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Electrical Conductivity and TDS
Electrical conductivity (EC) is the ability of a material to transmit
(conduct) an electrical current and is commonly
expressed in units of milliSiemens per meter
(mS/m). Soil EC measurements may also be
reported in units of deciSiemens per meter
(dS/m), which is equal to the reading in
mS/m divided by 100. (1 dS/m = 0.01 mS/m)
TDS or Total Dissolved Solids is a measure of the total ions in solution.
EC is actually a measure of the ionic
activity
of a solution in term of its capacity
to
transmit current. For water used for agricultural and
irrigation
purpose the values for EC and TDS are
related
to each other and can be converted
with an
accuracy of about 10% using the following
equation:
TDS (mg/l) = 640 x EC (dS/m or mmho/cm or
ms/cm).
With the reverse osmosis process, water is
forced in a semi-impermeable membrane
leaving
the impurities behind. This process
is capable
of removing 95-99 % of TDS, providing
pure
or ultra-pure water.
Use Lenntech calculators to calculate the TDS content from water
analysis and to convert TDS in EC ou
visa
versa
Measuring salinity
- Most meters measure the electrical conductivity
of water.
- The unit of electrical measurement used is
Siemens. The meter we use in the
field measures
milliSiemens (mS) per centimetre
(the electrodes
are one centimetre apart).
- Often the literature will use microSiemens
(uS/cm) and EC standing for electrical
conductivity.
These two units are the same. To
convert
milliSiemens (mS) to EC's or uS multiply
the reading by 1000. The meter in
the diagram
shows 1.65 mS which is 1650 uS or
EC's.
| Water Salinity Levels for Use (divide by
1000 to get ms/cm) |
| Source/Use |
uS/cm or EC |
Source |
uS/cm or EC |
| distilled water |
0 |
limit poultry |
5 800 |
| Murray R. (mouth) |
790 |
limit beef cattle |
16 600 |
| desirable limit people |
830 |
limit adult sheep |
23 000 |
| absolute limit people |
2 500 |
Pacific Ocean |
58 000 |
Conversions
1 deciSiemens per meter (dS/m) = 1
mmho/cm
= 1 milliSiemens per cm (mS/cm) = 1000
microSiemens
per cm (uS/cm)
1 mhos/cm = 1 S/cm = 1000 mS/cm
Electrical Conductivity units (EC's) IF measured
in uS/cm: 1 EC= 1uS/cm = 0.001 dS/m = 0.001 mS/cm = 1 mS/m
(Example 1.5 dS/m = 1.5 mS/cm = 1500 uS/cm
= 1500 EC)
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