Rookie Efficiency Calc questions

[tdfoste]

I'm having trouble calculating efficiencies for a high gravity Dark Strong I did last weekend. I'm going to post the data and some questions and hopefully some of you more experienced guys can get me straightened out.

First, Potential gravity from the malt bill:
Belgian Pils 12.25 lbs x 36pts
Munich 3lbs x 35pts
flaked wheat 1lb x 35pts
special B 0.75lbs x 30pts
caramunich 0.63lbs x 33pts
caravienne 0.37lbs x 34pts

Total potential gravity units = 637 GU

After a Single Infusion mash and sparge I got 7 gallons and measured the gravity (corrected for temperature using Beersmith; temperature measured before and after grav measurement and averaged = 126.5F) 1.071. 7x 71 = 497 GU

Efficiency = 497/637 * 100% = 78%.

This much seems straightforward (I didn't make a mistake did I?). Here's where it gets weird. I added 1lb Turbinado sugar (supposedly potential gravity of 44, 100% efficient) to the boil (after taking the above measurements), hops (1oz Northern Brewer 75 mins, 0.5oz Saaz 5min), Irish moss (15 min), boiled for 75 min, then chilled with immersion chiller to 75F (I know, I should have held out for 68F, but our chill water wasn't cold enough and/or we weren't patient enough), we dumped the chilled wort through a strainer into a conical. Then I measured the gravity out of the conical at 1.085 (temp corrected it should be 1.087). Volume approx 5.75gal.

This only gives 5.75 x 87 = 500.25 GU, which seems to indicate that I only got 3 grav points out of 1lb of Turbinado sugar!?

Or, do I need to take into volume shrinkage into account, since the 7 gallons was measured hot, and multiply it by 0.96? That changes the first equation to:
7 x 0.96 x71 = 477.12 GU, Eff = 477.12/637 *100% = 74.9%
But then I still only get 23 grav points out of my Turbinado. Granted, I got the Turbinado at Walmart, but it should be the same, right?

So, my three main questions are:
1) Where did my Turbinado sugar grav points go?!
2) When calculating the efficiency, do I correct the wort volume for temperature? (i.e. 7 gallons x 0.96?)
3) Did I make a mistake somewhere?

[Beer Martin]

Well, your equations look fine. Perhaps your boil-off rate was lower than you thought. How accurate is your 5.75 gallon measurement?

Say you actually have 6 gallons. That could potentially give you more points.

You hydrometer readings could be slightly off due to error as those things dont have a very accurate scale. your reading is probably about ? 2 points. When you multiply that error by volume (in your case 7, then 5.75) you could be of by as many as 14 points (then 11.5).

Also, those equations for adjusting specific gravity for temperature are a dirty science at best. They are watered down and simplified. There's an error associated with that of probably a few points. Add that to your reading the scale error and you've got a pretty wide margin of error there.

I wouldn't read too far into the missing points. The sugar is there after all.
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[tdfoste]

The 5.75 gallons is a bit of a guess. It was 6 gallons with the immersion chiller in the pot. We moved quickly after pulling the chiller and I forgot to look at the markings, so I estimate the final volume at about 5.75. It definitely wasn't as much as 6 and may have been as low as 5.5, but if that's the case, then 5.5*87 = 478.5, meaning I got either zero or less than zero gravity points from the sugar!

Ok, so there is may be error in the hydrometer, but if I always measure the same, then the relative error should be nearly zero. My point is that while the overall gravity may be inaccurate by as much as 11-14 points, the difference (assuming I measured the same way, which I try to) between measuring the pre-sugar wort to post sugar wort should reflect something closer to 44 points change, right? I mean, assuming the error is linear, then if the instrument is miscalibrated 5 pts too high, for example, all my readings will be high, but the difference will be correct and reflect the addition of sugar.

Is it possible that the hydrometer is really, really inaccurate at higher temperatures and that I need to take a sample and cool it down before reading it? I thought the whole point of temperature correction was so that I wouldn't have to do that!

[Beer Martin]

Ok, so there is may be error in the hydrometer, but if I always measure the same, then the relative error should be nearly zero.

Well, not really because if you read it off by 1 point your first measurement you multiply the error by 7. The second time if you make the same error it's only getting multiplied out by 5.75. Yeah it's small, but there's still a chance you could have read low the first time then hi the second time, which would have made the error up to 20 or 30 points.


Is it possible that the hydrometer is really, really inaccurate at higher temperatures and that I need to take a sample and cool it down before reading it? I thought the whole point of temperature correction was so that I wouldn't have to do that!

That is the point, but it gives you a reasonable answer not necessarily an accurate answer. Cooling down would remove that error.

There's also the chance you looked at the hydrometer and wrote down the wrong number. I've done that on several occasions.

Maybe you measurements were all perfect. Any chance you could have forgotten to add the sugar? (been there done that)
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[tdfoste]

Well, I definitely added the sugar (empty box and witnesses to prove it!), so scratch that off the list.

Hmm, well, I let the wort cool down to about 126F before I measured it, but I guess next time I'll let it sit out longer and cool even more. I was really hoping to not have to do that... Unfortunately, I wouldn't have uncovered the error if I wasn't adding sugar in later and rechecking the gravity and I don't have any more Belgians lined up in the near future to reexperiment with. I guess I could do the same experiment by measuring the preboil wort hot (around mash temps), then let it cool in the test tube (say as I'm boiling the rest), remeasure it. Then after the boil check the gravity again and see how the total GUs match up since they shouldn't change. That should give me the same kind of experiment - to test whether I need to measure my preboil gravity cold or hot, right?

Thanks for your advice. Pretty frustrating to find out my 'scientific' equipment may be unreliable!

[Beer Martin]

Thanks for your advice. Pretty frustrating to find out my 'scientific' equipment may be unreliable!

I wouldn't say it's unreliable, but it's not perfect. It's cheap and really as accurate as we need. The more I think about it, the error probably has more to do with the temperature correction than the hydrometer readings. I just thought you should be aware of any potential sources of error because it's probably a little bit of both.

If it bothers you that much, you could always get a refractometer, but I feel unless you've got the cash laying around it's overkill.

In the end you're still going to have beer no matter how off your measurements were. If you're worried about replicating the recipie, you have it written down. Just use the same mash tun and techniques and you'll make the same beer.
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