How do you use a hydrometer?

Home brew equipment hydrometer

Measuring the specific gravity of your wort and beer is good brewing practice.

Home brewing, because it is a hobby, suffers from what I can only describe as equipment overload.  You will discover this for yourself if you walk into any home brew shop.  Take a look at the shelves and they are packed out with gizmos that claim they can take the effort out of home brewing.  I am a strong advocate of the idea that you don’t need much equipment to brew at home.  But that does not stop people coming up with new ways of doing things, re-inventing the wheel some would say.  Now before I am accused of being a head in the sand progress-a-phobe I am not anti innovation.  In fact if it makes my life easier and makes the hobby of home brewing more fun I am all for it.  But let’s face it we have all been suckered into buying a piece of home brew equipment only for it to sit and gather dust at the back of the cupboard.

Now I am glad to say that there are some pieces of home brew equipment that, because they work so well, have missed out on an innovative make over which generally makes it more expensive.  The home brew hydrometer is one of those pieces of equipment.  Ingenious in its simplicity the hydrometer is also, in my opinion, a must have piece of equipment in any home brewers tool kit.

What is a hydrometer and how do you use a hydrometer?

Before we can answer that question it is perhaps worth discussing what a hydrometer measures.  An important analytical parameter to know, and therefore to measure, in brewing is the specific gravity of your wort and beer.  The specific gravity, in scientific terms, is the density i.e. weight per unit volume of a substance divided by the density of water.  Water in this case is used as the standard reference point.  The measurement of specific gravity also requires a constant temperature and pressure which is usually 20ºC and 1 atmosphere of pressure (101.325 KPa).  The density of water is defined as being 1.000 and the specific gravity of a solution is then either higher or lower than water.  Thus a solution with an SG of 1.050 is 5% more dense than water so will be 5% heavier.  Typically the specific gravity is used as a simple way of defining the concentration of simple solutions such as brine and sugar solutions such as brewer’s wort.

As I mentioned earlier in this article the hydrometer itself is brilliant in its simplicity.  Manufactured from glass it consists of a long cylindrical stem which is weighted at one end with lead.  The cylindrical stem usually has a scale inside it so that the specific gravity can be read directly.  The sample of wort or beer that is being measured is placed in a measuring cylinder.  The hydrometer is lowered gently into the liquid making sure that it is able to float freely.  The hydrometer will sink into the liquid and once it has come to a rest the place where the surface of the liquid touches the hydrometer is noted and the scale at this point read.  This is the liquids specific gravity.

Why is it important to know the specific gravity of your wort or beer?

It is always good brewing practice to measure the starting specific gravity of your wort before you pitch your yeast.  By measuring the starting or original gravity you are establishing the starting point of your fermentation, which can be used to give you an idea of the progress of your fermentation.  If you also measure the final gravity of your beer once fermentation is complete these two figures can be used to calculate the theoretical alcohol content of your brew.

It is also good to do this if you are brewing with liquid malt extract.  The gravity of liquid malt extract will vary and the manufacturer usually provides the home brewer with an idea of the specification.  For example take a look at the specification on the side of a can of liquid malt extract and you may find that the specific gravity can vary from 1.421 to 1.440.  Therefore to prevent over or under dilution it is always a good idea to measure the specific gravity.  I always add less water than is requested in the instructions for the kit, I then measure the specific gravity and then dilute further if I need to.  That way I am able to tightly control the starting gravity of my wort and prevent over-dilution.

At the end of fermentation it is always a good idea to measure the gravity just to make sure that the fermentation has stopped.  Most home brew kits suggest fermenting until you reach a gravity of 1.014 or lower.  This is because you don’t want excess fermentable sugar, which should have fermented out during primary fermentation, going into your bottles.  If you add priming sugar, and there is still some residual fermentable sugars remaining in the wort, it could give rise to excessive carbonation of your beer.

how to use a hydrometer

Read the specific gravity value from the bottom of the meniscus.

Finally to finish off here are some of my top tips for the accurate use of your hydrometer.  In essence  a how do you use a hydrometer quick guide.

  1. Always clean and sterilise your hydrometer before and after use to avoid introducing unwanted spoilage microbes.  I tend to put the hydrometer in a solution of cleaner and steriliser 15 minutes before I need it.  Something like Miltons is ideal for this job.
  2. I like to use a glass measuring cylinder/trial jar to hold the sample I am measuring.  I don’t know whether there is some form of electrostatic interaction between the plastic measuring cylinder and the hydrometer but the hydrometer never seems to float freely and always pulls to the side of the trial jar.
  3. Give the hydrometer a quick spin as you put it into the wort or beer.  This knocks off any bubbles which impacts on the buoyancy of the hydrometer and can lead to inaccurate readings.
  4. When measuring the final gravity of beer the carbon dioxide in the beer can form bubbles on the surface of the hydrometer which again can give rise to inaccurate readings.  I tend to degas the beer first by either stirring or tipping the beer between two glasses to release the CO2.
  5. Always take the reading on your hydrometer from the bottom of the meniscus.

That is it for this quick article How do you use a hydrometer.  We hope that you see the value in having such a crucial piece of home brewing equipment.



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