EUTECH INSTRUMENTS CYBERCOMM PRO FOR CYBERSCAN DO 1500 Manual de usuario descargar pdf (Pagina 2)

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About Dissolved Oxygen Measurement

What is Dissolved Oxygen?

Dissolved Oxygen (DO) is a measure of the amount of dissolved gaseous

oxygen in a solution. Some gases, such as ammonia, carbon dioxide and

hydrogen chloride, react chemically with water to form new compounds.

However, gases such as nitrogen and oxygen merely dissolve in water

without chemically reacting with it, and exist as microscopic bubbles

between water molecules.

There are two main ways in which dissolved oxygen occurs naturally

in water: From the surrounding atmosphere, where oxygen in the

surrounding air dissolves readily when mixed into water, up to saturation,

during water movements; Via photosynthesis when oxygen is produced

by aquatic plants and algae as a by-product of photosynthesis. The

amount of oxygen dissolved in water is usually measured in percent

saturation, or expressed as a concentration in milligrams per litre water.

Accurate measurement of dissolved oxygen is essential in processes

where oxygen content affects reaction rates, process efficiency or

environmental conditions, such as biological wastewater treatment, wine

production, bio-reactions, environmental water testing.

Basic Principle in DO Measurement

In theory, the amount of DO in a solution is dependent on three factors,

namely temperature, salinity and atmospheric pressure.

1. Water Temperature

Solubility of oxygen reduces as temperature increases. Hence, the

colder the water, the more dissolved oxygen it contains. Since

temperature aﬀ ects both the solubility and diﬀ usion rate of oxygen,

temperature compensation is necessary for any standardized

DO measurements.

All Eutech DO meters come with automatic temperature

ompensation for accurate readings even in varying

temperature conditions.

2. Salinity

The amount of dissolved oxygen increases as salinity level decreases.

In other words, freshwater holds more oxygen than saltwater. Since

the presence of dissolved salts limits the amount of oxygen that can

dissolve in water, the relationship between the partial pressure and

concentration of oxygen varies with the salinity of the sample.

Eutech meters feature manual salinity correction to compensate

or variations in ionic concentration. Simply enter the salinity

of the sample in parts per thousand (ppt) to ensure the correct

DO measurements.

3. Atmospheric Pressure

There is a direct proportional relationship between the solubility

of dissolved oxygen and the surrounding atmospheric pressure. As

pressure decreases with increase in altitude, the amount of dissolved

oxygen found in water reduces.

To ensure that your dissolved oxygen is not aﬀ ected by atmospheric

pressure, Eutech meters come with manual barometric pressure

compensation, with an Atmospheric Pressure Correction Chart

included in the manuals for convenient referencing.

Eutech DO instruments automatically compensate for

temperature, salinity and barometric pressure. The salinity

value and barometric pressure are either measured by the

instrument or entered by the user.

DO Electrodes

The measurement of DO requires a special DO electrode that is made

up of an anode, a cathode, electrolyte solution and a gas permeable

membrane. The material of the membrane is specially selected to permit

oxygen to pass through. Oxygen is consumed by the cathode which will

create a partial pressure across the membrane. Oxygen then diﬀ uses

into the electrolyte solution. In short, a DO meter actually measures the

pressure caused by movements of oxygen molecules in water or any

other medium. Currently, galvanic and polarographic electrodes are the

predominant methods for measuring dissolved oxygen.

The Galvanic Cell consists of two metals, the positive anode and the

negative cathode, connected by a salt bridge between the individual

half-cells. As the metal electrodes leave electrons behind as they dissolve

in the electrolyte. The diﬀ erent properties of the two metals causes them

to dissolve at diﬀ erent rates, hence a pressure is created when the number

of electrons in either side of the cell diﬀ ers. The pressure is translated

into an electric current proportion to the oxygen concentration in the

electrolyte if an electrical circuit is created between the two electrodes.

The galvanic electrode does not need polarising time and is able to

assume operation immediately.

During this process, ions of the more active anode are transferred through

the electrolyte to the less active cathode, and deposited there as a plating.

In this way the anode is corroded. When the anode material eventually

corrodes away, the potential drops and the current halts.

Galvanic elec t rod e s are av a i l a b l e w i t h m o s t Eutech

nstruments DO meters such as the EcoScan DO 6, CyberScan

DO 110 and DO 300.

The Polarographic Cell consists of t

wo electrodes placed in the

electrolyte: One with ﬁ xed potential called the reference electrode,

and the other with a variable potential called the polarizable electrode.

As voltage is applied to the polarizable electrode, a redox reaction

occurs, where electrons break away from the electrode to bond

with oxygen in the electrolyte. The rate at which the electrons break

Electrolyte

Solution

Cathode

Anode

Gas Permeable Membrane

About Dissolved Oxygen

Measurement

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