A hydrometer is a relatively simple measuring device.
It works on the same principle as floating in the dead sea. The dead sea is so natural to float in because it is full of dissolved minerals. Much like in the Dead Sea, the more dissolved sugar there is in the solution, the higher the hydrometer floats, giving a higher reading on the hydrometer's scale.
If you fill the trial jar, there will be no room for displacement when you add the hydrometer. Lower in the hydrometer and give it a spin.
It is important to remember to spin the hydrometer to release any trapped gasses before taking a reading.
Once the hydrometer is steady in the liquid, you can take a reading.
A reading is taken from where the bottom of the meniscus crosses the scale on the hydrometer.
This reading is called the Start or Specific Gravity (SG)
The meniscus is the concave or convex upper surface of a liquid in a tube.
In brewing, the meniscus will be concave, U shaped, which is why we take the reading from the bottom of the meniscus.
As the sugar is fermented and turned in to CO2 gas and ethanol, which is a liquid, the hydrometer will sink lower in the solution, giving a lower reading.
When fermentation stops, either by stabilising or all the sugar is fermented, measure the gravity again.
This reading is called the Final Gravity (FG).
A hydrometer can be used in a few ways. The most valuable is to measure the SG and the FG and use the simple equation below to measure the ABV. (Alcohol by Volume)
SG - (minus) FG= Variance (V) then multiply the variance by 131.25. (V x 131.25)
e.g if SG = 1.055 and FG = 1.010
(SG) 1.055 - (FG) 1.010 = (V) 0.045.
The variance is then multiplied by 131.25 as follows.
(V) 0.045 x 131.25 = 5.9 so the drinks ABV will be 5.9%
The greater the variance, the higher the ABV.
If fermentation is not stopped artificially, the fermentation will naturally stop between 1.000 and 1.010 when all the sugars have fermented.
Fermentation is only stopped early, by stabilisation, when the brewer wants to retain some sugars in the drink. Stopping the fermentation is more common in wines but, for home brewers, this is quite a complicated exercise.
Most brewers will allow full fermentation then add sugars after stabilisation to reach the desired sweetness.
TOP TIP: Once the wine has been sweetened to the desired level, measure the gravity with your hydrometer for future reference.
For more help with making sweet wines, please feel free to contact us either by
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The other use is merely measuring gravity when fermentation has stopped to ensure fermentation is complete.
If fermentation is not complete, i.e. the hydrometer is not reading 1.010 or below, then fermentation has stuck and needs re-starting.
If fermentation is not complete, it typically requires a "re-start" yeast. If this does not work, contact us on (0)114 2746850 for further advice.
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