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Conductivity measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport.
Therefore, with an increasing amount of ions present in the liquid, the liquid will have a higher conductivity. If the number of ions in the liquid is very small, the solution will be “resistive” to current flow. AC current is used to prevent complete ion migration to the two electrodes.
CONDUCTIVITY/RESISTIVITY
MEASUREMENT
Conductance = 1/Resistance
Conductivity: mho = Siemen
Normal unit of measurement is:
1 micromho (
µ mho) = 1 microSiemen (
µ
S)
1 millimho (mmho) = 1 milliSiemen (mS)
= 1,000
µ
S
Resistivity : ohm (
Ω
)
Normal unit of measurement is: megohm (M
Ω
) = 1,000,000
Ω
Example - Conductivity to Resistivity
Conversion:
Conductivity = 20
µ
S
= 20 x 10 -6 S
= 2 x 10
= 2 x 10
=
[
-5
-5
S
1 ohm mho
______
-5 ]
Resistivity = 1/conductivity
-1
=
[ ______
-5
1 ohm
]
= 1 ohm
2 x 10 -5
= 1 ⁄
2 x 10
= .5 x 10
= 5 x 10 4
5
5 ohm ohm ohm
= 50,000 ohm
CONDUCTIVITY/RESISTIVITY
CONVERSIONS
Conductivity Resistivity Dissolved
(Micromhos-cm) (Ohms-cm) Solids (ppm)
.
056
.084
.167
1.00
2.50
20.0
200
2000
20,000
18,000,000
12,000,000
6,000,000
1,000,000
400,000
50,000
5,000
500
50
.0277
.0417
.0833
.500
1.25
10.0
100
1,000
10,000
Note: ppm x 2 = Conductivity
Current
Source
If d = 1 cm, probe constant = 1.0
PROBE CONSTANTS
Probe constant defines the volume between electrodes.
Solutions with an extremely high conductivity require a sensor with a probe constant greater than 1.0.
d d d
Solutions with extremely low conductivity require a sensor with a probe constant less than 1.0.
The greater the distance between the electrodes, the smaller the current signal.
CONDUCTIVITY AND RESISTIVITY
(NaCI AND CaCO
3
SOLUTIONS AT 25°C (77°F)) ppm as CaCO
3
1700
1275
850
425
170
127.5
85.0
42.5
17.0
12.7
8.5
4.25
1.70
1.27
0.85
0.42
0.17
0.13
0.085
0.042
0.017
0.012
0.008
0.004
0.002
0.001
None ppm
NaCI
2000
1500
1000
500
200
150
100
50
20
15
10
5.0
2.0
1.5
1.00
0.50
0.20
0.15
0.10
0.05
0.02
0.015
0.010
0.005
0.002
0.001
None
Conductivity micromho/cm
3860
2930
1990
1020
415
315
210
105
42.7
32.1
21.4
10.8
4.35
3.28
2.21
1.13
0.49
0.38
0.27
0.16
0.098
0.087
0.076
0.066
0.059
0.057
0.005
Instrumentation Technical Services - ITS
20 Hagerty Bld. - Suite 1 - West Chester PA, 19380 - (610)436-9703
Resistivity megohm/cm
0.00026
2.65
3.70
6.15
10.2
11.5
13.1
15.2
16.9
17.6
18.3
0.00034
0.00050
0.00099
0.0024
0.0032
0.0048
0.0095
0.023
0.031
0.047
0.093
0.23
0.30
0.45
0.88
2.05