Psychrometers

For measurement of atmospheric humidities the sling psychrom-eter is widely used. This is composed of a wet- and dry-bulb thermometer mounted in a sling which is whirled manually to give the desired gas velocity across the bulb. In the Assmann psychrometer the air is drawn past the bulbs by a motor-driven fan. 

In addition to the mercuiy-in-glass thermometer, other temperature-sensing elements may be used for psychrometers. These include resistance thermometers, thermocouples, bimetal thermometers, and thermistors. 

A psychrometer measures the dry-bulb and wet-bulb temperatures simultaneously. The measurement of the wet-bulb temperature is achieved by means of a wet wick placed over the thermometer bulb. The thermometer can be practically of any type. A cylindrically shaped sensor is preferred. The wet-bulb temperaturesensing element, covered with the wick, and the dry-bulb temperature sensor, are placed in the airstream to be measured. The stream, generated by a small fan, should have a velocity of 3-5 m s and can be either transverse or axial. The... 

The humidity can be determined using either charts or equations provided by the psychrometer manufacturer. The partial pressure of water vapor provides a ni(2re general approach and can be calculated from the psychrometer equation ... 

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. 

Moisture and Humidity International Symposium on Moisture and Humidity. Washington, DC, 1985. R. G., Wylie, T. Lalas. Accurate psychrometer coefficients for wet and ice-covered cylinders in laminar transverse air streams. In Moisture and Humidity 1985 Measurement and Control in science and Industry, pp. 37-56. 

Assmann psychrometer An apparatus which uses a clockwork or electrical fan located above wet and dry bulb thermometers, positioned in a cylinder to provide a set airflow rate over the thermometer bulbs for accurate temperature readings of both bulbs. 

Sling psychrometer An instrument used to measure the dry-bulb and wet-bulb temperatures of the air, from which the humidity of the air can be determined by means of calculations, tables, or charts. 

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

In order that consistent conditions can be obtained, the air speed over the thermometers should be not less than 1 m/s. This can be done with a mechanical aspiration fan (the Assmann psychrometer) or by rotating the thermometers manually on a radius arm (the sling psychrometer). If the thermometers cannot be in a moving airstream, they are shielded from draughts by a perforated screen and rely only on natural convection. In this case the wet bulb... 

On this chart, the wet bulb temperatures appear as diagonal lines, coinciding with the dry bulb at the saturation line. If measurements are taken with the two thermometers of the sling psychrometer, the condition can be plotted on the psychrometric chart by taking the intersection of the dry bulb temperature, as read on the vertical line, with the wet bulb temperature, read down the diagonal wet bulb line. 

Figure 23.4 Psychrometers. (a) Assmann. (b) Sling, (c) Screen housing (Courtesy of Casella London Ltd)...

Modem electronic replacements for the Orsat apparatus include equipment manufactured in the United States by Bacharach, Inc. (www.bacharach-inc.com). This company produces handheld and boxed instruments that can be employed as comprehensive combustion and environmental analyzers, gas burner combustion testers, single gas analyzers, draft gauges, smoke test sets, sling psychrometers, and the like. 

Cosgrove, D.J., Van Volkenburgh, E. Cleland, R.E. (1984). Stress relaxation of cell walls and the yield threshold for growth Demonstration and measurement by micropressure probe and psychrometer techniques. Planta, 162, 46-52. 

In addition, there are hand-held psychrometers for measuring air temperature and relative humidity available, and also hand-held wind meters, which measure wind speed. When using such hand-held equipment, readings should be taken each hour during the course of the day when worker exposure volunteers or re-entry worker volunteers are performing the test. 

Psychrometer Measures relative humidity (soil moisture) within a soil Generally consists of a thermocouple, a reference electrode, a heat sink, a porous ceramic bulb or wire mesh screen, and a recorder... 

Field methods of measuring soil water are primarily designed to measure water in the range of -10 to -1500 kPa of pressure. However, different instruments have different ranges as shown in Table 5.1. Tensiometers, porous blocks, and thermocouple psychrometers are usually installed in the field and measurements are taken on a regular basis. Neutron probe and time domain reflectometry (TDR) equipment are usually carried to the field each time a... 

There are a number of other types of measurement made in soil that involve electrodes that are not directly in contact with the soil. An example is the thermocouple psychrometer, which involves a Thomson thermocouple in a ceramic cell buried in soil. The thermocouple cools when a current is passed through it, causing water to condense on the thermocouple. When the electricity is turned off, the condensate evaporates at a rate inversely proportional to the relative humidity in the soil. A voltage generated by the cooling junction is measured and related to the soil moisture content. This moisture content is related to both the matrix and osmotic potentials of the soil being investigated. 

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