# Help with COM100 TDS meter



## lyonchen (Nov 7, 2012)

Hi MikeHag,

I am very impressed by MikeHag's post long ago about using a refractometer to improve brewing. So I bought a COM100 as a ticket to enter a numerical method of brewing, and also a book of "Everything but espresso" is on the way.

However, the reading of COM100 is very confusing for it varies as temperature cooling down, pretty fast. It may read 1100 when hot, and 1400 when 25 degree C (though COM100 claims to have an automatic temperature compensation) when the coffee itself tastes not that bitter nor strong.

So, which reading should be trusted? hot or RT?

I brew coffee with a cloth filter, start at 87 degree C and 1:16 ratio.


----------



## Glenn (Jun 14, 2008)

Room temperature. Try to take readings at the same temperature.

Are you calibrating with distilled water?


----------



## lyonchen (Nov 7, 2012)

Thanks, Glenn. I will keep reading TDS at RT of the same temperature.

I just bought it and did not calibrate it myself. According to manual, one should prepare a calibration solution of TDS range near one's application. If I need to calibrate my COM100, what calibration solution would be recommended to near my 1200 ppm coffee sample?


----------



## SZA (Jun 28, 2012)

Did you read MikeHag's blog? I think he found out that he could not trust the measurements. Basically the temperature compensation in the tds-meter does not have time to compensate in the limited time you're doing the measurement, or something. Room temperature measurements are definitely the way to go.

Another thing, shouldn't the coffee ppm be more like 12000? Try diluting your sample 1:10 (exactly) and do the measurement again...


----------



## lyonchen (Nov 7, 2012)

Hi SZA,

I don't see MikeHag's blog. Would you mind quoting the URL here?

The coffee TDS should be around 1250 +/- 100 ppm if trying to achieve SCAA optimum balance (Golden cup).


----------



## SZA (Jun 28, 2012)

I couldn't find the blog post where he realizes that 1.25 % is really 12500 ppm (parts per million), but here's the post about the TDS-meter: http://haggieslab.blogspot.dk/2011/11/com-100-tds-meter-2.html

However, if percent and per million are based on the same units, e.g. mass/mass calculations, then 1 % = 0.01 = 10000 ppm.

I'm not sure how important it is, but the professional gear for measuring TDS in solutions are based on refractive index, which is not necessarily the same as the electric read-out on the com-100. But maybe the measurements are still reproducible enough that you can use it to navigate?


----------



## lyonchen (Nov 7, 2012)

Thank SZA for the URL.

COM100 can show TDS in PPM directly. I take more than 20 samples since I bought and they all read 1100~1400ppm.

The manual said that COM100 needs some time to "run in", which is proved right for the reading now is much more stable and independent of temperature variation. Though I still follow Glenn's suggestion and record reading only at 30 degree C.

According to my blind test to friends, TDS and a SCAA golden cup chart can improve my brewing very well. At least I can take a numerical reference along side with perceived experience of observing, tasting and smelling (aroma) which I used to depend on. After about 10 tests working on the same roast, now a near 20% yield can be obtained stably. I'm really amazed and urged to know more about brewing knowledge.


----------



## SZA (Jun 28, 2012)

Interesting! I have been looking in to getting a small reverse osmosis system and with that I would get a small TDS-meter, which might then actually be able to do the same measurements. It still bugs me that the SCAA chart converts 1 % to 1000 ppm instead of 10000 though... To my knowledge most tap water would give a TDS read-out of around 100-400 ppm, does that mean that brewed coffee is only 3 to 10 times as concentrated as the natural mineral content of tap water?! Did you try diluting a sample? I still think it could be interesting to see what happens...


----------

