How to Use a Coffee Refractometer
In this video, Matt Perger walks you through how to use the refractometer to measure the TDS of a coffee sample.
BH Protocols and User Advice:
We recommend you read and absorb the VST literature thoroughly, but here is a summary of our best practice recommendations:
Samples must be filtered below 0.5um — a paper filter does not provide a fine enough filtration (the holes are 20 times wider than you need!) If you are purchasing syringe filters, it is recommended they have a diameter of not less than 20mm — too small and they are blocked almost immediately.
You should test samples that have cooled to room temperature but you also need to avoid water evaporating. A good practice here is to coordinate sensory analysis into your TDS measurement process. Remember the TDS% readings are your GPS, and you are the driver. So follow this order:
Brew the coffee
Stir it thoroughly
Immediately remove the sample into a syringe to prevent evaporation
Taste the coffee
Finally, take the reading and record your findings
If you notice TDS reading drifting in a particular direction, keep pressing the read button until it stabilises. The drift is caused when there isn’t parity established between the temperature of the crystal on the refractometer and the sample itself. A difference between the crystal and sample of 5°C can impact your extraction yield calculation by up to 1%
Only use a 0.5g sample (approx half a ml) so that it cools faster. Pour the sample onto the stainless steel wall of the sample well. The steel will conduct heat and cool the sample, rather than changing the temperature of the crystal directly.
If you are taking readings from cupping bowls, draw out the samples after a specific period of time — we recommend 4 minutes after breaking the crust to allow the brew to have clarified a little. Hold the liquid in the syringes until you have completed your cupping. The tiny quantity of suspended solids in the syringe will not impart a measurable difference to the TDS reading of the liquid because these microscopic particles (having a diameter in the region of 0.001mm) can be assumed to have reached maximum extraction in 8 minutes.
If you obtain a reading you are surprised by, test the calibration of the device by taking a reading of some deionised water (this is easy to get your hands on from most hardware stores.) If you obtain a reading that is not 0.0%, then you need to recalibrate and/or clean the crystal with alcohol wipes. (People often use deionised water in their clothes irons since it is free of minerals so will not cause scale, though it will cause them to rust eventually.)
The protocols suggested by VST to apply to the coffee refractometer are listed here: