Tedious Meters

There persists a deluded, quasi-philosophical resistance to the use of electronic devices, and scientific methodology for improving the production of brewed coffee. While coffee preparation certainly falls into the culinary arts, like it or not, water is a solvent, coffee a solute, and there is much benefit to be gained in the adoption of scientific apparatus and some light math. I implore those holding out, to at least once, measure brew strength and extraction percentage. This should include roasters, retailers, consumers and all in between. It is the only practical way that the occupants of this consumption chain have of communicating reference points and explaining what is in their cup. I’d be very happy to see an end to the widespread use of wishy-washy, meaningless terms like “filter grind” and near universal prescription of 4mins as brew time. This offers that possibility. That spectrum of end-users though, probably reflects very different perceptions about what would be an acceptable amount to spend on a device for the purpose. It is important though, that an option is available for all price points.


The tool of choice for many a year for this purpose was a TDS meter. Hydrometers and dehydration were considered more accurate, but for in the field assessment the TDS meter was used. Ted Lingle, author of The Coffee Cuppers Handbook, The Coffee Brewing Handbook, and former SCAA Executive Director, updated their use with coffee in the early 90s, with Carl Staub and Agtron Inc (they of the bean brownness scale) assisting on the research end. Of course, the correlation between actual coffee concentration and conductivity had been established some time earlier. For example, in the 1979 tome, Coffee Technology (Sivetz & Desrosier), the authors state that “Specific electrical conductance is directly related to solubles concentration up to about 10 percent solubles; at this point the proportionality deviates”. The two examples, shown in the figure, confirm the linearity of the correlation, suggesting at least that conductivity is an appropriate technology for the purpose. TDS meters since became an integral part of the Gold Cup programmes, in both the US and in Europe.

Linear relationship between coffee strength and electrical conductivity.

Linear relationship between coffee strength and electrical conductivity. (A) Nicoli et al., ASIC, 14e Colloque, San Francisco, 1991. (B) Cammenga et al., ASIC, 17e colloque, Nairobi, 1997.

My first hand experience with TDS meters has been broadly positive. After the SCAE Brewmaster course I picked up a cheap ($16) one on Ebay, and based on its readings I found that my choice of grind size and brew time was resulting in a very weakly extracted cup. Some adjustments to my grind, and some more tests I was brewing a markedly more satisfying cup. $16 well spent.

Anecdotal evidence, however, points to a widespread level of frustration of users of the devices, their accuracy frequently questioned. While I have no way of confirming their accuracy beyond agreement with my palate, I observed that the TDS reading would take several minutes to stabilize, and even then it was only relatively stable. There was a big requirement on the user for patience and faith. At times the results would seem counter-intuitive, and some who would use them extensively would rely on the average of several readings to even out variability. Certainly at least part of the blame for the perceived failings of the US Gold Cup programme have been layed at the door of the humble TDS meter.

At the SCAA in April, in the midst of the barista and cup tasting competitions, some notable events took place. The SCAA adopted the Extract Mojo refractometer as the preferred tool for in field assessments of brew strength. They also changed the brew chart, adding a zero to the TDS scale, corresponding to the strength percentage (more on this below). The Mojo (r2 mini refractometer), a $339 tool, with the accompanying software suite (extra cost) represents the first concerted effort in over a decade to improve the accuracy and usability of a coffee strength measuring system. As with many exciting new products that hit the coffee industry, the hype built perceptions of a device that could be used almost casually, and would deliver never before seen accuracy and precision. I would liken the device closer to something to be used in a laboratory, with a requisite for care and consideration not adequately highlighted in much of the online discussion. Calibration, filtration and temperature seem keenly important, with the requirement that both the device and the coffee sample be in thermal equilibrium.

Even then, I have observed, in my short time hands-on with some of the r2minis, that readings can fluctuate somewhat. A typical experience would be an initially consistent reading (maybe varying by a couple of one-hundredth percentage places), then leaving the sample on the lens, and coming back 5 or 10 minutes later to find a very different reading. Nonetheless, in a recent side-by-side use of the mojo and a TDS meter, while at times they agreed, overall the results from the mojo more consistently matched our palate and expectations based on the adjustments we made to the brew. That alone, is progress. Money being no object, I would prefer to have a mojo as part of my tool kit than a TDS meter.

From The Coffee Brewing Handbook, Ted Lingle, 1996.

From The Coffee Brewing Handbook, Ted Lingle, 1996.

I remain a little confused as to the reasons for the change to the brew chart. In email correspondence, Ric Rheinhart, current SCAA executive director, said it was the restoration of the correct decimal placement for the TDS values, after the loss of much institutional memory in recent years. A TDS meter gives a reading in PPM. However, it directly measures conductivity, not TDS, and infers TDS based on an assumed ratio of conductive to non-conductive dissolved solids. For measuring water purity a TDS meter is roughly accurate, where the dissolved solids are mostly charged. In coffee though, there are many organic solids that do not conduct electricity. So the ratio must be changed. Though it seems implausible, the multiplying factor is 10. This is given short shrift in The Coffee Brewing Handbook, where an expanded explanation would have been most useful and appropriate (see the figure for the full account). The best articulation of this issue I could find was from Barry Jarrett in response to Jim Schulman on on, “so, there are 12,500 ppm of solubles, but only 1/10th of them conduct electricity“. Therefore a TDS meter reading of 1500ppm for coffee does equate to 1.5% coffee strength, despite the fact that strictly speaking 1500ppm means 0.15%.

That the multiplying factor appears to be exactly ten has contributed to the confusion (one of the better examples of the confusion can be seen on the CoffeeGeek forums here). The prescribed method for measuring the strength is to measure a sample of the brew water and the coffee, and to subtract the brew water TDS from the coffee TDS. This has led to questions over whether, the water TDS (a true PPM) should be subtracted from the coffee TDS or the coffee TDS times ten. Questions have even been raised about whether TDS meters have an inherent 10x scaling ability that has remained unadvertised. Lack of faith in the meters, combined with the lack of clarity on this point, have created this clusterfuck of confusion.

If we think about the reading from the TDS meter, not as TDS or PPM, but rather an indication of conductivity, things become simpler. The brew water does contribute to the conductivity of the final coffee beverage. This contribution to the conductivity should be subtracted to leave us just with that contributed by the coffee solids. In those terms it makes no sense to first multiply the coffee reading, because you are also multiplying the water contribution, then in essence only subtracting one tenth of its contribution to the reading.

The 2009 SCAA Chart Change.

The 2009 SCAA Chart Change.

So the new chart is both correct and incorrect. It is correct in the technical sense that 1.30% does mean 13000ppm, however, in terms of TDS measured by a conductivity meter it is incorrect, 1.30% would be 1300ppm. Some clarity on the intention of the change would be most welcome.

I think the last hurrah of the TDS meter could be bridging the price gap for the consumer, and the skeptics, until a time when the better tools become more affordable. For everyone else, (and perhaps some hardcore consumers), the Extract Mojo should be the (portable) tool of choice. If, like me, the TDS meter can get people into the ball-park of desirable extraction, then it is mission accomplished. If the Extract Mojo eventually sends the conductivity TDS meter the way of the dodo, then we should send it off with our thanks, for filling the void, getting us on the road to making this practice widespread, and for duty served in the name of good coffee.


13 thoughts on “Tedious Meters

  1. Ben Helfen says:

    Great, great stuff.

    It seems like the specialty coffee world’s progression would just explode if there wasn’t a lack of ability to communicate specifics in brewing, whether it be particle size, extraction percentages, soluble yield, etc.

    I dream of the day Octane is big enough to have a coffee laboratory. Oh the toys i’ll have….

  2. We have been using the R-2 mini refractometers for both filter coffee and espresso in our R&D labs for several months now, and we see significantly more consistent and accurate readings with them, as compared to the TDS meters we used to use.

  3. We’ve ordered the $16 TDS meter and have been awaiting its arrival for a while now. Maybe a little tutorial on what you have found to be the best process of using it would be a great blog post?

  4. Pingback: Virtual Memories » Unrequired Reading: Jan. 29, 2010

  5. Hey David,
    I’m looking for a cheap way to get into the extraction measuring game, trying to find a cheaper meter to use with the MojoToGo, is it possible/worth it to use a cheaper TDS meter? Any specifications/a specific one you would recommend?

    • Hey Collin. You can of course pick up one of the cheap ebay TDS meters (conductivity meters) and give it a go (for coffee, not espresso). Personally having used a mojo refractometer for the last 8 months or so, I couldn’t go back to a conductivity meter.

      I’m not sure about other (cheaper) refractometers. I believe there are specifications in the mojotogo literature re compatibility. The refractometer has to read at a certain wavelength and be able to report nD and temperature. The resolution of the specific refractometer will then determine the accuracy (more resolution required for coffee than espresso).

  6. Dan Kennedy says:

    Hey David,
    do you feel refraction is across the board more accurate with better resolution than conductivity with coffee (& water) or are there conductivity meters that are better than the r2 mini or VSTs new lab refractometer? I’ve been using the r2 mini since July and am looking to upgrade to something more accurate/higher resolution/better all around. Do the Myron L Ultrameters fall into your opinion of conductivity meters or are they a real step up? Seems like there should be a HUGE difference between a $16 tds meter and an $800 Ultrameter II. I’m also curious about your background in coffee and these analyses. I’ve been seriously studying brewing and roasting for about a year now and just got SCAA certified as a GC brewing technician and am trying to build a lab.

    • Dan Kennedy says:

      after I posted I realized you surely have an ‘about’ on your site and read it, so unless you want to elaborate further (I’m always interested), feel free to skip your background

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