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Technicl Index
Conductivity and Resistivity
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.
Conductance = 1/Resistance
Conductivity: mho = Siemen
Normal unit of measurement is:
1 micromho (µmho) = 1 microSiemen (µS),
1 millimho (mmho) = 1 milliSiemens (mS) = 1,000 µS
Resistivity: ohm
Normal unit of measurement is:
megohm = 1,000,000 ohm
Example - Conductivity to Resistivity Conversion:
Conductivity:
= 20 µS
= 20 x 10-6 S
= 2 x 10-5 S
= 2 x 10-5 mho
Resistivity:
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 |
Resistivity |
Dissolved Solids |
.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 |
