Controlled water dilution

by Simone Forgia
Controlled water dilution

Much has already been written on the subject of water for coffee, and thanks to research carried out in recent years we are now beginning to understand the most important characteristics that water must have in order to best extract the substances we want from the coffee grounds and not alter the taste. According to the Specialty Coffee Association (SCA), the water should be odourless, clean, chlorine-free, ideally have a pH of around 7, an alkalinity of around 40 ppm (parts per million), a general hardness of around 68 ppm, etc. Some of these characteristics are clearly subjective and different values may be preferred depending on taste preferences, but at the same time they have certainly helped to create a standard and ensure that everyone can speak the same language. With this came however the problem of how to arrive to those values. Many solutions have been developed, from filters directly connected to the water line to filtering jugs and up to the complete remineralisation using distilled water or reverse osmosis, each with its own advantages and disadvantages.

The filtering jugs, for example, are easy to use but often represent a generic solution with little customisation. They are fine for removing chlorine and other undesirable substances, softening the water and in some cases for adding magnesium, but they unlikely allow to reach very precise predetermined values.

Filters connected to the water line on the other hand certainly allow greater customisation, with many choices available for various needs and the possibility in some cases of choosing a "bypass"; meaning a percentage of water that remains in its natural state and is then mixed with the filtered water. The limitations of this method, however, are mainly three; the first is that this system usually works with ion exchange resins and therefore calcium and magnesium are not simply removed but are replaced with sodium (SCA recommends to keep it at values around 10 mg/L) and therefore does not allow to change the TDS (total dissolved solids) of the water. The second limitation is that the filters act uniformly on the hardness of the water and therefore it is not possible to only lower the temporary hardness and compensate to keep the general hardness constant. Finally, the third limitation is that they are only suitable for those who want to lower the values that are too high, and not also vice versa.

The complete remineralisation of water is certainly the most attractive option for those who like to experiment and try out different values and their effects. By preparing several solutions, it is possible to calculate how much of each solution should be added to achieve the desired result. The disadvantage of this system is that you cannot use your own tap water and you need to have a continuous storage of distilled water unless you have a reverse osmosis system; but this type of filtration also has its downsides as it is often inefficient. In fact, with small domestic systems, it can take more than 5 litres of water to obtain one litre for use.

What we want to introduce to you is a concept that is a bit different from the ones we have seen so far, which also has its limitations and still needs a large pool of users with various types of water for its validity to be confirmed, but which represents a good compromise between the current systems and which we will call Controlled Dilution System (CDS). This system is based on the principles of complete remineralisation mentioned above, but assumes that we often already have more minerals in our water than we need, and we can therefore use our tap water as a starting point to dilute instead of just using distilled water. In most cases, however, we do not want to dilute the concentration of all the minerals in the same way and that is why we call this system "controlled dilution" because with the help of precise formulas it is possible to subsequently add small quantities of previously prepared solutions to compensate for values that have dropped too low. These solutions, which are the same as those used for the classic remineralisation, work mainly to counterbalance the temporary hardness, the permanent hardness and also have a direct impact on the TDS of the water, which are the three most important parameters for coffee extraction; but let's start with a brief simplified overview of what these terms mean. 

With the term hardness we mainly indicate the amount of calcium and magnesium salts dissolved in water. In the latter, however, there are also other substances, including bicarbonates. When the water is brought to the boil, the bicarbonates interact with the calcium and magnesium salts (ions) and cause them to precipitate. This means that these salts are no longer dissolved in the water, but fall to the bottom of the water and create deposits such as limescale. If enough bicarbonates are present, then the whole amount of calcium and magnesium will precipitate, but this is not usually the case, in fact in most instances a percentage of them remains dissolved in the water and represents the permanent hardness, while the other part that has reacted with the bicarbonates is called temporary hardness. 

In the world of coffee it is common to reach high water temperatures, normally above 90°C or in the case of pressurised systems as on some lever machines even up to 120°C; at these temperatures the bicarbonates react with the calcium and magnesium salts causing them to precipitate as explained above and therefore the part of permanent hardness is of particular interest to us because it is mainly the one that will then extract the substances from our coffee. The temporary hardness part, on the other hand, is of interest to us more from the appliances point of view because a value that is too high would lead to an excessive formation of limescale in our equipment, but a value that is too low is not good either because it could actually make the water corrosive, ruining also in this way our machines. In addition, a high amount of bicarbonates can affect the alkalinity and therefore the perception of acidity in the coffee, dampening it.

The term TDS refers instead, as the acronym says, to the Total Dissolved Solids, also known as fixed residue. This value represents the sum of all the mineral salts present in the water and is therefore largely influenced by hardness, but also takes into account all the other organic and inorganic elements that may be present. To measure the fixed residue, a given quantity of water should be completely evaporated and, after this process the remaining dry solids should be weighed with a high precision scale. Fortunately, there is a close correlation between the total solids dissolved in the water and its electrical conductivity, and there are therefore fairly inexpensive devices that exploit this characteristic to give an approximate indication of the TDS in short time. Distilled water is simply water from which all these elements have been removed and therefore has a TDS equivalent to 0 and conducts almost no electricity.

Now that the basic concepts are clear, the question is: how do we measure these values and how do we change them? As explained above, for the determination of the TDS there are rather inexpensive devices that measure the electrical conductivity and multiply this value by a certain factor in order to give an estimate. The multiplication number varies from reader to reader and the use for which they are designed. For a better standardisation of the results, we therefore recommend manually multiplying the value of the electrical conductivity in µS by 0.65 (which is also the value recommended by SCA). To give an example, if we measure a conductivity of 220 µS and multiply it by 0.65 we get a TDS of 143 ppm. Alternatively, you can buy a TDS reader that already works with this ratio if you can find one but they are not easy to find.

Measuring kit for GH, KH and Hardness and Buffer solutions

Measuring kit for GH, KH and Hardness and Buffer solutions

For general hardness on the other hand there are little kits usually called GH Test. Simply take a small amount of the water you want to test, usually 5 ml, and add the liquid of the kit one drop at a time until the solution changes colour. By counting the number of drops you are then able to know the degree of hardness of the water and if you want to increase the accuracy of the measurement simply double the volume of water used (10 ml instead of 5) and finally divide the number of drops counted by 2. The same applies to the measurement of temporary hardness but this requires a further clarification. There are no commercially available kits for measuring temporary hardness in the same way as general hardness, however, there are kits for measuring the carbonate hardness (KH Test). Speaking of drinking water and without going into too much technical detail, in most cases carbonate hardness is the same as temporary hardness, so KH tests are also used to measure this latter value. The kits for measuring general and carbonate hardness can often be found in aquarium shops as they are also widely used in this area, but it is important to point out that the values are usually expressed in German degrees (°dH) and it is therefore necessary to convert them subsequently into ppm.

Controlled Dilution Water Calculator

Screenshot of the calculator that can be found here

Once the values of our own water are known, the only thing left to figure out is how much we want to change them. To lower them we have distilled water, to increase carbonate hardness the "Buffer" solution and to increase general hardness the "Hardness" solution. The calculator on The Lever Magazine website allows you to identify exactly in what proportion to add these three elements. As for the solutions, they can be recreated with ingredients easily found in pharmacies or supermarkets, such as sodium bicarbonate and magnesium sulphate (also known as Epsom salt) and of which details can be found on the calculator page.

Let's now take the real example of a water with a total hardness of 80 ppm, a temporary hardness of 53 ppm, a TDS of 120 ppm and let's assume that we want to bring it to the parameters recommended by the Specialty Coffee Association with the help of the Controlled Dilution System. Using the calculator on our website it is possible to enter the data we have measured, and then by playing with the various cursors it is possible to modify the quantities of "Hardness" and "Buffer" solutions to be added and see in real time how they react. The amount of distilled water to be added to dilute our tap water will be calculated automatically in order to obtain a total of 1 litre of water to use. For example, in the case we examined (visible on the image above), we found that by taking 650 ml of our starting water, diluting it with 328 ml of distilled water, adding 16 ml of Hardness solution and 6 ml of Buffer it is possible to reach the optimal SCA values for general and carbonate hardness and to fall within the recommended range for the TDS.

Clearly this system is not suitable for high volume coffee shops and a good tasting base water is required first, but for those who are at home and have time on their hands it is certainly very interesting to experiment and try out how the coffee taste changes using different parameters. In addition, there are more options to achieve a specific result because as you can see from the picture below, you could reach almost the same values as in the previous case with 760 ml of tap water, 7 ml of Hardness solution and 233 ml of distilled water, avoiding the use of the Buffer and using even more of the starting water.

Alternative water recipe without buffer

Alternative water recipe without buffer

Finally, it must be said that the flexibility of the Controlled Dilution System in our opinion also lies in the fact that it can optionally be used in combination with a filtering carafe or other filters to remove chlorine and other undesirable substances. Moreover, it leaves up to the user to choose the parameters he or she wants to achieve as, at the time of writing this article, even the SCA is reviewing the standards published back in 2009 and there is still much to discover. The important thing for our system is that the water that will be diluted is of good quality. What do you think? Try this system and let us know your experience.

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