salinometer

Water purity

• Water purity may refer to the absence of

dissolved salts, a measurement in terms of

acidity or alkalinity, or the presence of oil in a

sample.

• Various instruments which make these

measurements, their operating principles and

the need for the measurement, will now be

described.

Water purity

• Water purity, in terms of the absence of salts,

is a necessary requirement where it is to be

used as boiler feed.

• Pure water has a high resistance to the flow

of electricity, i.e. a low conductivity, so much

so that it is considered non-conductive.

Water purity

• . Water containing dissolved salts, in particular sea

water, has a high conductivity.

• A measurement of conductivity is a measure of

purity.

• A pair of electrodes of known area are placed in

the sample solution at a known distance apart.

• The resistance between them is measured and

expressed in conductivity units which are siemens.

Siemens/cell constant

• . A siemen is a reciprocal ohm and represents the

passage of one ampere at one volt potential

through one cubic centimeter of the liquid.

• Where electrodes are smaller than one square

centimeter and spaced closer or farther than one

centimeter then a cell constant is used to correct

the readings obtained.

Salinometer

• . One common type of salinity indicator uses a small

cell containing two platinum electrodes.

• The liquid sample passes through the cell and the

current flow as a result of any conductance is

measured.

• The reading given assumes all conductivity is due to

sodium chloride which, although not strictly accurate,

is acceptable for most practical purposes.

Salinometer

• Electrical conductivity rises with

temperature and thus a temperature

compensating resistor is incorporated in

the measuring circuit.

Salinometer

• In the dionic water purity meter Conductivity is

measured between two platinum wire ring

electrodes and two gunmetal ring electrodes

• . The liquid flows through these rings which are

separated by insulated lengths of tubing, see

Figure .

• In effect measurements are made on two

columns

Salinometer

Salinometer

• . In effect measurements are made on

two columns of liquid.

• A bimetallic strip operates a plunger

which insulates the two columns and

provides temperature compensation to

20°C.

Salinometer

• . Specific conductivity in siemens/cm3 is the

conductance across a centimeter long column

of mercury whose cross-sectional area is one

square Centimeter.

• A much smaller unit, the microsiemen/cm3,

when corrected to 20°C, is known as a dionic

unit..

Salinometer

• Pure distilled water has a Conductance of

about 1 dionic unit while fresh water is about

500 dionic units.

• A de-grassfire should be fitted upstream of

this unit to remove dissolved carbon dioxide

since this will cause errors in measurement

Acidity/alkalinity/p h meter

• Water, when acidic or alkaline, can cause corrosion.

Boiler feed water when acidic or strongly alkaline will

cause corrosion.

• It is usual to monitor and maintain boiler feed water

slightly alkaline. A scale of measurement using pH values

indicates the degree of acidity or alkalinity within a

solution. A solution which is neutral is neither acidic nor

alkaline. The pH scale is a measure of the concentration

of hydrogen ions in a solution using numbers from 1 to 14