s. The dimensions of the anemometer probe fabricated are as follows; the diameter of collector electrode is 20 mm, and the spacing distance of parallel plate electrodes having a size of 100×100 mm is 25 mm, which corresponds to the range of alpha particles emitted from the ²⁴¹Am source. The most sensitive condition for low wind velocity was achieved when the mean electric field strength between the electrodes was adjusted at 100 V/cm, which is in the region of ion recombination. In the optimized anemometer, the ionic current at a wind velocity of 100 cm/s decreases to about 70% of that at 0 cm/s, regardless of the α-ray source strength. By using a 1.85 MBq (50μCi) α-ray source, the low wind velocity was determined with a precision of 2 cm/s. Since atmospheric pressure is beyond control even in air-conditioned rooms, the influence of that on the ionic current was checked in a range of 96 to 102 kPa It was revealed that the effect of atmospheric pressure on the ionic current was minimized at the electric field strength of 100 V/cm, which corresponded to the ideal condition in the sensitivity for low wind velocity measurement. To compensate other atmospheric conditions a differential probe is useful. The wind velocity distribution in an IC manufacturing clean room measured by the anemometer was finally demonstrated.