 So, we have looked at all the 7 parameters, now we will look at the lifting gas purity Y. So, for this once again you will have to go back to your notes and bring out this particular formula, this bigish formula which tells you about the weight of the lifting gas in terms of the purity of the gas Y, ratio of pressure plus super pressure, temperature plus super heat into I into K into V. And this RD PG is the relative density of the pure gas, this is a fixed number, it depends on the whether you use hydrogen or helium or any other lifting gas. So, the value of 1 minus RD PG is a fixed number depending on the LTA gas that you use. So, in other words, it will be 1 minus K, some constant K into Y into the pressure upon temperature into I into K into V, this will be the WLG. If that is the case, let us calculate. So, now I want you to derive this expression yourself. Just work out the expression for delta WLG, change in the lifting gas weight because of the change in the purity. So, now you will have Y1 and Y2, but these other things like delta PSP, IKV will remain the same. But recall, I is going to change. No, it does not change, sorry, I does not change with humidity. There are two parameters, humidity and lifting gas purity due to which I was not changing. I was changing because of the other parameters. So, I changes when either the pressure or temperature changes, super heat or super pressure. But I did not change with the relative humidity and with the lifting gas purity. So, can you derive the expression please? It will be 1 minus RD PG Y1 PS plus this minus same thing, same thing, same thing into Y2, rather Y2 minus Y1. So, what will happen is that this constant quantity 1 which is in both the quantities will subtract and 1 minus RD PG Y2 and 1 minus RD PG Y1 will come inside the bracket and other things are all constant. Now, if we ignore delta PSP and also we consider that there is no super heat, then this equation simplifies into 1 minus RD PG Y2 minus Y1 PS by TA IKV. So, that is it. It is a simple expression which simplification is possible only if you neglect super heat and super pressure. Otherwise, you get the expression which is a slightly larger expression.