 The last thing that we're going to take a look at in this lecture is the idea of the way that you can measure the relative humidity of the air using two thermometers and one thermometer is just measuring standard atmosphere conditions it's what we call a dry bulb thermometer and the other one is a thermometer with which there is it could be a cotton swab that is wet and consequently that would be measuring what we call the wet bulb temperature which would be temperature with a hundred percent relative humidity fully saturated air so the concept or what we're going to take a look at is dry bulb wet bulb temperatures now there are electronic ways that we can measure these things probably using things like conductivity of the air but this is kind of an older traditional way it's a low-cost way of measuring the specific or the relative humidity I should say the air so what it would consist of is one thermometer so there we have what we call the wet bulb and the dry bulb temperatures the dry bulb temperature is just measuring standard room air and the symbol that we use for that is TDB and the wet bulb temperature what we do is we take our thermometer we put a cotton swab on it we put the cotton swab then into water and water will go up through this wick through a wicking effect and what it will do is it will create fully saturated conditions or we have a hundred percent relative humidity for that measurement and that is what we call the wet bulb temperature with a combination of these two we can then go ahead and do calculations specifically what it enables us to do is to determine the relative humidity of the air so we may not know it but with this measurement we can determine it so I'll give you the equations now that enable us to do this so there are equations that enable us to do this and in setting this up what we will say is we have state one and that would be our dry bulb and for this state we're trying to find the relative humidity so we do not know the relative humidity there we do know the temperature at state one is equal to what we will call the dry bulb temperature and then state two this is the wet bulb temperature here the relative humidity is a hundred percent it's fully saturated and temperature at state two is equal to the wet bulb temperature so those are the two states what I will now do is give you the equations that you can use to determine both the specific humidity as well as the relative humidity so those are the equations you can use you can see here that it's going to require going into the steam table and getting different parameters from the steam table and with that however you will be able to determine the specific humidity as well as relative humidity for your room conditions with only two temperature measurements and so that's the idea of a wet bulb and a dry bulb temperature measurement with that that will conclude today's lecture what we'll be going into next time is looking at a way to make these calculations a little quicker and more efficient and that's using a tool or a chart called the sacrametric chart and the equations that we've looked at have been a little on the laborious side I mean there are other things that you can work with however engineers like shortcuts and the sacrametric chart enables us to make those shortcuts that's what we'll be looking at next time but with that that concludes today's lecture thank you very much