Humidity hygrometers

Mechanical Hygrometers Materials such as human hair, wood fiber, and plastics have been used to measure humidity. These methods rely on a change in dimension with humidity. 

The dewpoint hygrometer is claimed to be the most accurate instrument tor measuring air humidity. When properly calibrated, the inaccuracy can be 0.5% RH.- On the whole, the dewpoint hygrometer is a reliable lundamen-tal instrument suitable for many ventilation applications, but is more expen sive than other humidity instruments. 

The simplest way to calibrate a hygrometer is to use ambient air. The humidity of ambient air is measured using both the calibrated meter and the reference meter. A simple psychrometer or an Assmann hygrometer can be used as a reference. Provided the air conditions are stable, this method is used to check hygrometers in field measurements and other not too demanding purposes. 

If the one-point calibration in ambient air is not sufficient, the next best approach is to use the calibration box method.- The air state is created in a closed box made of nonhygroscopic material, like metal or plastic. A controlled state of humidity is maintained by exposing the air in the box to a liquid surface of a saturated salt solution. In practice, a dish containing the saturated water solution of a salt is placed on supports at the bottom of the box. The air in the box is circulated by means of a small fan. The box should be airtight and positioned in a constant temperature environment. The calibrated instruments are placed in the box. A dewpoint hygrometer can be used as a reference. A wide range of humidity can be created by using solutions of different salts. Table 12.5 shows a few examples of equilibrium humidities achieved with different salt solutions. 

For most practical purposes, however, humidity is expressed as the relative humidity (RH), the percentage of moisture in the air at a given temperature in relation to the amount of moisture that the air could hold at that temperature before condensing as dew. Since the latter amount is dependent on the temperature, the relative humidity is a function of both moisture content and temperature. A value of 50% RH, for example, means that the air holds half the water vapor it can hold at the prevailing temperature. At 100% RH, moisture condenses and falls as rain. The relative humidity is measured with instruments known as a hygrometers. 

A TSI Condensation Nuclei Counter model 3020 is used to continuously monitor the aerosol concentration in the chamber atmosphere. The chamber air is drawn through a port in the chamber wall into the counter and returned to the chamber through another port. The chamber interior temperature and humidity are monitored with a commercial hygrometer system. 

Hygrometer relative humidity used in wildland firefighting to determine the amount of moisture in the atmosphere and therefore in the fuel percentage of water in air... 

The air stream finally enters a large collection chamber where contaminant concentration is verified by a calibrated hydrocarbon analyzer and the humidity monitored by a hygrometer. 

Relative humidity is the ratio of lhe actual vapor pressure of the air. at any temperature, to the maximum of saturation vapor pressure at the same temperature. It expresses the vapor content as a fraction or percentage of the concentration necessary to render the vapor saturated at the given temperature. At the dew- point, the relative humidity is 100 1. A rise of temperature without die addition of mote vapor reduces the relative humidity (but not the absolute humidity), while a fall or temperature increases it and may bring about saturation. Relative humidity is measured by the hygrometer. 

Saturated Salt Dew Point Sensors. The saturated sail (lithium chloride) dew point sensor is widely used because of its inherent simplicity, ruggedness, and low cost. Both the United Slates and Canadian government weather services use this type of sensor lor must official groundbused humidity measurements. However, some of these are being converted to the more accurate condensation hygrometers... 

Descriptive of a substance, the physical characteristics of which arc appreciably altered by effects of water vapor. The hygroscopicity of certain materials has been advantageously utilized ill humidity measurement and control devices for example, the hair clement of a hair hygrometer. 

Smith, PR. 1971. The determination of equilibrium relative humidity or water activity in foods A literature review. The British Food Manufacturing Industries Research Association, U.K. Stekelenburg, F.K. and Labots, H. 1991. Measurement of water activity with an electric hygrometer. Ini. J. Food Sci. Technol. 26 111-116. Stoloff, L. 1978. Calibration of water activity measuring instruments and devices Collaborative study. J. AOAC 61 1166-1178. 

On the air side calibrated thermocouples to measure temperature at different locations in the cell two capacitive hygrometers on the evaporator a humidity controller designed to deliver a maximum vapor flow rate of lOkg/h have been installed. A liquid flowmeter to measure the volumetric flow rate of the aqueous desiccant solution at the membrane contactor outlet was used. All instruments are connected to a data logger. 

Many standard lab activities use simple measuring devices such as a thermometer, a clock (or wristwatch, chronometer or stopwatch), a ruler (or micrometer or tape measure), or an electrical meter (e.g. voltmeter, ammeter). Humidity is measured with a hygrometer. 

B A eudiometer is a straight tube used to measure gas volume by liquid exclusion. A calorimeter is a device used to measure changes in heat. A manometer is a U-shaped tube used to measure pressure. A hygrometer measures humidity (Answer B). Mass is measured with a balance, radioactivity is measured with a Geiger counter or scintillation counter. Viscosity is measured with a viscometer, surface tension is measured by several different techniques. 

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