9.2 Liquid Level Using Ultrasound 1935 2立 ) Figure 9.4 Liquid Level Measurement Using Ultrasonic"Range-finding" Techniques -The "radar"liquid level sensors use similar principles but with microwave energy rather than acoustic().Two versions,noncontact and guided wave(导行波)，are available. 9.3 Humidity -Concepts 20/35 Absolute humidity:an amount of water vapor,usually discussed per unit volume.Generally the mass of water vapor,per unit volume of total moist air. Relative humidity:the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air depends not only on temperature but also on pressure of the system of interest. Specific humidity:the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio.Specific humidity ratio is expressed as a ratio of mass of water vapor,per unit mass of dry air. o Partial pressure is the pressure which the gas would have if it alone occupied the volume.The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture. In common use are the relative humidity,dew-point()temperature, mixing ratio or specific humidity,and volume ratio (parts of water vapor per million parts of air).(j) Figure 9.4 Liquid Level Measurement Using Ultrasonic “Range-finding” Techniques ▬ The “radar” liquid level sensors use similar principles but with microwave energy rather than acoustic(声学). Two versions, noncontact and guided wave(导行波), are available. 9.2 Liquid Level - Using Ultrasound 19/35 z Absolute humidity: an amount of water vapor, usually discussed per unit volume. Generally the mass of water vapor, per unit volume of total moist air. z Relative humidity: the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air depends not only on temperature but also on pressure of the system of interest. z Specific humidity: the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio. Specific humidity ratio is expressed as a ratio of mass of water vapor, per unit mass of dry air. z Partial pressure is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture. z In common use are the relative humidity, dew-point(露点) temperature, mixing ratio or specific humidity, and volume ratio (parts of water vapor per million parts of air). 9.3 Humidity - Concepts 20/35