EUTECH INSTRUMENTS CYBERSCAN DO 1500 DO METER Manual do Utilizador descarregar pdf (Página 2)

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 aects both the solubility and diusion rate of oxygen,

temperature compensation is necessary for any standardized

DO measurements.

All Eutech DO meters come with automatic temperature

compensation for accurate readings even in var ying

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

for 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 a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 di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.

Galvanic Electrodes produce a millivolt output directly proportional to

the oxygen present in the sample. The electrode reaction is instantaneous

and a result is obtained immediately.

Galvanic electrodes are av a i l a b l e w i t h most Eutech

Instruments DO meters such as the EcoScan DO 6, CyberScan

DO 110 and DO 300.

Polarographic Electrodes require voltage input from the meter to

polarize the electrodes. Since the voltage from an external source may

take up to 15 minutes to stabilize, polarographic probes usually need to

warm up before use to ensure proper polarization of the electrodes.

Polarographic self-stirring DO/BOD probes are available

for use with the Eutech CyberScan research-grade bench

meters DO 6000 and PCD 6500 in US EPA-approved auto

ve-day BOD testings.

Biochemical Oxygen Demand (BOD)

The BOD test measures the molecular oxygen utilized in the

biodegradation of organic material and the oxidation of inorganic

material. By measuring the amount of oxygen dissolved in samples at

the beginning and end of a specied incubation period, the relative

oxygen requirements of wastewaters, euents, and polluted waters

can be determined.

BOD

(mg/L) =

- D

BOD

= Oxygen uptake during incubation period t

= DO of diluted sample immediately after preparation (mg/L)

= DO of diluted sample after incubation period t (mg/L)

P = Decimal volumetric fraction of sample used

BOD is similar to the Chemical Oxygen Demand (COD), which

also measures relative oxygen-depletion. However, the possible

presence of non-biologically oxidisable may render the COD test to

be less accurate.

DO | About Dissolved Oxygen Measurement

page | 46

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