Dew Point Meter Working Principle

Dew point can be simply understood as the temperature at which the water vapor content in a gas reaches a saturated state. It is one of the ways to express the absolute humidity of a gas. It can be seen that the dew point temperature is a unit system for measuring the moisture content of a gas. The dew point meter is an instrument that measures the absolute moisture content of a gas based on this unit system.

When the measured moisture enters the dew point measurement room, it will pass through the cold mirror surface. When the temperature of the mirror surface is higher than the dew point temperature of moisture, the mirror surface is dry. At this time, the light emitted by the light source in the photoelectric detection device is almost completely reflected on the mirror surface. The photoelectric sensor senses and outputs the photoelectric signal, compares and amplifies it through the control loop, and then drives the thermoelectric pump to cool the mirror surface.

When the temperature of the mirror surface drops to the dew point of moisture, dew (frost) starts to appear on the mirror surface. The light has diffuse reflection on the mirror surface, and the reflection signal induced by the photoelectric sensor decreases with it. After this change is compared and amplified by the control loop, the thermoelectric pump excitation is adjusted to reduce the cooling power appropriately. After that, the mirror temperature is maintained at the sample gas dew point temperature. The mirror surface temperature is sensed by a platinum resistance temperature sensor directly under the cold mirror surface and displayed on the display window.

During the measurement, as the temperature decreases, the water vapor in the measured gas approaches the saturation state. Due to gravity, water molecules are adsorbed on the mirror surface to form a thin water film. This is the stage where dew is formed. When the temperature of the mirror surface continues to decrease, the thickness of the water film gradually increases, which is the second stage of dew formation.

During this phase, the force contrast between the gravitational force of the free surface on the water molecules and the surface tension of the water film begins to change, and the influence of the latter gradually dominates. At this time, any unstable factors on the cooling surface, such as tiny scars on the mirror surface, will cause the water film to condense into droplets. As the mirror temperature drops further, dew drops begin to appear. Through the microscope, you can see isolated dew droplets that grow in isolation and distribution, and then the dew layer spreads on the surface at a rapid rate. At this time, it can be considered that the liquid-vapor equilibrium begins, that is, the dew point is reached.