When you weigh something on a scale you measure two things, the sample weight (mass * g) and its bouyancy in air (unless you have an evacuated chamber). If the densities of the sample and calibration weight are identical there is no correction. Steel has a density of about 7.9 g/cm^3 and coffee has a true density of....? I'm not sure, but I believe it's something a little greater than water.
For example, if you were to measure out 100.00g of olive oil (density of 0.9 g/ml) on your calibrated 0.01g scale the actual sample mass would be about 100.12g. Would this matter? It depends.
I think I'm "good enough". Class E1 and E2 weights at 500g are in the kilobuck range, the scale is $25 - I don't need an albatross weight if I have four 500g weights that are repeatable within the scale resolution.
Could they all be off in the same direction? Maybe the statistics are on your side, but if you really wanted to you could buy a class F1 or F2 weight for about $20. It doesn't sound like that's necessary, especially if all you need is to measure coffee to something a little better than 0.1g.
Isn't TDS concentration typically defined as dissolved solids divided by solution volume? The equation above is TDS / solvent.
For normal concentration coffee that doesn't matter much but for higher brew strength it becomes more significant.
The most convenient way to measure coffee concentration is mass of dissolved solids divided by mass of solution. I'm pretty sure that's what a VST refractometer reports and I'll bet it's what you've been measuring with oven dehydration.
If you do this it changes the "standard" equation a little:
Posted Thu May 10, 2012, 7:02pm Subject: Re: Coffee Extraction Discussion, Questions for the membership:
I expected reduced extraction and was absolutely shocked to find that the normal-high(ish) brew parameters all tasted pretty much the same (when diluted to similar strength) - MAYBE a bit less extracted for the 18% brew, but not severely underextracted as the brew charts would predict.
Posted Thu May 10, 2012, 7:07pm Subject: Re: Coffee Extraction Discussion, Questions for the membership:
If you use 55g/1 liter of hot water, and achieve 1.24% strength, your calculated extraction is 20% on the classic brew chart. This is actually the same thing as using 55g/962.5g of hot water.
BUT if you use 55g / 1000g of hot water and achieve 1.24% strength, your calculated extraction is only 19.2% (because you achieved the same strength but used a higher brew ratio relative to volume to get there). On the chart, it's the same thing as using a brew ratio of 57g/1 liter and achieved 1.24% strength.
Yes, this error was identified and incorporated into VST's Extractmojo and MoJoToGo software. Many people nowadays would say that 19%, not 20%, is the center of their "sweet spot" for extraction yield.
There are a number of different ways to represent concentration and mass/volume is certainly one of them. Illy uses g/L to represent espresso strength, but I wasn't sure what the standard is for normal coffee.
The SCAA literature I could find only shows it as a percentage. The brewing control chart gives the brew ratio in g/L so it didn't seem crazy to assume that strength was in similar units.
Revising this to your form makes it impossible to find a constant for the absorption
It's possible. I believe VST uses the same form of the equation (do you use mojo?). What isn't possible to is EXACTLY match what the mass/volume formula for strength produces.
If you choose 2.3 for the absorption constant (2.068 + 0.2 extraction) the difference between the two charts is insignificant over the pictured range. It's only when you're well outside what is shown on the chart that it starts to make any noticible difference.
But if you're going to use mass/mass concentration your formula has to reflect that or else it's nonsense... assuming you believe that absorption really is constant for a given mass of grounds.
I don't do pourovers, but my Aeropress and moka pucks are always well below 2.0 absorption.
jpender Senior Member Joined: 11 Jul 2011 Posts: 742 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Fri May 11, 2012, 10:33am Subject: Re: Coffee Extraction Discussion, Questions for the membership:
By the way, I'm looking for an inexpensive scale. I read a comment on Amazon.com about the one you purchased. The person was claiming that it locks into 100g increments. I'm familiar with the annoying but common tactic of reporting zero when small masses (presumed drift) are added after the tare is used. But does your scale also do this at 100g increments as the reviewer claimed? I'd like to avoid that "feature" if possible.
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