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Conductivity and Resistivity

Introduction to Measurement and units

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.

In this article how the conductivity is measured is more detailed explained.

Conductance = 1/Resistance
Conductivity unit: mho = Siemen
Normal unit of conductivity measurement is:
1 micromho (µmho) = 1 microSiemens (µS),
1 millimho (mmho) = 1 milliSiemens (mS) = 1,000 microSiemens (µS)

Resistivity unit: ohm
Normal unit of resistivity measurement is:
megohm = 1,000,000 ohm

Conductivity units conversion:

20 microSiemens (µS)
= 20 x 10-6 S
= 2 x 10-5 S
= 2 x 10-5 mho >

Resistivity units conversion:

1 ohm/2 x 10-5

=1/conductivity
= 1/2 x 10-5 ohm
= 0.5 x 10-5 ohm
= 5 x 10 4 ohm

Conductivity and Resistivity (NaCl and CaCO3 Solutions at 25°C)

ppm as CaCO3
ppm NaCl
Conductivity micromhos/cm
Resistivity megohm-cm
1700
2000
3860
0.00026
1275
1500
2930
0.00034
850
1000
1990
0.00050
425
500
1020
0.00099
170
200
415
0.0024
127.5
150
315
0.0032
85.0
100
210
0.0048
42.5
50
105
0.0095
17.0
20
42.7
0.023
12.7
15
32.1
0.031
8.5
10
21.4
0.047
4.25
5.0
10.8
0.093
1.70
2.0
4.35
0.23
1.27
1.5
3.28
0.30
0.85
1.00
2.21
0.45
0.42
0.50
1.18
0.88
0.17
0.20
0.49
2.05
0.13
0.15
0.38
2.65
.085
0.10
0.27
3.70
0.042
0.05
0.16
6.15
0.017
0.02
0.098
10.2
0.012
0.015
0.087
11.5
0.008
0.010
0.076
13.1
0.004
0.005
0.066
15.2
0.002
0.002
0.059
16.9
0.001
0.001
0.057
17.6
none
none
0.055
18.3

Probe Constants

Probe constant defines the volume between the electrodes. Solutions with an extremely high conductivity require a sensor with a probe constant greater than 1.0. 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
(Micromhos/cm)
Resistivity
(Ohms-cm)
Dissolved Solids
(ppm)
.056
18,000,000
.0277
.084
12,000,000
0.417
.167
6,000,000
0.833
1.00
1,000,000
.500
2.50
400,000
1.25
20.0
50,000
10.0
200
5000
100
2000
500
1,000
20,000
50
10,000
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