There are multiple questions in this one question. So let us first understand the key concepts.

Dew point and Relative humidity

The air has a weight. This weight also contains some amount of moisture. So say 100 gms of air may contain 1 gram of moisture (water).

Similar to a piece of cotton that has a limit to hold water, air too has a limit. This limit changes with the change in temperature.

Dew Point

Dew point tells us the how much water the air actually has. Lower the dew point lesser is the water content in the air. 

The instrument that measures the dew point is called Hygrometer. On board ship the type of hygrometer used is the wet & dry bulb type. 

It measures the wet bulb temperature and dry bulb temperature. Dew point of the air is obtained by comparing the two temperatures on a dew point table.

Relative Humidity

Relative humidity tells us how much percentage of water the air has compared to how much it can hold at this temperature. 

So if we say 100% relative humidity, it means that the air has maximum amount of moisture that it can hold at this temperature. That is the air is saturated.

For the steel cargo, our concern is the formation of sweat as steel can get stains because of that.

So when can there be sweat in the cargo hold ?

Answer is when dew point of the air inside the hold is higher than the outside temperature.

In this case the formation of sweat can be avoided in two ways

• Removing the moist air inside the hold and replacing it with dryer outside air by ventilation. 
• Using dehumidifier inside the hold

With both ways the idea is to keep the dew point inside the hold equal to or lesser than the outside temperature.

When relative humidity inside the hold is 100%, this means that the air is saturated and holds the maximum water content it can at this temperature.

We need to either ventilate this moist saturated air with ventilation or use the dehumidifier inside the hold.