YSI 3100 - Manual (Page 27)

23
Conductivity Law
Solution Instrument Cell
Conductivity Conductance Constant
S/cm or mho/cm = S or mho
ЧЧЧЧ
1/cm
mS/cm or mmho/cm = mS or mmho
ЧЧЧЧ
1/cm
µµµµ
S/cm or
µµµµ
mho/cm =
µµµµ
S or
µµµµ
mho
ЧЧЧЧ
1/cm
CELL CONSTANT =
SOLUTION CONDUCTIVITY
METER CONDUCTANCE
METER CONDUCTANCE =
SOLUTION CONDUCTIVITY
CELL CONSTANT
Low Conductivity Measurements
When measuring reagent grade water (deionized) or other substances having extremely low
conductivity, it is recommend that a flow-through cell having a constant of 0.1/cm be used for
the best accuracy. If a flow-through cell is not practical, then extraordinary precautions must be
taken in regard to equipment setup, cell cleanliness, electrical interferences, etc. Therefore, when
operating on this range, some instability in the least significant digit is normal and should be
averaged or ignored.
Error Sources
•
Solution temperature coefficient may be upwards of 7% per
°
C
•
Absorption of atmospheric CO
2
may account for 1.3
µ
S/cm at 25
°
C
•
Platinization ions may leach into the solution from the electrodes
•
Glass ions may leach into the solution from the cell or container
•
Organic substances may leach into the solution if plastic is used
•
Electrical noise
•
Contact resistance
•
Cable series resistance and shunt capacitance
•
Cell series and shunt capacitance
•
Galvanic effects
Only the first four are of major concern for typical measurements, although the user should also
be careful to see that cells are clean and maintained in good condition at all times.