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Experiments of Accuracy Air Ion Field Measurement

K. Bartusek1, P. Fiala2, T. Jirku2, and E. Kadlecova2

1Institute of Scientific Instruments Academy of Sciences of the Czech RepublicKralovopolska 147, 612 64 Brno, Czech Republic

2Department of Theoretical and Experimental Electrical Engineering, University of Technology BrnoKolejni, 4, 612 00 Brno, Czech Republic

Abstract— An analysis of the electric state of air shows the presence of various ion sorts. Thetherapeutic effect of negative high-mobility ions of proper concentration is known. This positiveeffect was observed in caves that are used for speleotherapy. This article presents the capabilityof methods for measuring ion concentration and for ion spectral analysis. Here are new gerdientube design steps based on numerical analysis and its experimental verification. Article describeof any experiments with different air ion generator.

1. INTRODUCTION

Air ion concentration and composition belong to the frequently monitored parameters of the at-mosphere [5]. Their influence on living organisms has been the subject of intensive studies. Earlierresearch has demonstrated the positive influence of light negative ions and air cleanness on humanhealth. The Department of the Theoretical and Experimental Electrical Engineering of Brno Uni-versity of Technology and the Institute of Scientific Instruments of the Academy of Sciences of theCzech Republic are involved in the research of ion field in office and living spaces. The objective isto increase the concentration of light air ions in these spaces. Another task is to set up a simulatedtherapy room, with conditions similar to speleotherapy caves. It sets the requirements for accuratemeasurement of ion field with good repeatability. The article deals with the design of gerdiencondenser and peripheral measuring devices. An optimal design is important for eliminating theinaccuracy of ion concentration measurement.

2. MEASURING METHOD

Several methods are currently used to measure air ion fields: the dispersion method, the ionspec-trometer method, the Faraday cage method, and the gerdien condenser method, whose principleis shown in Figure 1. Here is d1-inner electrode diameter, d2-outer electrode diameter, l-length ofgerdien condenser, M -air flow volume rate, v-air flow velocity, e-elementary charge of electron, ⊕positive air particle (ion), ª negative air particle (ion). The gerdien condenser consists of two elec-trodes. There is an electric field between the inner electrode (the collector) and the outer electrode.The field is imposed by voltage source U. Air ions flow from the fan through the gerdien condenser.Negative ions in the electric field impact the collector, and the current produced is measured by apA-meter. The current measured is proportional to air ion concentration. The model of the mea-suring system is shown in Figure 2. The values measured carry systematical measurement errors.This is due to leakage currents and parasitic capacitances (modeled by ILEAK in Figure 2) [6]. Wehave to consider leakage resistances RAK of gerdien condenser, leakage resistances and capacitanceof the pA-meter input (REH , CEH , REL, CEL), insulation resistance (RV ) of the collector voltagesource. The current measured is further affected by the input resistance of pA-meter and the inputresistance of voltage source (RU , CU ). To minimize the measurement error, RAK , and RV shouldbe much larger than RI , and REH , and REL should also be much larger than ROUT . Time constantRUCU has to be much larger than the measuring time.

3. NEW DESIGN OF GERDIEN CONDENSER

The inner and outer electrodes are elliptical in shape. This shape ensures that the flow of air islaminar. Air flow turbulence can distort the accuracy of measurement. The surface of the electrodesis required to be as smooth as possible. These aspects make the design of gerdien condenser quitedemanding (fine grinding, lapping, etc.). The new design of gerdien condenser is shown in Figure 3.

Since in the measurement of air ion concentration very small currents are detected, it is necessaryto eliminate the influence of ambient electric charge. The influence of magnetic fields has to beminimized too.

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Figure 1: Principle of gerdien condenser method.

II

IOUT ILEAK

ROUT RAK RV

RU

REH

REL

CAK

CEL

CEH

CU

Figure 2: Model of a system for measuring air ionconcentration - the gerdien condenser method.

Figure 3: New gerdien condenser.

4. WEAK CURRENT AMPLIFIER

The current flowing through the gerdien condenser is due to the ion concentration. Current intensitydepends on polarization voltage, on the dimension and parameters of gerdien condenser, and on ionconcentration. The specific current range for the designed gerdien condenser is 10−10A–10−13A.For the following measurement it is suitable to convert the current to voltage. Because the currentis very weak, it is suitable to do this near the gerdien condenser. The low-level amplifier is realizedwith INA 116-Figure 4. The INA 116 has a very low input bias current Ib,max = 100 fA. The designof the amplifier is shown in Figure 5. The gain of INA 116 is set by resistor R10.

Figure 4: Principal scheme of INA 116.

Figure 5: Design of low-level amplifier.

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5. NUMERICAL MODELING, VERIFICATION

It is possible to carry out analysis of a MG model as a numerical solution by help of Finiteelement method (FEM). The electromagnetic part of the model is based on the solution of fullMaxwell’s equations. It was solved like simply electrostatic field. This results showed to newfacts in gerdien condenser design. The new knowledge were tested in many experiments and ourmeasurement system had approximately 50% better characteristics. In Figure 6 is shown one effectof light negative ion inside of gerdien condenser. There are showed the non-primitive moving ofone electron. Therefore the sensor has higher noise then sensor with filter. New design of gerdiencondenser was made with filter for the specific particles. Result of new experiments are showed inFigure 7.

Vliv konstruk ní úpravy na m ený proud I celk

se sítí

bez sít0 100 200 300 400 500 600

t [s]

-0,3

-0,2

-0,1

0,0

0,1

0,2

V0

[V]

Effect of design changes to measured corrent

With filter

Without filter

Figure 6: Result-characteristics of gerdien condenserwith filter, time depend.

Figure 7: Result-characteristics of gerdien con-denser with filter.

6. TESTS OF GERDIEN CONDENSERS

The gerdien condenser of new design was tested according to the special measuring methodology.The scheme of this method is shown in table 1. The test of the gerdien condenser method wastested on several sources of ion generators. The experiments are shown in Fig. 8 and Fig. 9.

Table 1: Air ion concentration methodology.

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Figure 8: Gerdien condenser measuring method- test by electronic generator.

Figure 9: Gerdien condenser measuring method- test by water “natural” generator.

7. CONCLUSION

The new design of gerdien condenser and the optimization of peripheral measuring devices haveminimized the systematic error of measurement. The new system allows measuring air ion concen-tration with a sensitivity > 100 ions/cm3. The ion mobility is in the interval 0.3− 100 cm2V−1s−1.The system is used to measure ion field distribution in living and office spaces.

ACKNOWLEDGMENT

The paper were prepared within the framework of VUT Brno of the Czech Republic and with thesupport of the research plan MSM 0021630516 and grant GAAV No. B208130603.

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VUT Brno7. Kafka, V, Saturacnı a spektralnı charakteristika iontoveho pole, Diplomova prace 2004 VUT

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