Many aspects of brewing require reasonably accurate measurements of liquids. Volume markings on vessels and jugs may not be accurate. If you are measuring water into a vessel with a jug, every time you fill the jug there is the potential for reading error.
The easiest way to measure liquid volumes is to use a stainless steel ruler as a dipstick. Work out the area of the vessel in square centimetres and that is the volume in millilitres per centimetre of depth.
For a round vessel area = π r2
So for example, a round vessel with a diameter of 30 cm has an area of
π * (30/2)2 cm2
3.1416 * 15 * 15 = 706.86 cm2
The volume of the vessel is 706.86 mls per cm of depth. This rounds nicely to 700 mls per cm. If you are adding water to the vessel work out how many centimetres of water are needed to give the desired number of litres. If you want to check the volume of liquid in a vessel, e.g. how much wort you have collected in the kettle, measure the depth in centimetres and multiply by the volume per centimetre. If the liquid is hot, close to boiling, then multiply the volume by 95% to take account of the expansion.
To calculate evaporation rates when boiling the wort simply measure the depth of wort before and after the boil – there is no need to calculate the volume, the difference in depth divided by the original depth multiplied by 100 will give the percentage evaporation rate.
For vessels with an irregular shape at the bottom – say conical or domed, an extra step is needed. Fill the bottom of the vessel with water to where the shape becomes regular. Measure the depth in centimetres from the lowest point of the vessel. Calculate the volume as if the vessel were of a regular shape.
Next drain the water from the vessel and measure it. Subtract this volume from the calculated volume for that depth to get the difference. If you want to work out how many centimetres of water are needed to achieve a particular volume, first add the difference to the desired volume and then divide by the number of mls per centimetre. If you are measuring the amount of liquid already in the vessel, measured depth multiplied by the number of mls per centimetre minus the difference will give you the correct volume.
Using a dipstick and calculator is a quick and accurate way to measure volumes.