I've spent quite a while studying the way the extraction is calculated from TDS in the MojoToGo app. One thing that really confuses me is the way the water loss ratio setting alters the calculations. When you change this value and apply a brewing preset, the app adjusts the brew ratio but continues to display the extraction as calculated with the default 2.0 g/g loss ratio. The same behaviour applies when switching between brew weight and beverage weight, as well as when applying a beverage size preset.



The figure above shows three brew ratios calculated with 1.0 g/g, 2.0 g/g, and 3.0 g/g water loss ratios. The brew water weight is fixed at 6 kg, the TDS is fixed at 1.30%, and the extraction is supposed to be fixed at 19%. However, you can see the calculated extraction is actually different for each one.

This is because the displayed extraction yields are all calculated using the default 2.0 g/g formula, as follows:

1.30 * (6000 - 389 * 2) / (389 * (100 - 1.30)) = 0.176812704
1.30 * (6000 - 365.3 * 2) / (365.3 * (100 - 1.30)) = 0.189993041
1.30 * (6000 - 344.3 * 2) / (344.3 * (100 - 1.30)) = 0.203188061


The brew ratios are calculated taking into account the water loss ratio to yield the desired 19% extraction, as follows:

1.30 * (6000 - 389 * 1) / (389 * (100 - 1.30)) = 0.18998393
1.30 * (6000 - 365.3 * 2) / (365.3 * (100 - 1.30)) = 0.189993041
1.30 * (6000 - 344.3 * 3) / (344.3 * (100 - 1.30)) = 0.190016835


So my question is why does MojoToGo keep using the default water loss ratio for all calculated values (extraction and beverage size) when the setting is changed? Is it a bug, or is there a reason for doing it this way?

It's a bug, to be fixed in the next version.

Kind of a big bug to leave in the software for 4+ months. I spent the last couple weeks working out formulas and trying to figure out why it didn't work properly in the app. I hope the next version still runs on iOS 3.

The 2 g/g default works with most regular brewing methods, but some things (Aeropress, cold brew) are really far off. It would be good if you could enter the actual brew and beverage weight to check the results, but I guess I can do that in my spreadsheet.