Guys, I’m confused now. I did those disconcerting experiments trying to determine the correction factor for my particular instrument, you recall. I think I’m still leaning toward sticking with my hydrometer, but: am I now to understand that I didn’t have to do that? Is 1.04 in fact not a WAG but a reliable universal correction on any instrument calibrated to 0?
I’m a bit confused too, but, I’m pretty sure each individual instrument has its own quirks such that I plan to continue to use 0.99 for my correction factor even if it’s “wrong” cuz I’m sure it’s right, for me.
Nope that’s his maximum first wort gravity chart, which is our gravity estimation engine in our spreadsheet. We like Kai! 
No. You guys are right. There is a different correction factor for everyone. 1.04 is a generally accepted default value but empirical data says it varies wildly. What I was trying to say, and what is now erroneous, is that I thought an additional factor needed to be applied to convert from Wort °Bx to Wort °P. Not the case.
You guys are on point. Everyone’s correction factor is different. I wouldn’t be surprised if ST just used Kai’s as the default.
My hydrometer doesnt make my head hurt like this. But nothing makes my head hurt like something that seems like it should be straightforward, but you just can’t find a clear answer in all the information out there! Or even whether sources are using terms consistently.
It’s really simple: determine your units correction factor and you are good to go.
What’s more, there is going to be a different correction factor for each gravity. That’s why we do so many test samples, to determine the average correction factor.
The end result difference between a 1.04 and 1.02 is quite minimal. Nothing to get in a twist about.
If you recall, I was cautioned about using my outliers. Well the difference in the final results between with outliers or without is like 1/2 of a gravity point in a moderate beer
That was the problem. My tests were all over the place (consistent w/in gravity, erratic between gravities) so I don’t see a factor for my unit I can trust. I can’t get my head around why an average would be valid when the apparent correction did not skew along a smooth curve or line as gravity increased: just bounced around. So I couldn’t call anything an outlier, there was just no pattern.
EDIT And how, anyway, could I trust that a given reading on a given brew day was not itself an outlier?
Here is the way we do it and why I feel, empirically that it works:
1.) We calculate a “target” first wort gravity using Kai’s idealized equation, i.e. 100% Mash Efficiency, 80% DBFG, and 4% moisture. This gives you the maximum theoretically possible first wort gravity. Since we assume no-sparge this is the maximum theoretical pre-boil gravity.
2.) We then calculate the “estimated” first wort gravity by subbing actual malt parameters and mash efficiency into Kai’s equation. We know that this will deviate from the “target” as mash tun Losses and absorption increases and converge with the “target” with less loss and smaller grain bills.
3.) We use a static value for Refractometer correction and always measure very close to our estimates, so it must be nothing if not consistent.
One caveat though: We (the royal We as in many of us at LOB forum) are in a pretty tight gravity range. Even me, who only brews Trappist “style” ales, stay pretty moderate on wort gravity and supplement with sugar.
Near as I can figure, my variations are largely due to my ability to read the hydrometer. I suspect that if I used an easier to read precision hydrometer the swing would tighten. To me it’s fine. I can’t tell, by looking at my hydrometer in wort, the difference between 1.060 and 1.0605… I also have no way of knowing if suspended solids are effecting the hydrometer. So there is still a degree of trust involved if all you use is a hydrometer.
All good folks. I don’t sell refractometers, lol
I guess I’ll stick with my hydro (again as I sparge, I use iodine to confirm conversion in the tun anyway), but over time I may occasionally take a refracto reading on occasion and see if it’s consistent in normal wort range. Just for fun, if I can remember. I have a hydro that even my crappy old eyes can read the 0.0005 lines on (actually 3 narrow range hydrometers), and I don’t use it on mash, only wort, so floaties aren’t a factor. And I don’t sell hydrometers either!
UPDATE AND APOLOGY FOR QUESTIONING YOU GUYS’ METHODS
So I had some time today. Thought about some methodology you guys suggested. Got out the stir plate, used Jim’s method, and did a MUCH more extensive series of tests. I also remembered that Jim thought that a lack of homogeneity might show up big time in the refractometer sample, so I took those readings FIRST right out of the beaker on the stir plate and THEN checked with the hydrometer. Well!
Result: very consistent for the most part, and even the apparent outliers, when included in an average of all tests, fall precisely in line with the result excluding them. I get a correction factor of 0.97, which, having gone through this, I accept. I will now trust the refractometer enough for on-the-fly assessment of wort in the mash and boil. But I’m still sticking with my precision hydrometers for OG and beyond.
Big Monk, Jim, guys, thanks for helping me get my head around this!
There ya go. Cool
Another test i want to do is hydrometer in distilled water at calibrated temp, compared to the same water with a obviously visible amout of yeast in suspension. Palmer claims starch is suspension threw his off by 10 points. I hope to not ever have visibly starch in mine, but occasionally there’s visible yeast.
I’d be very very interested to hear anyone’s results for FG using the refractometer, once they know their correction factor. Use the “Old Cubic” or the OP’s calculator, and watch just how close you come. You may find as I have that the refractometer is every bit as useful for FG as it is for OG.
Cheers all.
10 points sounds like a lot, but I know mine often has dropped (apparently) about 1 point pre- to post- cold crash. I never know whether that’s a change in suspended yeast, or just in bubbles lifting the hydrometer,or the temperature fudge factor. I guess at that point there’s not a huge amount of yeast still in suspension anyway. Either way, I just assume the accurate FG is the post crash. (In fact I don’t even do the pre crash anymore, but that’s to avoid opening the carboy now that I’m hyper cold side O2 conscious! In fact I don’t see a need for any gravity readings but OG and FG taken after the fact. And the only reason for that is, if attenuation was off, I’d reculture instead of repitching. I know my yeast.) Anywho, if you do the experiment I’ll be curious to see the post!
I have the “New Cubic” and “New Linear” calculations coded into our spreadsheet on a toggle. "New Cubic"is the one I’ve gotten the best results from. “Old Cubic” always seems low to me.
I just signed up to use the OP’s calculator. I’ll report here!
…And we’re officially back on-topic! ;D 8)
Try the OP’s calculator. I think you’ll be impressed.
EDIT: FYI, I add a constant 0.001 to every result when using the Old Cubic. A little tweak like that can make a real difference in our judgment of it. The curvature is not affected, just the vertical placement on the graph.