 I am supposed to take one hour of psychrometry and we will look at it mostly from the point of view that you already or at least I would say that you should as far as possible try to emphasize that at least in this part of the course if you are teaching students that you should have the students practice psychrometry through the steam tables rather than you know telling them to go to a psychrometric chart. So, once they are confident with the steam tables and they know that the psychrometric chart is usually drawn I mean normally at one atmospheric pressure then they know that you know they can always come back to steam tables and do whatever calculations if the ambient atmospheric pressure is not one atmosphere because if you go to a hill station for every around 1000 or 2000 meters in height you know the pressure drop significantly you know close to 10-15 percent and then no longer your psychrometric charts you know as they are right now can be applied and you will need a psychrometric chart which is you know made for that pressure. So, it is better if you just go for steam tables at least in this course and then if necessary I am sure that if the students will take a class on refrigeration and air conditioning they will you know be very well versed with the psychrometric chart I think that should be the overall you know guiding principle when you talk to them about psychrometry if it is going to be a part of thermodynamics. So, let me just begin and go over things fast. So, I mean normally if you know begin psychrometry again standard way to start is you go for the TS diagram of the water substance and draw a typical pressure line and then you can tell the students that you know as far as normal air is concerned it is just not absolutely dry and maybe only in you know very much in the interior of the country the weather is reasonably dry, but you know close to the coast there is lot of moisture and they know what moisture means you know and that they sweat mostly a lot of it because of moisture and you can tell that the state of a water inside or in the surrounding air is basically it is vapor and once it is vapor it means it is a kind of superheated liquid sorry a superheated state of water and if I would try to plot it on a TS diagram it would be somewhere here the state and then you can tell them that this pressure line which on which probably it is lying is a very very small pressure I mean or a very pressure which is of very less quantity I mean compared to one atmosphere this is the value of this pressure is pretty negligible and this is the pressure at which water vapor is existing in the atmosphere as if you draw this then I think a lot of things become very clear and you know after this I do not think there is too much if you just continue with this diagram there is not too much that we can you know go wrong by when you are teaching psychrometry. So, I will just draw this pressure line on which water vapor is existing and this is let us call this line as PV where V represents the or some kind of subscript for vapor. So, this is how we begin with telling where the state of water vapor is and once you say this you say that now the pressure is so less that if you mix this with regular dry air the dry air at one atmosphere reasonably behaves like an ideal gas and water vapor in superheated state and at such low pressures also can be reasonably approximated as an ideal gas and if this is again emphasized then you realize that they can be told that water vapor will follow as long as it is in the vapor state most of the properties of ideal gas then only between steam tables and ideal gas relationships you can derive a lot of psychrometry and hopefully that should be good enough as a part of this course. So, if we begin by saying that the water vapor pressure is PV then we say the net pressure at any location is P let us say it is P total that is made up of pressure of dry air plus PV. So, at any point this P T is the total atmospheric pressure or the barometric pressure and this is made up of let us say two components and here probably you can tell them about things that they have learned in chemistry that is Dalton's law of partial pressures. So, if you have many gases which do not react and if they are mixed and if they are all behaving ideally then they will exert pressures in proportion to how many molecules are there of that gas and that is what is the Dalton's law of partial pressures. This is what will happen that I can just then add the pressures of the two components and say that is the total pressure. So, this is if you emphasize this I mean that would be good enough and then you can just come back and you know write down ideal gas relationship for each of these. If you had written them in terms of molar or in terms of moles then I would have said number of moles of dry air times. So, let me just write this times universal gas constant times the ambient temperature divided by some volume that we are considering. So, this is this is number of moles of vapor multiplied by ideal gas constant divided by same volume. So, this is using the ideal gas law P v is equal to N i v and I could write it in terms of mass basis where I would say that this is in a particular volume mass of dry air times specific gas constant for dry air. So, this is capital V it is not specific volume once this is set up then what you can of course, tell them is that and they should know this that for any gas a specific constant is just the universal gas constant divided by the molecular weight. So, at this point if you have already taught them combustion or if you have if they know about mole mass ratio then you can go ahead and you know roughly you can derive what the molecular weight of dry air would be and this is mostly based on telling them that you know dry air is mostly made up of nitrogen and oxygen. And though there are small components of other species these are negligible and just considering nitrogen and oxygen you can derive roughly that the molecular weight of dry air is around 29 it is 28 point something, but you can put it as 29 and for water it is you know it is just H 2 O. So, it is just above 18, but writing 18 is good enough and if you can if they would of course, know all this because of their background in chemistry and at this point you know it is just to reemphasize what the specific gas constants would be for water and air. So, once this is done so, this of course would here be mass of total air R of you know total moisture let me call it M A E upon 1. So, once this set is done I think we can then it is just ok to go ahead with telling them of the various terminologies in psychrometry. So, the first terminology that they should be aware of is something called and it is always good to just stick I mean a lot of times I mean I am sure if you have taught this you must have taught a lot of times I see the students just remember some kind of a formula for specific humidity using point 6 to something like this. So, if we just stick to the saying that specific humidity is just you take a particular volume then the mass of water vapor or M V upon mass of air that is the definition for specific humidity. Then I think a lot of things if they remember only this then that is good enough and if I just use now ideal gas law I can go ahead and write it in terms of partial pressure. So, now you know I know that P V sorry P V is equal to M V R U by M V T and I will just write M V is P V V by R V by M T. So, if I just substitute here this would be P V by V R V by M T and for mass of air it would just be mass of dry air I should write it would be P air V by R U by M air T. So, this P goes off the V goes off the R U goes off and I am just left with M V by M A P V and this is 18 upon roughly 29 P V by P A would be roughly 0.62 P V by. So, I am sure all of you remember this relationship, but rather than students remembering such a relationship it is ok if they remember you know just the basic definition of what specific humidity means. And then as long as they remember that they have to use steam tables and ideal gas relationships they cannot go wrong. I mean everyone would know roughly what is the mass or molecular weight for water what is the molecular weight for air. So, you know getting this 18 upon 29 is not going to be that difficult at all for people and what it should be and once they get the number they will obviously remember, but rather than trying to remember this as some formula you know it is always good to just tell them to remember that it is just a ratio of masses and that is good enough. Of course, people sometimes write this as 0.62 P V by P A is just 0.62 P V by P total minus P V. This is another way people write this because the pressure in air we already discussed Dalton's law we discussed that the two pressures add up to the total pressure and hence the pressure of air is just the total pressure minus the wave pressure. But I would say that you know if the emphasis is on ideal gas law and just the pure definition you know it just takes one minute to come up to this formula and there is no need to remember anything more than that. If the next concept that is normally taught is what is called as the dew point temperature at this point it is again best to go to the T S diagram and remind them that this is where the state of water vapor is. It is lying on a particular pressure line that is at T V it is a very small value and this is where the water vapor is in. So, if at the same pressure the water is just cooled down sorry the air is just cooled down then this is of course you know this is where the temperature is right now and at this point you can tell them that this is the temperature of both the water vapor as well as air and this is something which is normally measured with a thermometer and this is what is normally called as the regular temperature or the dry bulb temperature. So, this is where things are as they are existing and if you start cooling the water vapor or the air down then the pressure is the same, but the temperature starts going down and this is how normally dew points are measured you know you keep a vessel and you start cooling it down slowly and see when condensation occurs. So, this is what is occurring here that at the same pressure as I start cooling down the temperature start decreasing and I just move along the same pressure line like this till I reach some point here where you know the water is just become saturated and it will start condensing out and this is this temperature is just the saturation temperature corresponding to P it is pretty straight forward that I drew a curve like this and this is the existing temperature which is much much more than the saturation temperature that is why it is a superheated state and once I start cooling it down obviously, at some point I am going to reach the saturation temperature and the saturation temperature is nothing, but the dew point temperature for that PV. So, if I had a different PV I would have a had a different you know dew point temperature and of course, now at this point you can tell them that they should take out the steam tables and realize that you know for most of the low pressure the saturation temperature is given. So, if they have the steam tables knowing PV you can immediately get what the dew point temperature and at this point you can draw various PV curves like this. So, at if I draw a higher pressure line here then let us say this is PV 2 then obviously this pressure is more, if this pressure is more then here the specific humidity is more because PV is higher and if I go along this pressure line then the dew point temperature is definitely going to be a higher value. So, the more the moisture in the air the higher the dew point temperature will be until you get up to this line here where you know maybe the air is completely saturated with water vapor at this point completely the dew point temperature is the same as the existing dry bulb temperature. So, if you draw via this diagram I mean the concept of water dew point temperature is you know they do not really have to think too much if they have the T s diagram of water pretty much clear in that way. So, various pressures various dew point temperatures till you reach this and obviously beyond this you cannot put in water into the air because it will just start condensing out. So, you can tell them this is the maximum capacity of what the air can contain. So, beyond this at a particular temperature I cannot put in more water vapor than this vapor pressure. If I want to go higher then I need a higher temperature. So, I will need to heat the water or heat the air and then put in try to make it you know gather more air because now I can go to a higher vapor pressure here. So, if this concept is also told to them that beyond a particular vapor pressure you cannot just push in water into the air because it will start condensing out. So, I think that is something that can be well explained just by using a T s diagram. Yes, so depending on what your so if I draw this I mean which pressure? So, if there is a particular vapor pressure then there is a particular dew point temperature corresponding to it. So, if I if there was more if so if the air was more moist the vapor pressure would have been higher and the dew point temperature would also have been higher because corresponds. So, this is let us say P v 1 this is P v 2. So, if the air is relatively dry then there is lesser water vapor in it and it corresponds to this lower pressure line and let us say this is the dry bulb temperature. So, this is the existing temperature at any place is that clear and then if I am on this pressure line then to get to the dew point temperature I have to cool down till I reach the saturation temperature corresponding to this pressure line. And obviously, this temperature is low compared if I have a higher pressure let us say I am existing here definitely the you know specific humidity is higher in the air I come down till I reach the saturation temperature corresponding to this pressure. Obviously, this temperature T 2 is greater than T 1 this is the T you know this is the T axis. So, the higher the water vapor if the overall pressure is the same it does not matter this water vapor pressure is what matters and as long as you reach the saturation temperature corresponding to the vapor pressure you have reached the dew point. So, the air pressure could have been anything it did not matter it was all depending on the saturation temperature corresponding to the vapor. See the total you see this is what is assumed normally the total pressure is rarely assumed to change that is because overall the pressure is around one atmosphere people believe that PV is so negligible that overall P does not change it does not really matter. And in general if something changes there is enough you know pressure this small pressure differences to just create a small flow that overall pressure would not change. But that is not of concern here it would not change the dew point temperature of course what will happen is let us say that I come here and I start condensing things out then you know I can start condensing things out and then I would have to move along the saturation line and keep on condensing. If let us say you keep a body which is at a temperature below T 1 then first initially the air will come up to T 1 and then it will start going below T 1 and you start condensing out all water. So, you would move down in PV and PW and let us say I remove all the water and keep it separately and still the air is there and I bring it back to the older temperature obviously I would have gone to an even lower line here and if I bring it back. So, my PV you know star would be even lower than PV 1 because I would have removed all the water I mean that is pretty straight forward I would have made it even more drier. In fact this is a reasonable way of thinking what happens in an air conditioner you just put it over cool coils remove the water and now it is become even drier. So, you have actually moved to a lower PV line that is exactly what you have done in air conditioner which one yes see if the regular atmospheric pressure is the same then higher PV would have been higher water vapor pressure or more moisture or more RH we will come to RH. The right side of the bell cow yes the line will completely the saturated vapor line is not it? Yes this is the saturated. Saturated vapor line yes X is equal to 1 that is dryness fraction is equal to 1 here X is equal to 0. Correct. So, particularly on the dryness fraction line sorry X is equal to 1 saturated line. Yes. The point is 100 percent saturated dry. Correct. Yes. Vapor is 100 percent saturated dry. Yes. But in the definition of dew point temperature what you have studied is the temperature of when which the first droplet of water begins to condense. Correct. So, just 100 percent dry temperature may be taken as dew point temperature sir. Yes. So, unless the temperature has reached the saturation temperature you cannot condense out finally this is the same temperature line is not it? This is completely saturated liquid this is completely saturated vapor once it reaches here only then you can achieve this state. So, that is the same temperature. So, finally it has to be the same dew point temperature it is the saturation temperature. Yes for this vapor pressure this is the boiling temperature that is for sure. So, if the pressure was reduced so much that is where it would have boiled. So, it is already superheated it is beyond boiling. So, the next concept is what is called as degree of saturation. So, again you draw the T S diagram. Now, this is addressing that other question. So, this is where we are this is the vapor pressure line on which we exist and this is the dry bulb temperature the temperature of the surrounding and here is water vapor in superheated state. So, as I said you know you can have different pressure lines here. So, this would mean more moisture. So, at any point if let us say you are here obviously the air can take more moisture it is a it is available that you can put in more moisture into the air. So, you can actually put in steam and try to increase the moisture of air and a lot of time this is done. So, I mean it is not done probably at least in Mumbai it is not done, but in wherever in air conditioning in the US etcetera where it is very dry weather you know if you just try to cool an existing or sorry a heat an existing atmosphere in dry condition. So, it gets very cold in the winter and if I just put a room heater all you have done is just heated the room is already dry and you will feel your skin really not able to take this kind of dry weather at that time people would actually start pushing in moisture by you know pumping in steam into the air and it is necessary because your skin cannot take it otherwise it becomes too dry. So, that means at any point if things are dry I can still go on putting more and more moisture into the air. So, at that point if I put more moisture obviously the vapor pressure is going to increase. So, I will now move from this vapor pressure to this vapor pressure at the same temperature. Obviously, I can heat an increase I could have moved up also, but let us assume that I can move at the same temperature to a higher vapor pressure. So, I can go on and on and on till I reach here. So, you can go on doing this till you reach this point and at this point you are saturated again you cannot put in more water vapor because it will start condensing out the air cannot take it. So, this obviously is the saturated condition corresponding to the DBT to the dry bulb. So, this is an unsaturated condition here this is an unsaturated condition this is an unsaturated condition I move through various unsaturated conditions till I reach a saturated condition. So, every time I am trying to keep the temperature same and pump in more and more water vapor to increase the moisture content in the air till I reach saturation. Obviously, here I will have some omega 1 that is specific humidity I will have some omega sorry I have called as Pb 2 some omega 2 which is higher specific humidity finally, I reach here I will get some other omega which let me call it as omega saturated. So, at a particular temperature I will have a vapor pressure which corresponds to the saturation vapor pressure corresponding to that temperature. So, there is two concepts. So, one is the saturation temperature corresponding to you know the current pressure that will lead me to what is called as a dew point temperature and there is a saturation pressure corresponding to this temperature. So, this vapor pressure line goes like this here. So, I have moved along different vapor pressure lines till I reach here. So, this is the saturation pressure corresponding to the current temperature. So, this is so corresponding to this I will call this as PS this is saturation pressure corresponding to P. Obviously, corresponding to this PS there would be a WS. WS would have been just of course, now I am writing the formula, but you know PS upon P minus PS. So, there is something called a degree of saturation is just omega existing upon omega s as far as I know they should be. This is what is called as a degree of saturation. Obviously, if you are if you have reached the saturation pressure your degree of saturation is 1 you cannot increase it. Otherwise, omega will be always less than omega f your degree of saturation would be less than 1. So, there is a saturation temperature corresponding to the current vapor pressure that is dew point temperature. There is a saturation pressure corresponding to the current temperature which is the saturation pressure which is normally I will call it as PS and corresponding to that I will get an omega s that is specific humidity and the ratio of the specific humidity again is the definition degree of saturation just ratio of specific humidity current to the saturated one. The saturated one is at the saturated pressure. So, most of these definitions are just going to be ratios. So, the next definition is again a ratio and it is a very commonly used parameter it is relative humidity. So, this is just defined as if I take a particular volume then what is the mass of water vapor right now upon what will be the mass of water vapor in the saturated state. Again corresponding to that saturation pressure now if I take put the ideal gas law here. So, we had that PV is equal to MRT. So, and then PS obviously, if I take at the same volume this is what I am doing if I take a ratio here this is RV is same at the same temperature I am getting the saturation pressure. So, obviously, MV upon MVS is just going to be PV upon PVS and normally people will use this for the definition of relative humidity this is going to be a ratio which is less than 1, but lot of times people express it as a percentage. So, expressed commonly as a percentage. So, RH would be just PV upon PVS. So, I mean again just to emphasize this is your PV you are here this is PV this is PVS you have moved along this till you reach here RH is just the ratio of this pressure to this pressure multiplied by 100. So, finally, we are just talking of either the saturation temperature corresponding to a pressure or the saturation pressure corresponding to the current temperature. So, between these two most of our definitions are done I mean whether it is the dew point temperature whether it is degree of saturation or whether it is relative humidity and definitely the relative humidity is going to turn out to be a number less than 1 if I am multiplied by 100 there it will be a number less than 100 and you can say if it is very close to 100 relative humidity is very high that means I am at a line which is very close to saturation and hence you will feel that the moisture is very humid whereas, if I move further and further away it is going to be very dry. So, between I just by this diagram it is good enough they know that all of these quantities can be just you know calculated using the steam table you pick up the steam table go to the current temperature in the room see what the saturation pressure is you will know immediately how to calculate the relative humidity or for the current vapor pressure get the saturation temperature you will know what the dew point temperature is. So, that is the good. Is it more easy to express in terms of specific humidity? So, I mean you will have to justify why do you say that? Because again it is a ratio of pressure but when I say that quantity of moisture divided by quantity of air isn't it more easy. So, let me try to see why you are doing this see whenever I am or you know going to do calculations regarding mixing of air at that is at that time the specific humidity is going to be the far more easy quantity to deal with that is because when I do calculations regarding moist air I will take everything in terms of per kg of dry air in which case how much quantity of moisture is there per kg of dry air it become very easy to me I take one stream of 2 kg of dry air with so much omega that is specific humidity another one kg is another omega 2 if I mix them I know what is the water here I know what is the water here I can just add them because I know it in terms of per dry air. So, at that time for such calculations it is very easy to deal with but if you just tell me the omega specific humidity just like that people do not know how far away from saturation I am. So, as far as weather and atmosphere and how much humid I am feeling that omega number is not going to give me a feeling whereas, a number between 0 and 100 gives me a good feel of it you know 100 means it is really humid. So, to tell the weather relative humidity is definitely the way you know the quantity to use but if you are going to do calculations using for mixing of moist air if relative humidity has been given immediately calculate the omega because all calculations are best done with omega if you are doing mixing calculation otherwise to tell the weather I think relative humidity is really one of the best ways to tell how far away or how immediate. So, you know we have done various concepts here and then finally, there is what is called as the wet bulb temperature again this is a concept that is used often and you would see that in psychrometric or in whenever people are discussing psychrometry they will tell you what is normally called as the wet bulb temperature and there is another quantity they will define normally as an adiabatic wet bulb temperature because the regular wet bulb temperature is defined in terms of just some heat transfer coefficient and mass transfer coefficient and they would not involve thermodynamics in it and hence they will say this is regular wet bulb temperature and what it is is that if I have a regular thermometer so, this is let me draw the T s diagram again to see what is happening. So, here we are at a particular p v now what I can do is this air I can you know out of this air I can start reducing let us say the temperature and one way that normally happens and if you have been in dry weather you know that there is something called as this cooler you know. So, you have this fan with some curtain of air in front of it and it blows air it will work in dry weather that is because what you are doing is you are essentially pushing in water into the air air is reasonably dry once you start putting in water if and the vapor pressure is pretty low once you put in less quantity of water the water will use up energy from the air to dry up and it will become water vapor. So, as it is using up you know water vapor sorry energy to dry the temperature of the air will drop down because it has lost energy in you know converting this water into vapor and that is why you will start feeling cool. So, this is the regular you know cooler that you know obviously, such coolers will not work in humid weather because already things are pretty humid and you want to put in more water into the air you know people are already feeling sticky you will start feeling stickier and that is not going to help too much whereas, if I have let us say this temperature this thermometer and if I you know surround it or you know put one let me change the color here. So, if I surround it with some kind of a wet cloth and push air on it then what is going to happen is if you know the state of air is somewhere here obviously, it can take more moisture and what it does normally is that if I push it over some wet cloth or wet region I mean that is completely saturated and normally that would have been saturated at the dry bulb temperature just before I begin anything. Now, once I start pushing air though it will it will know that there is water here. So, it will start absorbing water from this week and some water will get into the air. So, what you have what happens is that you move to a higher vapor pressure line here because you have put in more water but to vaporize the water because water in this cloth was in water form. So, you have you are going to vaporize it if it has to mix with the water. So, what it does it picks up some amount of energy from the air itself and some amount from the cloth and the cloth temperature drops down and the air temperature also drops down. So, that means that I no longer I am going to be at this temperature this is my dry bulb temperature I would have moved to a higher vapor pressure line but at a lower temperature because now the air has lost energy its temperature is going down. So, I will move like this. So, I will move in a sense between this line and this line. So, everything basically happens within this one is the saturation pressure it is this line here it is just the extension of the saturation pressure line at constant temperature and this is the same pressure line. So, I will move to a higher vapor pressure. So, I will move to one of these lines in between there are so many lines in between till I reach saturation I will move to a higher vapor pressure line, but at a lower temperature and this will keep on continuing I can go on moving moving till I reach. So, I will go on you know the temperature will keep on reducing and the vapor pressure will keep on increasing till of course, you know once I reach here I cannot cross this barrier because now I have reached a saturated state. But at this point you know you have reached a lower temperature and you have reached the saturated state corresponding to the lower temperature and this point normally would have been called what is the wet bulb temperature and if equilibrium is reached then the temperature of the temperature of this wick here this green cloth that I have drawn is the same as the temperature that this air will achieve only once it has passed over it. So, that means initially I will start going down and as the air starts passing over this wick the wick temperature also will drop the air temperature also will drop and now I have reached a temperature which corresponds to this. So, this that means at this point no longer the wick temperature is going to go down because you know the air temperature also you know it cannot go lower than the air temperature at this point. So, what is actually happening is that once you have reached some kind of steady state all the energy that is required to vaporize the water from this wick is actually coming from this air and you have just a balance between how much energy is required to vaporize the water vapor is entirely coming in as you know the energy to go from the dry bulb temperature to the wet bulb temperature. So, that is what is happening in this whole process and you reach this wet bulb temperature. So, now you can show that this is a pretty unique point, but this is not thermodynamic in nature you have just used heat transfer to do this and there are various you know empirical relations available to have a relationship between because this will obviously depend on the vapor pressure you are right now if you are at a different vapor pressure you will have a different wet bulb temperature and obviously if you are at a vapor pressure here you are already at the wet bulb temperature. So, if you are completely saturated you can no longer go down to a lower temperature or a higher vapor pressure line. So, when you are completely saturated your wet bulb and dry bulb temperature are same otherwise depending on the current temperature and current vapor pressure there will be a wet bulb temperature and there are empirical relationships and I have not copied any one of them here, but one of the famous equations is what is called as a carrier equation which relates the wet bulb temperature to T v and the driver. So, again if you show this on the T s diagram people will have a good idea and they will know between what limits you are operating overall the dry and dew point temperature, the saturation pressure and the wet bulb temperature. Once this is done you can probably tell them a brief about the psychometric chart the psychometric chart is normally drawn with dry bulb temperature here and omega here overall you realize that if I knew the temperature the total pressure and T v I can get omega correct because it is just going to be T v upon T t minus T v I can get take the steam tables get P s and if I know P s using the steam tables I can get P v upon P s and say this is R h I can use the steam tables and get the dew point temperature. So, basically if I know these three quantities I know all my other quantities and of course, even degree of saturation which is again ratio of omegas at T v and P s. So, that is conversely if I know let us say P t and T and I am always going to use P t and T because you will always have a pressure gauge and a temperature to measure ambient temperature and pressure and these are two of the most common quantities people will give you you realize that you require only a third quantity I will either I will require P v or let us say someone did not know P v, but they knew that I could take a vessel keep on cooling it and see when you know dew starts forming on it I know that the dew point temperature. If I know the dew point temperature I know that it is the temperature which corresponds to the saturation temperature of P v. So, if I know the dew point temperature I know P v immediately because I just have to go to the steam table figure out at the dew point temperature what is the saturation pressure correct. So, I will just go here I do not know what P v is, but I know what this temperature is I know what this temperature is I know what this temperature is, but I just go to the steam table find out what is the saturation vapor pressure corresponding to this temperature this better be this one. So, the moment I know P v you know that I can solve everything similarly you know if I know R H that is also good enough because I if I know the temperature I know what is the saturation pressure. So, you just go to the steam table get saturation pressure corresponding to D B T. So, I know that is this line if I know R H R H is just P v upon P s. So, P v is just R H into P s obviously, I will put this as a fraction less than 1 not express it as a percentage. So, I will get P v once I know P v again I know all quantity. So, dry bulb temperature and you know the pressure are normally given if I know what is P v I can find out everything I cannot normally get P v, but if you get give me R H that is slightly difficult to find, but always P D P T is pretty easy to find out and W B T is also pretty easy to find out if I know W B T I can get using the carrier equation what is P v and then I can go ahead. So, knowing these two quantities and P v is good enough and to get P v either I should know dew point temperature or I should know wet bulb temperature and that is good. I will get all other quantities required for what I want and then I can go and draw this psychrometric chart this is W this is B B T. So, obviously, if I look again at this T S diagram I take a particular temperature here this is T this is S I take a particular temperature. So, I will know that I can get an omega here I can get an omega here I can get an omega here and I get an omega which corresponds to this which is the saturation omega. So, for every temperature with the I can get a saturation omega. If I go to a higher temperature obviously this P v is a higher one and I get a higher saturation omega. So, if I know a temperature I can get a saturation omega and this is let us say this is increasing temperature this is increasing W. If I have a higher temperature I have a higher saturation omega. So, I will have this various saturation omegas and I will just draw a curve like this through them. And since this is the curve corresponding to saturation omegas it is the curve corresponding to when it is saturated at that temperature. So, it means it is the curve corresponding to 100 percent relative humidity. So, this is what would normally be called as the R H 100 curve. Now, I can similarly draw an R H curve for different omegas that is because I was sorry different relative humidity. So, all I have to do is say let me say relative humidity 90 percent. I will find out saturation I will find out vapor pressure take 0.9 times of it take calculate omega at 90 percent relative 90 percent of saturation pressure and connect all the points I will get a 90 percent R H. And hence I can draw various curve these are what is that yeah sorry I mean I have drawn things here I am sorry it will always start from the end here put R H and omega and T. So, there is one more quantity which is very important when you compare conditioning and which normally people will use we will put H here. And this is because whenever you are doing all kinds of mixing calculations the enthalpy will by default come in it is an open system process you mix things. So, the H calculation is something which people often do and they will put it on that curve H various H curve. And what you have to know is that you will just say that you take 1 kg of dry air if you know the state of the air you know what is omega. So, for 1 kg of dry air you will have omega kg of water. And you just add the enthalpy of this and the enthalpy of this to get the total enthalpy. Now, the enthalpy of this I can start you know 1 kg m C p delta T I can start put 0 somewhere. So, you already for all of these energies and enthalpy that some base you have to put. And you normally know and in most engineering calculation with air and water I will put the base at T is equal to 0. At this I will define the enthalpy of air as 0. So, here if I am just going to measure the enthalpy of air or the temperature is just greater than 0 I will just put it at C p. Of course, things will change if you have sub 0 temperatures and then you will just write it in terms of negative enthalpy or shift your base somewhere else it does not matter. So, it is up to you normally we will talk about positive temperatures that is above 0 you will put C p T for your enthalpy of air. Similarly, if I have water vapor I know it is here and at this point you can explain to people that here also I normally put the enthalpy 0 at saturated liquid condition at 0 degree this is a very common thing that people do. If I put 0 here then if I want this enthalpy obviously, I will first have to go all the way here this is just h f g and then I have reached vapor state. So, I will use whatever is C p for vapor into this T minus 0 this is if I am I was going along the p corresponding to 0 degrees here and reach this temperature. If I move along another line here like this you can show that the specific enthalpy for all these points is not different. So, for example, I need to go let us say to this temperature along another pressure line. So, there is this pressure line it has a different saturation temperature here which is higher than 0 degrees let us say it is 10 degrees. So, now how do I get the enthalpy for this point what I need to do is first I take water at 0 degrees I heat it up to 10. So, first I have to C w that is C of water 10 minus 0. So, first I have heated the water up to 10 degrees then I need to convert it into vapor. So, then I have to add h f g corresponding to 10 degrees because h f g varies with the temperature and then finally, I have to go from 10 degrees let us say dry bulb temperature is 30 degrees I need to go C v 30 minus 10 and only if I add up all this I would have gotten the correct enthalpy starting with the base correct or you know at this same temperature here I have the saturation vapor pressure line I can get directly the enthalpy edge from steam tables. Now you can tell students that they should do an exercise take the steam table take the 0 degree saturation vapor pressure line here I am starting with h 0 right here this roughly corresponds to 2 5 0 1 the h f g at 0 degrees and I will have this enthalpy as 2 5 0 1 plus C v times T minus 0. I will have this enthalpy corresponding to this calculation and I will have this enthalpy directly from the steam table it is the saturation enthalpy at. So, you can tell them to do all this calculation what happens is roughly in this regime since air or water vapor behaves like an ideal gas you will realize that h in a very good approximation is a function of temperature. So, whether I calculate using this method or this method or this method as long as the temperature is same your values of enthalpy should not be too much off they will be off because not really an ideal gas, but and especially close to the vapor dome is not really an ideal gas, but you can still tell them to do this exercise and see that the specific enthalpy is same. And once they are convinced then the easiest formula is to go for this because you have to remember only 2 numbers 1 is 2 5 0 1 which is h f g at 0 plus C v times T and this roughly people take as 1.88 and that is 0. So, once they know how to calculate edge then all you have to do when you come here with this line here is that I can get if I know the omega and a particular temperature I can get the enthalpy then I can join lines of same enthalpy and get equal enthalpy lines and that is something that you do not know. Then enthalpy this sign of enthalpy because negative. Correct. So, I mean it is up to you whether you want to work with negative enthalpy is it is up to you. But negative enthalpy is a stored energy. You should not think of it like this I mean I think you have already seen enough of this curve it just you are measuring some kind of thermodynamic quantity from a base. If you want you are free to shift the base to minus 100 degrees and do all the calculations from there. So, that is your freedom as long as you do it consistently there is going to be no problem because finally you will realize that I need to heat air from this to this I just need the difference that is all I need. So, and one thing which probably does not affect this course, but I have seen it affects lot of time in refrigeration courses is some refrigeration courses they will use this R 12 and they have a particular base for entropy. And some other books have another base for entropy and yeah it is the most common sensical way to look at it. We are used to what is called as 0 degrees it is just you know imbibed in our memory that 0 is some number which we figured out is where ice melts and we have used yes. So, you do you want to use 273.15 is the question it is up to you see the question the thing here is people always work in units which they are most comfortable with and this is you know I am not the person who is making standards here, but you are right if everyone decides that let us throw away Celsius and Fahrenheit scales and let us work only in Kelvin scale probably it will be simpler and if you work only in Pascal it will be simpler, but even now you will see some people work only in bars some people work only in tors. Similarly, if you go to US no one knows what Celsius means they still talk in Fahrenheit now I mean is that something that I have a hold on I do not have a hold on. From the triple point no it is in most of the engineering calculations people put it as 0 degree. So, that is what is the most common you know yes. During rainy seasons yes when we switch on the air condition in cars the dribble the dribble temperature comes down and no moisture formation, but when we are not switching on the AC the dribble temperature actually increases, but moisture is formed on the glass yes. So, this is the inside so this is the inside of the car is it not. So, definitely when you are you know at this temperature let us say you are saying that you are at saturation basically what you are doing is you are exhaling out water vapor and most of that is going to go and you know condense out onto those screen and when you are using an AC you are actually drying out things because you are you know condensing out the vapor elsewhere and putting in dry air and that is why once you have dry air inside the car you anything which is vaporized will just get sucked into the air and it will vaporize and then you are again condensing it out somewhere in the AC machine and that is what is happening. So, whenever you have dry air any existence for example, you know you know for sure that let us say you leave out some amount of water in the room and overnight and let us say it is fallen on the floor. Next day if you come and see it is totally dry you know dry you know that the weather was dry it just got sucked into. So, similarly you ensure a dry atmosphere in using the AC and you just suck out the moisture which has formed anywhere on the glass and that is what. The AC cools it down to much lower temperature yes and is supplied to the car at a higher temperature whereby the relative humidity has come into a lower value and hence that air can absorb. Correct. So, that is what I am saying the AC is supplying you dry air. But I mean what I am trying to say is on the cooling coils it has reached saturation. Yes. But later it gets heated heated to higher value as it comes into the inside of the car. So, that is why the R H value lowers and then able to. Yes. So, what you are of course saying is that if there is a heater along with it then it happens. No, see not all ACs behave like this. Without the heater itself the relative humidity lowers because the inside body temperature or whatever adds to the temperature. Yes. So, what you are saying is that you have actually removed the your cooled it and then heated it. Then that is always possible and that is the best solution in this case but that is not possible.