There are different definitions for "theoretical plates". In our calculators it is always valid, no matter whether theoretical plates are to be calculated or entered:

• Each theoretical plate means a complete step in the boiling point diagram. It does not matter how this step is done. So it doesn't matter how many real plates or packing height are needed for this.
• An insulated potstill with a low riser normally has exactly one theoretical plate. If there is reflux and a condensation surface, for which an uninsulated lid is sufficient, the potstill has more than one theoretical plate.
• A reflux still with 10 real plates has 10 theoretical plates at 100% reflux. These 10 real plates are either 10 real plates with an efficiency of 100% or a higher number of plates with lower efficiency, which together provide the same separation as 10 with 100% efficiency. However, these 10 real plates with 100% efficiency can of course also be a packing height with the same performance.
• Fractions of theoretical plates:
  This is more complicated than it seems at first. There is no rule, for example, what 1.5 theoretical plates are. But a definition is important. Especially when evaluating the separation performance of a potstill, you need the decimal place. And what it means exactly, you have to decide if you want to calculate something. There are three possibilities: according to alcohol strength, i.e. either without contraction (% by weight or % by mol, which is mathematically the same) or with contraction (% by volume), or according to temperature. Here is the test whether this is relevant at all:
  Assuming with 1 theor. plate you get 40%abv and with 2 theor. plates you get 79%abv, 1.5 theor. plates are the middle, so 59.5%abv.
  But now everything converted into %abw (alcohol by weight):
  40%abv = 33.3%abw, 79%abv = 72.3%abw, so 1.5 theor. plates are the middle: 52.8%abw. And this is converted 60.7%abv, which is significantly more than the previously calculated 59.5%abv.
  And if you calculate by temperature:
  Vapor with 40%abv has 96.1°C, with 79%abv has 83.8°C, so 1.5 theor. plates have the middle: 89.95°C (all of course related to normal pressure). And this temperature has vapor with 62.9%abv, which is also significantly more than the 59.5%abv calculated before.
  So you have to decide to what you refer the decimal places. Our calculators calculate with %abw.
  But there is a second problem. Namely, it is not really logical, if 1.5 theor. plates are exactly in the middle between 1 and 2 theor. plates. Because 2 theor. plates are also not exactly between 1 and 3. The boiling point diagram curves are not straight lines. Here is a diagram showing the %abw in the vapor as a function of the theor. plates, starting from 5%abw in the still and with linear interpolation between the integer theor. plates:
  You can see a kink in the curve for every integer theor. plate. Especially at 2 theor. plates. This means that the step from 1.8 to 1.9 theor. plates is the same as the step from 1.9 to 2.0, but the step from 2.0 to 2.1 is much smaller. This is illogical. Therefore, we have solved it this way on our calculators:
  Here, instead of a straight line, the points are connected by a monotone cubic interpolation.

The consideration of the air pressure has a significant influence on the result. If nothing is entered, the calculator assumes the local atmospheric pressure 1013.25 hPa.
Since almost no one has an absolutely accurate thermometer, an additional "thermometer error" can be specified. This can be determined with the help of the calculator Thermometer Error. Temperatures entered here are then those displayed on this thermometer, not the real ones.
However, there is of course no point in entering the thermometer error if you do not enter temperatures in this calculator but the alcohol strengths in both the boiler and the vapor. This is why this input field is then deactivated.

Information about our boiling point data and the influence of atmospheric pressure