Sling psychrometer

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. 

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. 

The selection of the design wet-bulb temperature is made on the condition existing at the tower site and measured with a sling psychrometer. The selected temperature is normally close to the average maximum wet-bulb for the summer months. 

The wet-bulb temperature is obtained by using a sling psychrometer, an apparatus containing a dry-bulb thermometer and a second thermometer wrapped in a wet sock or wick. Spinning the psychrometer around in the air (slinging) causes evaporation to occur and a lowering of the temperature to a minimum degree, at which point the vapor pressure of the water in the wick has been so reduced that no further evaporation takes place. This equilibrium point is the wet-bulb temperature. Evaporation takes place at the wet-bulb temperature of the air. 

A sling psychrometer is a device to measure the humidity of air. A porous cloth (the wick) is wrapped around the bulb of a mercury thermometer, which is then whirled around in the air. As the water in the wick evaporates, the temperature of the thermometer bulb drops, and finally stabilizes at the wet-bulb temperature of the air. The dry-bulb temperature is read from a second thermometer mounted on the sling. 

One summer day, the weather bureau reports a temperature of 33 C and a relative humidity of 40%. You wipe the sweat from your forehead and remark to a friend that you d bet 5 that the report is wrong, and the relative humidity is higher than 80%. He immediately puts a 5 bill on the table, accepting your challenge. You bring out your sling psychrometer. whirl it around, and read a dry-bulb temperature of 35 C and a wet-bulb temperature of 27°C. Who wins the bet ... 

Figure 4.23 Wet-bulb temperature obtained with a sling psychrometer.

SLING PSYCHROMETER - Measuring device with wet and dry bulb thermometers. Moved rapidly through air it measures humidity. 

Sling psychrometer slig si- kra-m9-t9r n. A psychrometer containing independently matched dry- and wet-bulb thermometers suitably mounted for swinging through the atmosphere, for simultaneously indicating dry- and wet-bulb temperatures. 

To obtain the two temperature readings, one can use a sling psychrometer. This is a special kit that holds two thermometers (one wet-bulb and one dry-bulb) on a sling. A user hangs onto a handle and whirls them around. Another device holds the two thermometers and sets on a surface. A built-in fan provides moving air to cool the wet sock of the wet-bulb thermometer. Again, the readings require use of a psychrometric chart to determine relative humidity. 

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