YSI 3200 - Manual (Page 45)

SECTION 8 PRINCIPLES OF OPERATION
8.1 RATIOMETRIC CONDUCTIVITY
The basic concept of microprocessor based ratiometric conductivity measurement is to place an
unknown conductance and a known conductance in a series circuit with an AC voltage source,
measure the voltage across each conductance and compute the value of the unknown. The object
of conductivity measurement is to measure the resistive portion of the complex impedance of the
cell. Measurement of AC voltage is done with a synchronous rectifier followed by an integrating
A/D converter that is synchronized with the signal waveform. The phase reference for the
synchronous rectifier can be selected to eliminate either series or parallel capacitance. Using the
voltage across the reference resistor as the phase reference eliminates series capacitance, which is
the dominant error source at high conductance. In this mode, the A/D only "sees" the portion of
the cell voltage that is in phase with the current thus eliminating the series capacitance. Using the
voltage across the cell as the phase reference can likewise eliminate parallel (cable) capacitance,
which is the dominant error source in ultrapure water measurement. In this mode, the A/D only
"sees" that portion of the voltage across the reference resistor that is caused by the in-phase cell
current thus eliminating parallel capacitance.
8.2 CONDUCTIVITY
PRINCIPLES
INTRODUCTION
Conductivity measurements are used in wastewater treatment, industry, pharmaceutical, and
military etc. as a measurement of the purity or the condition of a process. Conductivity is used as
a measurement of a solution’s ability to conduct electric current. The ability of a solution to
conduct electric current depends upon ions: their concentration, size, mobility, viscosity, valence
and the temperature of the solution. Inorganic solutions are relatively good conductors. Organic
solutions are poor conductors.
CONDUCTIVITY FUNDAMENTALS
Electrical conductance (k) is defined as the ratio of the current (I) in a conductor to the
difference in the electrical potential (V) between its ends (k=I/V), measured in mhos or siemens
(S). Conductance, therefore, is not a specific measurement. Its value is dependent upon the length
of the conductor. Conductivity (
ℵ), or specific conductance, is the conductance per unit of
conductor length. For our purposes, conductivity is defined as the conductance in mhos or
siemens measured across the sides of a one-centimeter cube of liquid at a specified temperature.
YSI Incorporated Model 3200 41