 Hello, Myself Ganesh Agalai, working as an assistant professor in Department of Mechanical Engineering, Vulture Institute of Technology, Solapur. Today, we will see the psychrometry. In the first session, the psychrometric properties will be studied. Learning objective, at the end of this session, students will be able to elaborate moisture, measure d-bit and w-bit of moisture, evaluate properties of moisture. So, what is the psychrometry? Before studying the psychrometry, we must know the definition of air conditioning. As per ASHRAE, the science in which moisture is cooled or heated, humidified or dehumidified, filtered and the velocity should be controlled. So, if we will be able to control all these parameters in the science is known as air conditioning. So, this is the definition of ASHRAE. Now, in the air conditioning, air is treated means the psychrometry plays the important role, because the science which deals with study of moisture is known as psychrometry. So, here moisture in the air conditioning is very important. So, what is moisture? You think over the composition of atmospheric air as in the air conditioning, atmospheric air is treated and its properties are changed by the air conditioner. So, you must know the composition of atmospheric air. The mixture of dry air and water vapour is known as moisture. So, what is happening here? The atmospheric air is first of all passed through the filter, the unwanted particles will be removed by the filter, such as odour also and remaining mixture will be of dry air. Dry air here will call it as a permanent gases and water vapour which plays important role in human comfort. So, here as the liquefaction temperature of permanent gases are less than minus 150 degree Celsius, these permanent gases will remain as it is in the moisture. So, variation of water vapour is the main part of the moisture. So, moisture as is the mixture of dry air and water vapour using the Dalton's law of partial pressure will get the total atmospheric pressure of that system air conditioning system that is the summation of partial pressures of water vapour and dry air. Now, here psychrometric properties are there in that main properties are temperature specific humidity, relative humidity and enthalpy. We will see step by step these properties in detail. First of all temperature in the psychrometry the moisture will have these three different temperatures. Now, here to understand these temperatures to know in detail the temperatures we must draw the temperature versus entropy diagram that is T s diagram of the water vapour it is not for the permanent gases. So, if we will consider simple thermometer from that we will be getting the temperature of the surrounding air. So, that temperature is known as dry bulb temperature. So, the temperature indicated by simple thermometer is known as dry bulb temperature. So, here on the T s plot this is the saturation curve and if this is the dry bulb temperature then suppose this state will locate the condition of the water vapour present in the moist air nothing, but surrounding air. Now, we may get saturation pressure by using the steam table. So, corresponding to dry bulb temperature from the steam table we will get saturated pressure of that water vapour. Now, next temperature is wet bulb temperature. Now, if this simple thermometer bulb is wrapped with the wet cloth or the sponge in which continuously water should be present. So, the temperature shown by that thermometer is known as wet bulb thermometer after studying the DPT I will locate the wet bulb temperature on T s 1. Now, next temperature third type of the temperature is dew point temperature. So, consider air conditioning room in that room will start AC. So, what happens at a constant pressure the air will be cooled air will be cooled and when it will touch to the saturation vapour curve the first droplet of water vapour will be obtained. So, that particular temperature is known as dew point temperature. So, you can see here the dew point temperature the temperature corresponding to the condensation of water vapour is known as dew point temperature. If we use small t for dry bulb temperature T suffix d for dew point temperature then it is experimentally found that wet bulb temperature will lie between dry bulb temperature and dew point temperature. So, corresponding to dew point temperature from steam table will get water vapour pressure the partial pressure of water vapour and will be using PV dash notation for the pressure partial pressure of water vapour corresponding to wet bulb temperature. So, these are the three temperatures of the moist air. Then next property is the specific humidity it is also known as humidity ratio or moisture content. Now, all the psychrometric properties are evaluated per kg of dry air. So, specific humidity is the ratio of mass of vapour in a sample mass of vapour in a sample to the mass of dry air in that sample at a constant temperature at a same temperature. So, the ratio of mass of vapour to mass of air in a given volume of air at a constant temperature is known as specific humidity. Now, as the vapour is treated as a perfect gas we can use PV is equal to MRT adelgas equation then substituting that for mass of vapour will be getting PV by VV here PV stands for partial pressure of water vapour volume of that water vapour by Rv gas constant of water vapour and temperature. So, this is nothing, but dry bulb temperature divided by mass of air. So, PAVA of dry air by RAT here the temperatures are same will get cancelled volume is also same will also get cancelled if we will put the vapour constant and air constant will get the constant 0.622. Then the ratio will be PV by PA now here as we have seen the atmospheric pressure is the summation of PA plus PV. So, instead of PA I can substitute here P minus PV. So, you have to remember this formula specific humidity is equal to 0.622 PV by P minus PV. Now, unity will be kilogram of water vapour as in the numerator we took mass of water vapour per kg of dry air. So, this will be the specific humidity of that moisture. First property is the relative humidity now this humidity is to be measured with some reference. So, the ratio of moist mass of water vapour to the mass of water vapour in saturated condition. So, mass of vapour in the suppose at this point MV to the mass of that vapour, but in saturated form is known as relative humidity. Now, it will be denoted by 5 and the percentage will be available. So, all these properties will try to show on the psychrometric chart. Now, after studying these psychrometric properties in next session we will study the psychrometric chart. Now, for properties to study in detail you can refer these books books refrigeration and air conditioning by CP Aurora, Micro Hill Publisher, refrigeration and air conditioning by Manohar Prasad New Age Publisher. So, in next session we will study about the psychrometric chart. Thank you.