# A Practical Treatise on Brewing/Lengths and Gravities

CALCULATING LENGTHS AND GRAVITIES.
From the erroneous mode generally practised of calculating the gravities of the taps or raw worts, as they are generally termed, a very great discrepancy appears betwixt what is called the raw and boiled gravity, or, in other words, the gravities shown by the taps, and the actual or real gravities in the gyle-tun.
It is now an ascertained fact, that little or no saccharine matter evaporates with the steam in boiling, and consequently the gravities of the taps, if properly taken, ought exactly to coincide with ​the gravities in the gyle-tun, making allowance only for What may be retained by the hops in the last wort.
We all know that the gravities of the taps, or worts running from the mash-tun, vary considerably, according to the times at which the samples may be taken; thus leading to erroneous conclusions. The only accurate mode, therefore, of calculating the gravities of the unboiled worts, must be by taking samples from the under-back after being thoroughly roused, or better, from the copper when the worts are therein collected, and after about ten minutes’ boiling.
The common practice is even then to calculate the gravity as when reduced to a temperature of 60°, without making any deduction for the decrease in quantity necessary to reduce the worts to that temperature, by evaporation and condensation. We thus calculate a gravity (taking in what is retained by the hops) of at least ten per cent. more than we actually have: making all the difference between the (so denominated) raw and boiled gravities.
The Excise, when taking the dip of hot worts in the coolers, always allowed ten per cent. for evaporation and condensation. The same allowance should be made when taking hot worts in the copper—when we shall find that the raw and boiled gravities will very nearly correspond.
​In making calculations for regulating the gravities per barrel of the different beers to be brewed, it will generally be found that by deducting ten per cent. from the apparent gravity of the taps, as shown by the instrument at a temperature of 60°, we shall come very near the truth, at least near enough for practice. Should extra quantities of hops be used, allowance must be made accordingly. This, however, is only for the first worts. Every 60 lbs. weight of hops retains nearly a barrel of worts of the same gravity as those turned out of the copper. For the second worts, therefore, after deducting the ten per cent., add the gravity of one barrel of the first worts for every 60 lbs. of hops used in the first boiling. This in the second boiling will be replaced by nearly an equal quantity of worts, of the same gravity as turned out of the copper, which, in making the calculation, is to be deducted from the aggregate of the second worts, and so on with a third wort if necessary.
Having thus made accurate calculations of the actual number of pounds gravity we have in the gyle-tun, the aggregate amount of pounds gravity is to be divided by the number of pounds gravity per barrel the beer is intended to be made; the quotient is the number of barrels at said gravity, which can be produced. All we have now to do is to turn out of the copper such a number of barrels as will make up our calculated quantity in the ​gyle-tun. The only difference that can then arise must proceed from the difference of evaporation from the wort in the coolers, which will be found to vary more or less according to the state of the atmosphere. Our beer, therefore, must be a little stronger or weaker accordingly; but the aggregate gravity will be found to correspond very nearly with the calculation made from the unboiled worts; and the brewer, by a little attention, will soon be enabled to judge of the quantities to be turned out of the copper, so as to correspond with the evaporation which he may expect.