After the thermometer, the densimeter is undoubtedly the most important measuring tool for the brewer. Its correct use makes it possible to know quite precisely the quantity of sugar present in the must of beer.
What is the purpose of a hydrometer?
A densimeter is used to measure the relative density of a liquid, i.e., the ratio of the thickness of the liquid to the mass of the water. On a densimeter, the value of the distilled water is 1. Any solution denser than water will have a value higher than one, and conversely, a solution less dense than water will have a cost lower than 1. The density of the must of beer before fermentation is most often between 1,040 and 1,084. Some densimeters are graduated in Plato degree (noted ° P) and allow to measure the density of the wort of beer regarding the percentage of extract by weight. In this case, a value of 1 ° Plato is equal to 10g of sugar per kilo of the must. The scale in Plato’s degree is convenient for the brewer because it allows knowing the amount of sugar present directly in need.
Attention is fragile!
The densimeter is a very fragile measuring instrument and must be handled with care. This must NEVER be immersed directly in the boiling wort. The thermal shock produced by the temperature difference could destroy the density meter.
Influence of the temperature of the must on the measure
Commercially available densimeters are generally calibrated at 20 ° C. It is therefore preferable to measure this temperature. In practice, the temperature of the must is not always at 20 ° C. For a temperature lower than the standard temperature, the need is more viscous, and therefore the densimeter sinks less. Conversely, for a temperature above the usual cold, the must is less viscous, and consequently, the densimeter falls further. There are measurement variations proportional to the heat of the need relative to the standard temperature. These differences must be corrected using the tables supplied with the hydrometer. For example, for a must sample at 30 ° C, the density read is 1.040, but the actual density is 1.043.
When to use a hydrometer?
The hydrometer is a valuable ally in the critical stages of beer making.
- Before cooking the must: the measurement of the density made on the must be collected after the filtration indicates the performance of the brewing plant. Practically, if the target density is not reached or otherwise exceeded, the brewer can then take corrective measures to adjust the concentration of the must.
- Before the start of the fermentation: the measurement of the must density after boiling and cooling and before the addition of the yeast gives a value called “initial density” – OG (Original Gravity) or more correctly “initial extract” – OE (Original Extract ). It is the percentage of sugar contained in the must that can potentially be fermented by the yeast.
- During and after the fermentation: the measurement of the density of the beer at the end of fermentation gives a value called “apparent extract” – AE (Apparent Extract). After removing the alcohol from the beer by distillation, the measurement of the density of the beer gives a value called “real extract” – RE (Real Extract).
How to use the densimeter?
- Depending on the size of the density meter, choose a graduated 250ml or 500ml test tube and put it in a container placed on a horizontal surface.
- Fill the test tube with must (cooled to around 20 ° C) until it overflows into the container. This filling method eliminates the foam formed by stirring the requirement.
- Take the temperature of the must and write it in your logbook.
- Make sure the densimeter body is clean and dry.
- Slowly lower the hydrometer into the must.
- To feel that the densimeter floats, to turn it on itself by releasing it.
- Avoid the densimeter sticking to the wall of the test tube, which would distort the measurement.
- Naturally, a concave meniscus will be created around the hydrometer.
- To make a correct reading of the density, look at the specimen precisely at the height of the meniscus.
- For clear liquids, read the value at the bottom of the meniscus.
- For cloudy or dark liquids, read the value at the top of the meniscus.
- Note the density value in your logbook.
- Rinse the hydrometer with water at 20 ° C, wipe gently and store in its box
- Store the box upright
Check the accuracy of the hydrometer.
Following the method described in the previous paragraph, first, measure the density of the distilled water. It should read 1. Then prepare a sweet solution by mixing 100g of white sugar with 900ml of water at 20 ° C. Measure the density. This must be 1.040. If your hydrometer measures a value that is slightly above or below, report this difference on your next density measurements.
Conversion between ° Plato and SG
Even if the conversion is not linear, for ease, it is considered that 1 ° Plato is approximately 0.004 points of relative density. For a more accurate conversion, use the following formulas:
Plato degree = 259 – (259 / SG)
SG = 259 / (259 – degree P)
Almost all the densimeters are calibrated to take a correct reading at 60ºF, which is about 15,556ºC, say 16ºC. So if you interpret the density of your sample when it is at 16ºC, the value you are getting is correct. If the temperature is different, you will have to add or subtract a correction factor to the density value that your hydrometer gave you. Below is a table with correction factors corresponding to each temperature. So if, for example, the initial density of your stout is 1,054 and you took the reading at 31ºC, the actual initial density is 1,058.
If you are using a hydrometer that already has an integrated thermometer, what you have to do is see what temperature marks on one side of the thermostat and what correction factor marks the other side of the thermostat.