Polymeter 667

The Polymeter is a hair hygrometer combined with a thermometer for measuring air temperature and relative humidity. From these two quantities, saturation vapor pressure, vapor pressure,
absolute humidity, dew point and saturation deficit can be determined.
Terms of moisture measurement
The air always contains a small amount of water vapor. This water vapor, like the air itself, exerts a certain pressure. This pressure is called vapor pressure. The water absorption capacity of the air is limited, i.e., the
Vapor pressure can only increase up to a maximum, the saturation vapor pressure. The saturation vapor pressure is strongly dependent on temperature, and it increases with increasing temperature. Relative humidity
is the ratio of the current vapor pressure to the saturation vapor pressure at the same temperature. When the air cools, the relative humidity increases while the water vapor content remains constant.
The temperature at which a relative humidity of 100% is reached is called the dew point temperature. Further cooling leads to partial condensation of the water vapor, i.e., the formation of dew or frost.
Air temperature and saturation vapor pressure
The left side of the thermometer scale indicates the air temperature in degrees Celsius. The right side of this scale indicates the saturation vapor pressure corresponding to this air temperature.
Example:
The air temperature is +8°C, corresponding to a saturation vapor pressure of 10.6 hPa.
Relative humidity
The outer division of the hygrometer scale shows the relative humidity in percent.
vapor pressure
The instantaneous vapor pressure in hPa is equal to the saturation vapor pressure multiplied by 1/100 of the relative humidity.
Example:
The saturation vapor pressure is 10.6 hPa. The hygrometer indicates a relative humidity of 50%. The vapor pressure is then 10.6 hPa x 0.5 = 5.3 hPa.
Absolute humidity
Absolute humidity is the water vapor content in grams per cubic meter of air. The numerical value of absolute humidity is equal to 0.75 times the vapor pressure in the temperature range from -10 'C to +20 'C.
Example:
The vapor pressure is 5.3 hPa. This means that one cubic meter of air contains 4 grams of water vapor.
dew point
The dew point in degrees Celsius is determined by finding the value on the right side of the thermometer scale that corresponds to the current vapor pressure and by finding the corresponding value on the opposite scale.
reads the temperature value.
The dew point can also be determined approximately by subtracting the numerical value shown on the inner division of the hygrometer scale from the air temperature.
Example:
The vapor pressure of 5.3 hPa corresponds to a dew point temperature of -1.6 °C. The number shown on the inner hygrometer scale is 1 °C for 50% relative humidity. This gives the dew point:
+8 ' C - 10(' C) = - 2 ' C.
Saturation deficit
The saturation deficit, i.e. the water vapor absorption capacity of the air, is equal to the difference between saturation vapor pressure and vapor pressure.
Example:
At 10.6 hPa saturation vapor pressure and 5.3 hPa vapor pressure we have a saturation deficit of 10.6 - 5.3 = 5.3 hPa.
Night frost forecast
The polymeter is particularly important for night frost forecasting, which is based on the following physical principles:
At night, the ground radiates its heat into the room, most strongly when the sky is clear. The cooling of the ground is transferred to the air layers near the ground. There is a limit to the cooling of the air,
when the dew point is reached and the water vapor contained in the air condenses (condensation releases heat). The nighttime cooling is therefore greater the lower the dew point is.
If the dew point in the evening is 0 'C or below, there is a risk of frost the following night.