Relative Humidity Sensors (Hygrometers) PDF

Introduction to Process Measurement • Chapter 2 Objective 5 Describe the types of humidity sensing and measuring devices. Relative Humidity Sensors (Hygrometers) Traditionally, relative humidity has been determined with a wet-and-dry bulb psychrometer. These devices are still available, and still...

Introduction to Process Measurement • Chapter 2 Objective 5 Describe the types of humidity sensing and measuring devices. Relative Humidity Sensors (Hygrometers) Traditionally, relative humidity has been determined with a wet-and-dry bulb psychrometer. These devices are still available, and still in use. Psychrometers could be either small portable devices, or permanently installed instruments with supplemental recording or transmitting devices. In either case, two thermometers are exposed to the same atmosphere. One thermometer is kept wet by a wick that draws water from a reservoir. The other thermometer is kept dry. There must be positive air movement around the thermometers so that they are continually provided with a representative sample of the surrounding atmosphere. After a short period of time, the thermometers read different temperatures. These are called the “dry bulb” and “wet bulb” temperatures. The difference between the two temperatures is a function of the relative humidity, which determines the rate of water evaporation, and hence the temperature of the wet bulb. The dry and wet bulb temperatures can be plotted on a psychrometric chart, and the relative humidity determined. Today, various other sensing methods are used to determine relative humidity, including the measurement of changes in resistance, capacitance, impedance, and frequency. One method that uses resistance to determine relative humidity depends on a layer of hygroscopic salt such as lithium chloride or carbon powder, deposited between two electrodes (Figure 52). The material absorbs and releases moisture as a function of relative humidity, causing a change in resistance of the sensor. An ohmmeter is connected to the sensor to measure the relative humidity. The device may be used as an indicator or as a controller. Figure 52 – Resistive (Resistance) Type Relative Humidity Sensor Conductive Hygroscopic Salt Nonconductive Base Gold Electrodes Wires to Controller Another style of hygrometer measures the change in capacitance on a metal oxide as the relative humidity changes. Capacitive hygrometers are fairly economical, durable, and have reasonable accuracy. The absolute humidity of air can also be measured by thermal hygrometers that measure the thermal conductivity of air. The calibration standard for hygrometers is the gravimetric hygrometer that measures the mass of a sample of air and compares that mass to the mass of an equal volume of dry air. 4th Class Edition 3 • Part A 2-41