Wick evaporation

In the third case, we are concerned with a considerable depth of water distributed in fine channels and small pockets where there is negligible stirring. Since the result is important and its explanation is often misunderstood, it is discussed separately under wick-evaporation. 

There is abundant evidence that water can accelerate the evaporation of pesticides from the soil. Two mechanisms operate one which depends on the evaporation of water, the other requiring only its presence. The first is best called wick evaporation and is the phenomenon which should be referred to by the agronomist when he speaks of steam distillation. 

Over most of the United States the potential evaporation is higher. That it may not in fact be higher only serves to emphasize the fact that the soil surface is wet only for a small part of the summer. Only by one mechanism (wick evaporation) does evaporation of water assist that of the pesticide. If the pesticide is on the surface and water is not, the... 

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. 

As the water from the wick evaporates, the wick cools down and continues to cool until the rate of energy transferred to the wick by the air blowing on it equals the rate of loss of energy caused by the water evaporating from the wick. We say that the temperature of the bulb with the wet wick at equilibrium is the wet-bulb temperature. (Of course, if water continues to evaporate, it eventually will all disappear, and the wick temperature will rise.) The equilibrium temperature for the process described above will lie on the 100% relative humidity curve (saturated-air curve). 

More recent versions of this type of probe include some refinements, such as the provision of a wick to aid evaporation of the solvent and matrix from the probe tip (Figure 13.5). Such improvements have allowed greater flow rates to be used, and rates of 1 to 10 ml/min are possible. For these sorts of low flow rates, minibore LC columns must be employed. 

For the air—water system, the humidity is easily measured by using a wet-bulb thermometer. Air passing the wet wick surrounding the thermometer bulb causes evaporation of moisture from the wick. The balance between heat transfer to the wick and energy requited by the latent heat of the mass transfer from the wick gives, at steady state,... 

The temperature of air as indicated by a thermometer when a water-wet wick encloses its bulb. If the surrounding air is not saturated water will evaporate, taking the latent necessary latent heat from the thermometer bulb which then gives a lower reading than a dry bulb in the same air. The depression in wet bulb temperature is proportional to the amount of moisture in the air. 

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. 

The effect can be observed and measured by using two similar thermometers (Figure 23.3), one of which has its bulb enclosed in a wet wick. The drier the air passing over them, the greater will be the rate of evaporation from the wick and the greater the difference between the two readings. In the case of air at 25°C, 50% saturation, the difference will be about 6.5 K. The measurements are termed the dry bulb and wet bulb temperatures, and the difference the wet bulb dqrression. 

FIGURE 6.2 Brenner and Niederwieser S-chamber (a) normal development, (b) continuous development 1 — mobile phase, 2 — wick, 3 — chromatographic plate with adsorbent layer face up, 4 — cover plate, 5 — support, 6 — heater, 7 — evaporation of the mobile phase. (From Brenner, M. and Niederwieser, A., Experientia 17, 237-238, 1961. With permission.)... 

One instrument for determining the relative humidity of the atmosphere. It consists of a pair of thermometers, the bulb of one of which is surrounded by a fabric wick kept moist from a small water reservoir. Due to evaporation the temperature indicated by the wet bulb thermometer is lower than that of the other. The difference in the readings compared to the dry bulb reading enables the relative humidity to be obtained from tables. 

The amount of soil residues found In the different soil depths, as shown In Tables VI to XI, Indicated that a major trend for the pesticides was to concentrate In the upper 0-1 Inch layer of soil. A theoretical explanation could be the "wick effect reported by Hartley. As water evaporates off the soil... 

Any liquid propane that remains on the sample should be wicked away from around the crystal with a lint-free tissue. Thawed liquid propane should be collected and allowed to safely evaporate in a chemical fume hood. 

Fuel volatility is an extremely important factor related to fuel combustion and burning efficiency. Evaporation, vaporization, and vapor pressure of fuel can all be reduced in cold environments. Poor startability and warmup of gasoline and diesel engines can be directly related to fuel volatility. Also, cold kerosene will not vaporize and bum as efficiently in wick-fed systems. 

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. 

Increasing separation toward the bottom Bottom electrodes Buffer shunt Evaporation Feeding by supplementary wicks Trickle feeding of background Good dissipation of heat (thin wall and cooling system) reduction of heat (lower n, thinner paper)... 

Wet- and dry-bulb temperatures are measured by exposing two temperature-sensitive elements to the atmosphere whose moisture level is to be measured. The wet bulb is wrapped with a wick soaked in water the other element, the dry bulb, is left bare. Water evaporating from the wick lowers its temperature, which is read as the wet-bulb temperature, whereas the other reads the dry-bulb temperature. The relative humidity can be read from a psychometric chart such as the one shown in Figure 3.25. 

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