Improving Refractometric and Gravimetric TDS Measurement (Part 1)

Method for cross-checking your TDS measurement

Have you ever been doubting the TDS measurement? Both refractometric and gravimetric ways? Especially if you are using cheap refractometers that are more and more often advertised on coffee websites, forums, and videos. As a chemist with analytical thinking, I always have these thoughts of double check my results and experiments.

This is the continuation of the gravimetrical TDS measurement project I started a long time ago and the first article could be found HERE. Here is how it started.

I tested the gravimetrical TDS method described in the previous blog post. However, I got unexpected results for the Aeropress coffee I prepared for this purpose. At first, I calculated the %TDS using the provided in the article formula and I got quite a low result - 1.21% TDS average value obtained by the three different measurements. This is a low result for an Inverted Aeropress method and the recipe I used. This made me think, is there something else I’m missing or not taking into account? How can I double-check the result? How should I do that without using the values obtained by the refractometer if I don't have one? Since I am trying to test the accuracy of the TDS refractometer with gravimetrical TDS measurement, using the refractometer to correct the result would be inadequate. Nevertheless, I checked the result with one expensive and one cheap TDS meter just to be sure and there was a difference.

I got an idea from analytical chemistry where everything is calculated based on a calibration curve or comparison using standards with known concentration. This is the basis of all measurements and experiments in a standard lab. So, I had to make a standard with a known concentration. But how? Dissolving salt will do the work, but salt and coffee are two very different things and variables will be too many. This will influence the accuracy of the experiment and I didn’t want that. And what is more similar to coffee extract than a “coffee extract”?

Instant coffee!! It is a solid that could easily be dissolved in water and because it is a coffee it will behave the same way. It is a solid, 100% soluble in water. Thus, it will be easy to create a standard with the exact calculated concentration. For this, I’ll need only the mass of the instant coffee and the mass of the solution and I could easily calculate the concentration in %.

Based on this idea there will be an easy way to make TDS result correction for the gravimetrical method as well as for the refractometry method. It doesn’t require expensive equipment, but only a scale and instant coffee… and a few calculation skills. After I conducted the experiment and calculated all corrections, I got a result of 1.37% TDS. Well, that was more realistic.

If you want to read more, visit my Patreon page HERE or go to www.patreon.com/npcoffeescience. At the price of a single espresso per month, you can read all my experiments, work-in-progress projects, and future ideas, and allow me to continue my work. Thank you!