Dew Point Meter Principle

Dew Point Meter Principle

By Cooling at constant pressure if the temperature of air is reduced, the water-vapour in the air will start to condense at a particular temperature. This temperature is called dew point temperature.

Description of Dew Point Meter

The main Parts of arrangement are

• A shiny surface (mirror) is fixed with a thermocouple.
• A nozzle is providing a jet of air on the mirror.
• A light source focused constantly on the mirror.
• A photo cell to detect the amount of light reflected from the mirror.

Operation of Dew Point Meter

• The mirror is constantly cooled by a cooling medium. The cooling medium is maintained at a constant temperature.
• To this mirror is attached a thermocouple whose leads are connected to a millivoltmeter.
• Constantly a light is made to fall in an angle on the mirror and the amount of reflected light is sensed by a photo cell.
• Now an air jet is made to fall in an angle on the mirror and the water-vapor (moisture) contained in the air starts condensing on the mirror and they appear as small drops (dew) on the mirror.
• This moisture (dews) formed on the mirror reduces the amount of light reflected from the mirror and it is detected by photocell. When for the first time, there is a change in amount of transmitted light; it becomes an indication of dew formation.
• At this instance (that is, when the due formation is detected first), the temperature indicated by the thermocouple attached to the mirror becomes the dew point temperature.
• Thus this arrangement is used to determine the time at which the dew appears for the first time and dew point temperature.

Application of dew point meter

• This instrument is used on ships to protect cargoes from condensation damage by maintaining the dew point of air in holds lower than the cargo temperature.
• Used in industries for determining dew point.

Limitations of dew point meter

There are limitations in cooling fluids and light measurement.