6-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية(التوصيلية الكهربية)

Physical parameter of water
Water Conductivity

Definition and description

Conductivity of a substance is defined as 'the ability or power to conduct or transmit heat, electricity, or sound'.

Its units are Siemens per meter [S/m] in SI and millimhos per centimeter [mmho/cm] in U.S. customary units.

Its symbol is k or s.

Electrical conductivity (EC)

An electrical current results from the motion of electrically charged particles in response to forces that act on them from an applied electric field.

Within most solid materials a current arise from the flow of electrons, which is called electronic conduction.

In all conductors, semiconductors, and many insulated materials only electronic conduction exists, and the electrical conductivity is strongly dependant on the number of electrons available to participate to the conduction process.

Most metals are extremely good conductors of electricity, because of the large number of free electrons that can be excited in an empty and available energy state.

In water and ionic materials or fluids a net motion of charged ions can occur. This phenomenon produce an electric current and is called ionic conduction.

Electrical conductivity is defined as the ratio between the current density (J) and the electric field intensity (e) and it is the opposite of the resistivity (r, [W*m]):
s = J/e = 1/r

Silver has the highest conductivity of any metals: 63 x 106 S/m.

Water conductivity

Pure water is not a good conductor of electricity.

Ordinary distilled water in equilibrium with carbon dioxide of the air has a conductivity of about 10 x 10-6 W-1*m-1 (20 dS/m).

Because the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases.

Thus conductivity increases as water dissolved ionic species.

Typical conductivity of waters:

Ultra pure water 5.5 • 10-6 S/m

Drinking water 0.005 – 0.05 S/m

Sea water 5 S/m

It is a measure of the inverse of the amount of resistance an electric charge meets in traveling through the water.

Distilled water has a conductivity ranging from 0.5 to 3 µS/cm, while most streams range between 50 to 1500 µS/cm.

Freshwater streams ideally should have a conductivity between 150 to 500 µS/cm to support diverse aquatic life.

Electrical Conductivity and TDS

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.

In dilute solution, TDS and EC are reasonably comparable. The TDS of a water sample based on the measured EC value can be calculated using the following equation:

TDS (mg/l) = 0.5 x EC (dS/m or mmho/cm) or = 0.5 * 1000 x EC (mS/cm)
The above relationship can also be used to check the acceptability of water chemical analyses. It does not apply to wastewater.

As the solution becomes more concentrated (TDS > 1000 mg/l, EC > 2000 ms/cm), the proximity of the solution ions to each other depresses their activity and consequently their ability to transmit current, although the physical amount of dissolved solids is not affected.

At high TDS values, the ratio TDS/EC increases and the relationship tends toward TDS = 0.9 x EC.

In these cases the above-mentioned relationship should not be used and each sample should be characterized separately.

For water 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).

This is a measure of the capability of a solution such as water in a stream to pass an electric current.

This is an indicator of the concentration of dissolved electrolyte ions in the water. It doesn't identify the specific ions in the water.

However, significant increases in conductivity may be an indicator that polluting discharges have entered the water.

Physical parameter of water
Electrical resistivity

Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity)

A low resistivity indicates a material that readily allows the movement of electric charge.

is a measure of how strongly a material opposes the flow of electric current.

It is known that the theoretical maximum electrical resistivity for water is approximately 182 kilohm-meters (or 18.2 MΩ•cm) at 25 degrees Celsius."

the resistivity of water to be approximately 182,000 Ωm at 25 °C. This makes pure water a horrible conductor. It would take a huge potential difference to just run a 1 A current through a body of water

Resistivity is an intrinsic property of a material that is measured as its resistance to current per unit length for a uniform cross section

Electrical resistivity ρ (Greek: rho) is defined by,

where
ρ is the static resistivity (measured in ohm-metres, Ω-m)
E is the magnitude of the electric field (measured in volts per metre, V/m);
J is the magnitude of the current density (measured in amperes per square metre, A/m²).
Most resistors and conductors have a uniform cross section with a uniform flow of electric current and are made of one material. (See the diagram to the right.) In this case, the above definition ofρ leads to:

where
R is the electrical resistance of a uniform specimen of the material (measured in ohms, Ω)
is the length of the piece of material (measured in metres, m)
A is the cross-sectional area of the specimen (measured in square metres, m²).
Finally, electrical resistivity is also defined as the inverse of the conductivity σ (sigma), of the material, or

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