How do you know how much liquor to prepare? There are three things to think about.
Suppose you want 23 L in the FV at the end of the day, you will clearly need to start with at least 23 L. The problem is that due to the process and equipment, if you start with 23 L you will end up with considerably less in the FV - you don't want to spend all day making a pint of beer!
You can't (unless your design allows for it) drain all the liquid from a vessel; some will be left behind - this is the 'dead space'. To determine the dead space is easy. Fill your vessels with a few litres of water and drain them. Make a record of the water left behind in each vessel. These volumes will be pretty constant so whatever they are, you need to add them to your total liquor volume. For instance, my boiler has a dead space of 1. 15 L and my mash tun 0.2 L.
The grist in the mash tun will absorb liquid. It's not a good idea to squeeze them because you can extract unwanted tannins, so you just have to live with it. Hops and trub will also absorb liquid.
I find that I lose around 1 L per kg of grain and around 250 mL per 10 g of hops.
When you boil the wort you will obviously lose liquid as steam. It's generally accepted that you lose around 5% every 30 min, which is 15% for 90 min.
This is easy. You decide what you're aiming for, depending on the capacity of your fermenting vessels.
So, let's think about how much wort you need in the boiler at the start of the boil.
Target volume + the loss to the boiler's dead space + the loss to hops and trub.
When you boil this volume it will evaporate. Therefore you also need to add the loss to evaporation of the above volume.
Put some values in:
23 + 1 + 2.5 = 26.5
If we boil 26.5 L for 90 min and lose 15 %, that will be 3.975 L , so we need to increase the Boiler Volume by at least that amount.
26.5 + 3.975 = 30.475 (say 30.5 for simplicity)
Okay that's straight forward enough; how do you make sure that after mashing and sparging you achieve that volume?
The Total Volume is Boiler Volume plus whatever we need for mashing in and sparging.
The first thing you need to consider is initial mash liquor volume, which is determined by the ratio of liquor to grain. A usual ratio is 2.5 L liquor to 1 Kg grain. For a 4 Kg grain bill you will need 4 * 2.5 L of liquor, i.e 10 L.
This is the volume you use to mash-in with. Fill your mash tun with this volume at the start of the day.
When you batch sparge, the prevailing wisdom is that the best efficiency is gained when you collect two equal volumes from the mash tun. Therefore you'll need to collect half Boiler Volume from each sparge. Easy.
After you've mashed, how much liquor do you need to add to the mash tun for the first sparge?
As already stated, you want to collect half the Boiler Volume, i.e if you're aiming for 36 L in the boiler, you want 18 L from each sparge. We've added 10 L liquor to mash-in with already, so we clearly need to add more. "How much more?" is the question; 8 L would seem about right. But it's not enough. Why?
The mash liquor has been absorbed by the grist and also filled the dead space in the mash tun. Therefore we need to add those two losses together and then add the extra 8 L.
How much liquor is absorbed by the grist? Around 1 L per Kg is a usual figure. The second batch won't need this extra volume, because the grain can't absorb it twice (same for the dead space; once it's filled - it's filled).
For instance we want 18 L per batch
Which results in a top-up volume of 12.5 L.
The second batch is simple. The losses have already been accounted for. All we need to do is add the same amount of liquor that we want to collect. In this case, just add 18 L.
When you have collected all the wort in the boiler, you can measure the SG and see whether or not you are on track to reach your target SG. To do this, you need to use 'gravity points', which is easy, just use the last two digits from the SG measurement. For instance, a reading of 1.030 will be 30 gravity points
The amount of sugar in the wort is fixed and can't be altered, but you can change the volume of the wort, thus altering its density and the SG. This is not strictly true, because you can add malt extract or sugar to increase the amount of sugar in the wort.
For instance if, after sparging, you have 30 L of wort with 30 gravity points and you have a target volume of 20 L after the boil, you will decrease the volume and increase the gravity points. How many gravity points will it now have?
You can calculate thus:
GUe = (GUb * Vb)/Ve. Which gives you: 30*30)/20 = 45 an SG of 1.045
Looking at the problem another way, say you have 30 L in the boiler at 30 gravity points and you want to end up with a wort of 1.038, what volume do you have to boil down to?
(GUb * Vb)/GUe = Ve. Which will give you:(30*30)/38 = 23.7
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