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.

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