TDS, Chemex and London’s terrible water

So thanks to the lovely people at Bunn I have a couple of TDS meters and have begun to poke them into various coffee drinks.

The first thing that upset me was just how hard London’s water is. Out of the tap I get 410ppm, which is pretty hard. This means it is not ideal for brewing coffee as it is less “hungry” for new solubles than softer water. I didn’t realise until I started testing just how badly it was affecting the coffee.

Chemex and TDS

Testing some of Tim’s coffee brewed on the skinny Chemex

I was struggling to get into the Golden Cup zone of 18-22% extraction which, through years of filter brewing, is considered a suitable target. Different countries might like different strengths of coffee but they all generally like 18-22% of whatever dose that is.

I have know switched to bottled water for brewing (with a much more attractive TDS of 130ppm) and the coffee does indeed taste much better and my extraction percentages are up even though I haven’t changed the grind or dose. I had gotten used to a very coarse grind on the Chemex (after being advised by Kyle in but have slowly been bringing it finer to get the percentage up. I will keep doing more but wonder how many other people out there have TDS meters and whether they have compared their brewers with different grinds and doses and compared the cups after.

To this end I’ve done a little spreadsheet that negates the need for a brewing chart as the maths is built in. It works on 2ml of water being absorbed by each gram of coffee but you can change that too. Input the amount of coffee you use, the amount of water you brew with and then TDS measurements. I’ve left columns in there for other useful data – such as brewer, coffee used and notes on grind settings. I probably should have found a way to include a cupping score/notes but haven’t. If anyone else fancies having a go then do download it and let me know how you get on. Once I have enough data I will post up about my Chemex experiments.

Thoughts and comments welcome….

XLSTDS Testing Spreadsheet


Mike Khan from Bunn sent me his spreadsheet which does percentages and graphs for each brew you do.
XLSMike’s TDS Calculator


Thank you Mike!

12 Comments TDS, Chemex and London’s terrible water

  1. Ian Clark

    Someone with programming skills should make the community a little program that takes TDS calculations and displays the result graphically on one of those solubles yield/concentration charts. Now that would be sweet!

  2. Jaime

    Water is very important and I learned a long time ago how much it can negatively affect a brew. Cirqua makes a fantastic water formula over here but if you don’t have a good roast, a good bean, and understand the brew method, what’s the point in quantifying the output? If the machine and method are flawed… I’m not sure I understand what get what you gain from measuring TDS in the brewed cup. How does it taste?

  3. Mike

    Ian, I have just made a rough version of what you are suggesting. It’s rough because I just got to the end of my programming skills….. I’ve sent it to Jim, if he thinks it’s worthy, maybe he’ll make it available for you to download?

  4. James Hoffmann

    Jaime – you make a good point about the work of TDS metering but I guess for me the revelations it has given me in regards to my home brewing are similar to a naked portafilter.

    I get sent a fair amount of different coffees and it is unlikely I will have dealt with them as green or had a chance to cup them previously. My expectations of them is based on what is on the bag (Assuming I can read the language) or conversations I had had with the roaster. If I get the brew a little bit wrong, and the coffee is disappointing I can point the finger of blame one of two ways – me or the coffee. The TDS meter just gives me a better idea of whose fault it is.

    A naked portafilter reminds you your espresso could always be better, but you soon learn to trust your tastebuds for the final say as some ugly shots taste great, and some beautiful pours taste deeply disappointing. A chemex of Tim Wendelboe’s coffee might taste very good, but maybe it could taste better….

    Mike – thank you for the e-mails! I’d love to upload the xls file!

  5. true

    As my own shop moves closer to completion, I’ve turned my attention to my tea service. Now I’ve probably been interested in quality tea longer than coffee, and I have a fairly fine-tuned sense of what makes good tea good. I understand and use the appropriate temperatures and brewing methods for each tea. But I’ve never really thought about fine-tuning my water for tea. When I was talking to a tea buyer the other day, I mentioned that I have a cirqua system for my coffee. The tea buyer was adamant that what was appropriate water for coffee was not appropriate for tea. Apparently, harder water makes for better tea– which makes sense if you think about the extraction process. It’s a bit of a stretch, but perhaps there’s a watery reason for a cultural/historic bias towards tea in London…

  6. jason Scheltus

    Not only that, but also in terms of exact location and time, London’s water changes a LOT. And because they’re upgrading the pipe network in central London, mains pressure is also an issue occasionally.

  7. Al

    Not about to add calcium to my soft tap water in an attempt to emmulate Thames Water’s product. Would agree certain teas are better suited to different types of water, although I’ve never personally found one to suit south east water!

    The “oily” scum of calcium on the surface only further detracts from the cup. Granted this is not visible if you add milk!

  8. Mike

    My experience is that soft water is much better for ICED tea, As it retains its clarity after it cools, and it has a ‘smoother’ taste I’m not sure that clarity is so important with hot tea, particularly if you add milk. Using soft water for hot tea will require a shorter steeping time, as the water is ‘hungrier’. If you use the same water regularly, you may be unaware that your usual steeping time is only appropriate for the water at your location. I am not a big tea drinker, so am reluctant to comment on taste…. but I have lived in a hard water area all my life, maybe that’s why I don’t drink tea?

  9. Ross "Home Brewer" Greening

    DUMB QUESTION ALERT FROM A LONDENER WITH HARD WATER – CORR BLIMEY MARY POPPINS: Jim, will a Britta water filter decrease PPM? i.e. Will the filtered water be more hungry for solubles? If so should i use Britta water in my trusty Bodum French Press? Thank you please.

  10. Peter

    “If the machine and method are flawed… I’m not sure I understand what get what you gain from measuring TDS in the brewed cup. How does it taste?”
    The taste cannot be measured with a measurement tool. There are only so many measurements that can be taken with instruments. If the machine and method are flawed then adjustments made to both can be then tracked using TDS. If it tasted too weak then what did increasing the temperature do to the strength? you have actual figures to note using TDS. This can help predict the next change or adjustment needed without wasting lots of coffee setting a machine up to produce a “Gold Cup”.

    I have made plots of the 55, 60 and 65g/L lines based on the Coffee control chart. This outputs a table of figures so the chart is not needed. I then created numbers using Mike’s formula and they are very close to each other, the 65g/L lines being almost identical.

    I still do not know how the original chart was produced, if it was a forumla or based on empirical data. I am not sure if it was based on dehyradtion methods, or actual TDS. The dehydration methods cam also have undissolved solids included, the amount depending on the amount of filtering etc. ECBC guidelines allow no more than 75mg of sediment per 100ml of coffee. This means a dehydrated “TDS” reading of 1.575% could actually only have 1.5% actual TDS. So if using dehydration methods it should be well filtered.

    I am currently testing to see if the filter paper itself can lead to increases in TDS, I also suspect it could increase nonsoluble weights.

    Most people tend to allow the coffee to cool before using a TDS meter, this will cause a reduction in volume as the cold coffee has a greater density. So a cold litre of coffee would have a higher TDS than a hot litre of the same coffee, by about 3%.

    ECBC guidelines have the grams per litre based on cold water at 15C. I expect many people using bulk brewers are measuring hot water output when calculating how much coffee to use. The density of 15C water is 0.991g/ml, at 95C it is 0.962g/ml.

    So say you want 60g/L and have a hot water output of 10L, so you use 600g. 9.71L of 15C water would have outputted 10L of 95C water. Therefore the coffee used should really be 582.6g, if based on cold g/L figures.

    I am not sure if the original graph was based on cold water, and TDS of cold coffee.

    These figures are only out by 3%, but they could add up if they were all in the “wrong direction”. e.g. you aim for 60g/L but use too much coffee which results in a high TDS reading, then you allow this to cool which further increases the TDS, so you end up 6% out.

Leave A Comment