You can see that there is quite a lot of error because of the way VLOOKUP works. It finds the highest value in the table that is not greater than the looked up value.

We will now use interpolation to get a better estimate. The easiest way is to add an extra column to the table for dP$/$dT which we can estimate by differences. Use forwards differences.

Then a better estimate of P at temperature T can be gained from

$\_()=\_1+(\_1)(\_)/$

where $\_1$ is the value of T that VLOOKUP chooses from the table.

To put this together:

$1.$ use VLOOKUP to get $\_1($you$\text{'}$ve already done this above$)$

$2.$ use VLOOKUP to get $\_1($i$.$e$.,$ the value found in the first column$)$

$3.$ use VLOOKUP to get $\_/$

$4.$ combine these to create formula to use to get the steam pressure for any temperature in the range.

What happens in T is less than or greater than any value on the table?