true wet-bulb temperature

evaporative equilibrium, true wet-bulb temperature

The condition attained when the wetted wick of a wet-bulb thermometer has reached a stable and constant temperature when exposed to moving air in excess of 900 ft (274.3 m) per minute.
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Both the true wet-bulb temperature and the measured wet-bulb temperature are closely related to the adiabatic saturation temperature.
The measured wet-bulb temperature is different than the true wet-bulb temperature. To measure the wet-bulb temperature with an aspirated psychrometer, a moist air stream is drawn across a temperature sensor that is kept wetted by a moist cotton sock.
In the actual measurement of wet-bulb temperatures, however, there are other forms of heat transfer to the temperature sensor that will cause the observed or measured wet-bulb temperature to differ from the true wet-bulb temperature. These "parasitic" forms of heat transfer to the temperature sensor can include radiation, lead wire/sheath conduction, and heat gain from the water source that is used to wet the sock.
Comparing Equations 2 and 1 shows that the measured wet-bulb temperature differs from the true wet-bulb temperature by the quantity [[delta].sub.wb,error], given by Equation 3.
Although the geometrically optimal radiation shield was helpful in reducing the radiation parasitic, the model still indicated that the optimized radiation parasitic is considerably higher than what is deemed "allowable" for a measurement error (relative to the true wet-bulb temperature) of [+ or -] 0.05[degrees]C ([+ or -] 0.09[degrees]F).
Combining the uncertainty in these measurements to determine an uncertainty in the true wet-bulb temperature and in the adiabatic saturation temperature produced a total uncertainty in the range [+ or -] 0.02[degrees]C-0.03[degrees]C ([+ or -] 0.036[degrees]F-0.054[degrees]F) for each of these quantities over the entire range of test conditions.
Figure 14 shows the difference between the measured wet-bulb temperature and the true wet-bulb temperature for each of the 30 test points.
Figure 16 gives an important result, as it shows that the measured wet-bulb temperature provides a much better prediction of the adiabatic saturation temperature than of the true wet-bulb temperature. The measured wet-bulb temperature can be used to predict the adiabatic saturation temperature to within approximately [+ or -] 0.05[degrees]C ([+ or -] 0.09[degrees]F), with a slight negative bias, over the entire range of test conditions.
The reason that the measured wet-bulb temperature is a better predictor of the adiabatic saturation temperature than the true wet-bulb temperature is because the adiabatic saturation temperature is larger than the true wet-bulb temperature by an amount that is almost exactly compensated for by the error associated with the parasitic heat transfer to the sensor.
In doing this, the heat transfer coefficient over the temperature sensor is reduced, which causes the measured wet-bulb temperature to deviate farther from the true wet-bulb temperature but lie closer to the adiabatic saturation temperature.