If you're serious about making these measurements, look up the engineering definitions of "accuracy," "precision," and "resolution." They have very specific meanings that may be different from colloquial language. If you're going to ask technical questions using these words, please be careful to say what you mean and mean what you say.
what the VST coffee refractometer instrument accuracy is on the raw refractive index
For the second time I'll tell you I don't know the accuracy in refractive index of the VST refractometers; VST does not offer that specification. The offered specs for the newest LAB model are in the link I previously supplied to you and refer to coffee TDS: "accuracy +/- 0.05% (0.00-4.99%) warranted, and precision +/- 0.02% (typical)"
If you want to know the instrument's accuracy in RI, you already have been given the info you need to guesstimate an answer. In doing so, you must bear in mind that that "typical accuracy" is significantly better than the "warranted accuracy." If that were not the case, Vince would lose time and money trying to satisfy warranty claims. To avoid such claims, I know that he personally runs a QC test procedure on every unit before it ships.
Ouch! Real name? You know I'm Dave - IMO, if you see something you don't agree with, there is no reason to take that tone with me
I'd suggest you grow a thicker skin and take responsibility for teaching us something we don't yet know.
RE: thick skins, if you read the posts that came before you entered this thread, you'll see that the previous poster pretty much accused me of doing paid work for VST and being less than truthful about it. I don't appreciate this, but he has as much right to post here as I do.
jpender Senior Member Joined: 11 Jul 2011 Posts: 732 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Thu Feb 23, 2012, 7:22pm Subject: Re: coffee refractometer
Yes, but I didn't have my notes with me. If I recall, dehydrating at 300°F ended up an extra 0.1g out of the grounds and not a measurable difference in the dehydrated liquid. Something like a tenth of a gram went missing and it didn't seem right to let it sit on the counter for an hour and let it absorb that tenth of a gram just to make the numbers come out right. Something like 22g original, normally weigh (after extraction and dehydration) 17.4-17.8g, the dried solids left behind on evaporation weigh the balance - except for high temperature drying of grounds, which lose a tenth of a gram between the two.
Doesn't sound like much, but in those quantities, every tenth of a gram lost in the grounds weight is around a half percentage of extraction.
Thanks for the response. I've tried dehydration numerous times and have always felt unhappy with the way the finishing weight seemed to be a moving target. Since, as you point out, an error of 0.1 gram translates into a difference of 0.5% extraction (for 20g of coffee) the fact that I saw the weight continuing to change by more than that bothered me.
I suspect my issue was impatience. Rather than wait hours to use an oven I tried using the microwave. Careful as I tried to be, it just got too hot at the end. Some of my samples actually smoked!
So I tried it in the oven again. I made two nearly identical 19+ gram batches in an Aeropress, mixed the wet grounds together and then divided them as equally as I could (difference of 0.03g). Then I dried one half at 215°F (which took forever) and the other at 300°F which took about half as long. There was still uncertainty as to when it was done. The weights seemed to rise for the first several minutes of cooling and then stabilize, something I had never seen with the samples I had microwaved. After more than an hour each of the oven dried samples (covered in a layer of foil) had a stable weight, to the precision of my scale (0.01g).
What I didn't see however was a difference between the 215°F and 300°F dried samples (other than time to dry). They weighed almost exactly the same, just a few hundreths of a gram difference. The size of 0.1g of dried coffee is quite small, enough so that it would be very easy to be off by that amount if one isn't very careful. Even the coffee that I couldn't quite scrape off of the AP plunger or that stuck to the utensil I scrape it with might be significant.
The fact that you are able to get consistent results that add up is impressive, both in your skill and your patience. I'm glad I don't have to oven dry my used grounds for hours in order to have good coffee. If I did I might switch to Folgers, or tea.
Kafeman Senior Member Joined: 18 Feb 2012 Posts: 24 Location: FL Expertise: Just starting
Posted Thu Feb 23, 2012, 9:13pm Subject: Re: coffee refractometer
Whatever, Andy, I prefer to enjoy my coffee rather than argue with someone who insults me at every point he can figure out how like a politician who seems to pump himself up by stepping on me and plenty of others as I look over your history - and then tells me if I don’t like it to grow a thicker skin. Just because you've been around a while gives you no right to do this.
Your discussion on precision, accuracy and resolution is only the latest example of your attitude to berate and discredit anyone who you decide to be nasty to. I discussed accuracy and *you* responded with resolution which was irrelevant right then, but no, I don't slam you for bringing up a different concept, I just say I'm confused as to why you would say that; so I followed your lead, and if you were kinder you just would have been nicer and mentioned what you knew earlier if you thought it relevant, but no, and then you slam me for being confused with your mashing of this all together as if I originated the mess.
If I held you to the standard you hold me, I could trash you on each statement you make as well, putting a spin on it of course like you've done to me.
Remember again, I said, you were a star contributor (it was after seeing how you treated others and for a genuine fear of you as a senior member and something like this happening) and you even slammed me even for that, 'we don't have a star system here, keep on topic' - I don't know where your head is coming from half of the time since you invented all by yourself that I wanted to implement a star rating system here – I still can’t believe how you made that up).
I feel you only say what you want when you can get another nasty word in this thread with me. If I wanted to get into this sort of banter, I will lose with you, because I see all you will do is dissect every single thing I say and modify it to suit your mood and then send your missiles. If I respond you do it again and if I don't you get the last word. I can't continue that, even after 10 cups of coffee.
Just for clarity in case someone else would judge me by these damning posts, I haven't accused Andys of working for VST, he's talking about someone else and I don't have any idea who that person is except for what is written in this thread.
You have made federal cases out of my sharing my efforts at improving my own evaluation of my coffees, as if you were my boss breathing down my neck before I finished a project, asks where I'm at and then blasts me because I'm not done. You tell me I am "giving instructing" to people on refractometers, man, you are so far off it would have been silly to respond to that on this planet, any chemist will think this way; how you dreamed up I am instructing others again completely escapes me, and this crap about last names - look: I'm ok by forum guidelines and that is all that counts, so you got under my thin skin with that comment of personal crap.
As a newbie, I have taken far more on the chin than I should have, my fault, just because I thought it would be nice to gain your acceptance but I failed, and as a result I will just cease with my participation in this thread as it has become unpleasant for me and I have no wish to take on such an endeavor, is now only detracting thanks to your nasty grilling and as for your disrespectful clincher
take responsibility for teaching us something we don't yet know
That last bit of nastiness sums you up. I assume it was the royal "we", and I don't care, yeah, according to you I’m an irresponsible slacking off teacher so far, whatever... Exit stage right (soapbox vacant for all time)
Posted Fri Feb 24, 2012, 6:55pm Subject: Re: coffee refractometer
Personally I'm very happy with de facto standards. For example, AndyS says he likes likes his espressi at 19-20%, personally I prefer 18% when at 65% brew ratio. We might have different tastes or be drinking different coffees. More likely it's because he's got better kit - and I have to jump through too many hoops to get yield up to 19% with that small amount of water.
Believe me, I too sometimes have to jump through hoops to get yields up around 19%. Some batches of particular coffees seem reluctant to give up their solids (lighter roasts especially). Grinding finer and underdosing the basket a lot helps (eg, 18g dose in a 22g basket).
I'm told that grinding too fine for a particular basket will produce a silty espresso (as extra-fine fines pass through the perforations). But at the underdose levels I use, I have rarely noticed this.
BTW, there is a term in psychology for one's mental state when others are able to consistently extract higher yields from a particular coffee than you can: "extraction envy."
Speaking of which, I brought my grinder to Vince's house last summer and we pulled side-by-side shots comparing my Robur to his K-10. Using identical doses, similar brew ratios, and with grinders tweaked to give matching shot times, the K-10 yields were consistently about 0.5% higher. A small difference, but enough so that I was helpless to suppress mild extraction envy symptoms.
Presumably the K-10 burrs produce slightly fewer fines, which means you grind a little finer to get the desired shot time. That finer grind gives a bigger yield.
Some people may not have seen Dave Walsh's fascinating piece where he sieved out the fines and the extra-coarse pieces from some grounds. He was then able to make pleasing, non-bitter coffee from the mid-size particles at a 24% extraction level. Since the generally accepted extraction range hasn't changed much in the last fifty years, his experiment suggests that there's been little progress in grind quality during that period of time.
It's nice to know that it's not because we have a different notion of what 19% is though.
Posted Fri Feb 24, 2012, 8:39pm Subject: Re: coffee refractometer
22 grams of coffee used. 385g of brew water in an Aeropress.
Press 1 yields 330g of coffee (because I don't press all the way through the puck, just until I hear air, and I have a wet puck left behind). I dehydrate the coffee, and the grounds, and get 4.2 TDS and 17.8g of dried grounds. The coffee is at target strength: around 1.27%, but the calculated extraction is only 19% (4.2/22).
Press 2 yields 360g of coffee (because I press every drop I can out of the puck, and yes you can get up to another 30g of coffee). I dehydrate the coffee and grounds, and get 4.5 TDS, and 17.5g of dried grounds. The strength of the coffee is the same for all practical purposes: 1.25%. Calculated extraction is 20.4%. Which one is right?
I like your methodology, but why not consider both results right? In the second case, you got an additional 30g of beverage, at 1.0% strength, containing 0.3g of solids. Not only was the strength of the last 30 g less than the first part, but the composition of the dissolved solids was likely different (more weak caramels, creosote-bitters, etc). Next time you could taste the two fractions separately and compare.
For me it would be nice to take press 1, squeeze out the last 30g of stuff, and check the strength - if that's the same strength (and why wouldn't it be? The extraction is for all practical purposes completed)
You actually proved that it wasn't the same strength, right? Meanwhile, the extraction is not "completed" until you approach the physical limit of solubility (somewhere around 30% yield), which takes a much bigger volume of water run through the grounds.
Why do larger grind sizes seem to retain more than finer grind sizes? Is this why espresso seems to have less absorption?
Posted Sat Feb 25, 2012, 8:38pm Subject: Re: coffee refractometer
I think I'm inside the discretion of my current measurement system. I have repeated this a few times, and I've also tasted the last squeezings - it tastes like the stuff in the cup, it doesn't seem to be weaker or stronger, and doesn't have any overbearing smokiness, creosote, and will also have the same basic fruits/florals as the stuff already in the cup.
I think I understand your rationale in doing these experiments. While the traditional method for measuring extraction yield focuses on what's in the cup (ignoring whatever liquid remains trapped in the grounds), I believe you are exploring whether the trapped liquid should also be considered as part of the extraction.
Following the latter procedure is a practical challenge, though, because we can only guess the composition of that extra liquid. We can't measure it accurately because by definition, we haven't pulled it out yet. And the physical process of pulling it out will change its composition, as agitation and turbulence come into play.
We know that the trapped liquid's composition will vary tremendously depending on the brew method. For cupping brews (where the grounds are gently spooned out of the cup), the trapped liquid will be almost identical to the liquid beverage. For full immersion style brews like french press or aeropress, the trapped liquid will be similar (but not identical) to the extracted beverage. In percolation style brews like pourover or espresso, it will be much, much weaker.* Exactly HOW much weaker depends on the brew ratio. Every brew method has a slightly different effect on this, and accounting for it will drive you crazy.
So I think it makes the art and science of coffee extraction far more complicated -- and due to all the ambiguities, ultimately less useful -- if we try and include the trapped liquid in our measurement process.
The gold standard of extraction measurement has always been comparing the mass of solids found in the evaporated beverage to the original mass of dry coffee. Although not a perfect method for every brew style, it is simple, repeatable, and gives a reasonable correlation to taste. The interesting thing is that when we use this classic, imperfect extraction measurement protocol, people's taste preference usually centers around 19% -- no matter what brew method is used.
*For example, the last bit of espresso that flows into your cup might be only 15% as strong as the first bit. The last bit of pourover coffee is also much weaker than the first.
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