Assignment
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Objectives:
The objective of this experiment is to use record the characteristics of air using different equipment. The characteristics obtained were dry and wet bulb temperature and relative humidity.
Introduction:
There are couple of building science terms that are used to describe the characteristics of the air; starting with the relative humidity (RH), which is ratio of water content of air to the saturation produced. It can be obtained from the following equation:
RH=P/P_vs x 100%
Where Pvs is the vapor pressure at the saturation
A relative humidity of 100% is reached at the dew point temperature, which is when the air becomes saturated by being cooled at a constant pressure.
The second term is the specific humidity, which is the quantity of vapor needed for saturation; it can be expressed as follows:
q=M_w/M_a
There are two different bulb temperatures; the first one is "dry bulb" temperature which can measured using a thermometer or a thermocouple. The second one is the "wet bulb" temperature; it's the lowest temperature that could be reached by evaporating water into air. The wet bulb temperature can be measure using a normal thermometer with a wet cloth covering the bulb. Moreover, the wet bulb temperature is reached when the heat loss due to evaporation is the same as the heat gain due to conduction.
Discussion:
1. It can be noticed that each tool gives slightly different air properties than the other. For the sling psychrometer, the air flow isn't constant due to the fact that its being rotated manually, thus this could result in slightly different measurements. Meanwhile, the humidity meter is said to give the most accurate results. On the other hand, the aspirated psychrometer does not have steady results, in other words, the reading changes constantly. For the second part, only the humidity meter was used.
2. In part 1, the measured relative humidity using the humidity meter was " 15.5% " and dew point was " -5.4°C ", on the other hand the second part, the relative humidity was " 12.8% " and the dew point was " -3.7 °C " for the condenser and " 22.2% " , " 9.1 °C " for the evaporator. The dew point for both the first part and the second part (for condenser) are negative and relatively close. The values change because of the presence of the moisture in the air.
The relative humidity indicated the percentage of the water in the air relative to the amount of air; and the dew point temperature indicated the temperature at which the amount of water in air will be saturated. Both of these parameters are good indicators of the air humidity. They both can be used to determine other parameters using the psychrometric chart.
3. Using the psychrometric chart, at Temp= 25 C and RH= 55%, the moisture content is 0.01078 kg/kg, the humidifier is said to reduce the RH from 55% to 45% and the Temp will be constant, therefor the new moisture content will be 0.0089. the amount of water removed is " 0.00188 kg/kg"
4. An instrument that could measure the moisture content is a capacitive humidity sensor; sensors work is such a way that detects the relative humidity of the air using a thin strip of metal oxide placed between two electrodes.