Figure 10-40.--Salinity cell.

The operation of the salinity indicator system is

(ppm); the terms are interchangeable. For purposes of

our discussion, we will use epm. The system is a neces-

based on the principle that an increase of the electrolytic

sity aboard ship because all fresh water, particularly

impurities (principally salt) in water increases the elec-

when underway, is made from seawater. Excessive

trical conductivity of the water and, conversely, that a

decrease in the impurities increases the electrical resis-

salinity in the boiler feedwater causes pitting of the

tubes and rapid deterioration due to electrolysis. Salin-

tance of the water. If two electrodes are immersed in the

water tested and a stable alternating voltage is applied

ity indicators are usually provided in the engine rooms

across the electrodes, a stable alternating current will

and the firerooms for checking the condensate from the

flow, provided the impurity content and the temperature

main and auxiliary condensers. They are also provided

of the water remain unchanged.

for the evaporator plants to indicate the degree of purity

The amount of current flow is indicated on a meter,

of the fresh water and condensate at various selected

the scale of which is graduated in equivalent parts per

points in the distilling system.

million. If the saline content of the water increases

The ship's water system (fig. 10-39) pumps water

because salt water leaks into the system or because the

aboard the ship from the sea, sends it through the

operation of the distilling plant becomes faulty, the

evaporator to remove the salt, and then stores the puri-

conductivity between the electrodes increases and the

fied water in freshwater tanks. The fresh water is used

meter reading increases an amount that is proportional

for showers, drinking water, cooking, and, most impor-

to the increase in salinity.

tantly, it is used in the ship's boilers to generate the

The salinity system is composed of three major

steam that drives the ship. The boiler feedwater must

components: the salinity cell, the salinity indicator

contain less than 0.065 epm or the boilers can be dam-

panel, and the dump valve and solenoid.

aged or "salted up." The salinity system monitors the

impurity content continuously; should the impurities

Salinity Cell

reach alarm proportions, the salinity system de-ener-

gizes the dump valve solenoid and redirects the

contaminated water to the ship's bilge or directly over-

The salinity cell (fig. 10-40, views A and B) is the

board.

device that does the actual detecting of impurity. It

Figure 10-40.--Salinity cell.

10-25