GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.1 341 cm 3 ; mobility 0.4 cm 2 /(V sec), accuracy ± 10%; flowing air velocity 40 cm/sec. This instrument samples the air ions by drawing the air through an air ion collector. The ion collector is a Gerdian chamber which consists of two groups of parallel plates, called polarizing plate and collecting plate, see Figure 2. A DC electric potential is applied between these two plates, generating an electric field. When the sampling air passes through the ion collector, the air ion will be deflected by the electric field and move towards the collecting plate or the polarizing plate. One polarity ions give their electric charges to the collecting plate, the other polarity ions is destroyed on the surface of the polarizing plate. The collecting plate is connected to an extremely sensitive Pico Ammeter, called electrometer. The output reading of the electrometer is calibrated by the sampled air rate and the collected ion current. In this way, the air ion number per cubic centimeter can directly read. Changing the polarity of the polarizing potential change the collected ion’s polarity. The concentration of the ions per cubic centimeter N is expressed by N = I / (q v A) , where I is the ion current measured in Amperes, charge of one electron equal to 1.6 10 -19 C, v is the velocity of air flow through the collector in cm/sec and A is the effective entrance area of the Gerdian chamber in cm 2 . Because the size and mass of the air ions are different, they have different rates of movement in a give electric field. Higher potential is used to collect the larger ions; lower potential is used to collect the smaller ions. By changing the potential strength V , the collecting plate length L and the velocity of air flow v , different ion mobility can be selected. In fact, the mobility threshold K of the ion collector may be computed from the equation K = v d 2 / (L V) , Fig. 2. The Gerdian chamber configuration of the DLY-6A2 air ion counter. Polarizing voltage is changing to collect both positive and negative ions. Fig. 3 - A first long time recording of air ions made by the Fermo Station with the new instrument air ion counter DLY-6A2 by Kilter from July 1, 2018. Positive air ions are indicated by red bars, while negative ions are indicated by blue ones.