Help with calculating mash efficiency please

[richl025]

First time I am trying BIAB after doing extract recipes for about 6 months...

I can't remember where I got the recipe from, but I am trying an ESB with 10.75 lbs Maris Otter, 0.5lb Caramel 20 and 0.25lb Caramel 120.

I mashed in about 7 gallons of water for 75 min, and kept the mash temp a around 152 (+/-2 degrees). Heated up to 168 for a mashout for 10 min and then pulled the turkey fryer basket with my grain bag.

After pulling the basket, I supported it over my kettle with a couple of 2x4s and let it drain, and pressed out excess wort with a pot lid. I was left with about 6.25 gallons and used my spiffy-new refractometer to get a SG of 1.051.

So here's where it gets hinky...if I use an online calculator to correct the SG for temperature, I get a SG of 1.070 (way too high for an ESB of course) and an efficiency of > 100%!

So when I reran the efficiency calculator using 1.051 (ie, not correcting for temperature) by efficiency was now at a believable 73%...

So can someone explain to me why you would not correct for temperature? Is that something you have to do only with a hydrometer (since it measures density) but not for a refractometer (which measures solutes bending light or whatever...)

 

[grv]

I frequently mess up the grain absorption calculation. If you've got a way to measure volume directly (markings on mash paddle, etc.), you may be sure of your pre-boil volume. With that much grain, I'd usually use at least a gallon to absorption, but you may have squeezed that bag for all it was worth. If you really had less pre-boil volume, the efficiency would be more plausible.

I usually cool my sample before I measure. At least down to ~75 degrees. The conversion accuracy for temp is not so reliable in the mashing range, from what I've heard, so I play it safe. I use a hydrometer. Not sure with the refractometer if that's a concern.

Those are a couple reasons I can think of.

 

[stpug]

Generalized maximum points per gallon (i.e. 100% efficiency):
11.5 lbs grain x 36 point per lb = 414 points

You got:
6.25 gallons x 1.051 points per gallon = 318.75

318.75/414 = 77.0% efficiency (probably plus/minus a couple percent)

 

[grathan]

The sample of a refractometer cools so quickly that the original temperature matters not so much.

 

[stpug]

Another way is break out your grainbill by point per gallon for each grain:
10.75 MO x 37ppg = 397.75
0.5 C20 x 34ppg = 17
0.25 C120 x 33ppg = 8.25

Total = 423 ppg possible

You got: 318.75

318.75/423 = 75.35% efficiency

 

[stpug]

The main point is:

YOU DID AWESOME, especially for a first timer

 

[IslandLizard]

Welcome to AG brewing, you've entered a fascinating world. I don't do BIAB, but see 2 things you can improve on.

1a. The refractometer sample is very small so there is no need for temp correction. Placing 2 drops on the relatively cold and large crystal almost immediately brings it down to ambient temps. I'd take that reading as firm. If you're doing high gravity measurements (say over 1.060), unless you have a truly corrected refractometer (rare), a correction factor may be needed since the SG scale is off. In that case use Brix and convert. Print out a small conversion table (Excel) and read the corrected SG.

1b. That small sample, although convenient, is also a problem. It tends to evaporate a bit when transferring it from your kettle to the crystal. I use the point of a butter knife and questioned the reading on hot wort. So now I ladle a little out (couple table spoons) into a metal or glass cup and allow it to cool a bit before I take my measurement. I transfer the 2 drops quickly, and dump the wort back into the kettle.

2. Did you sparge at all? There is some good heavy wort trapped in that bag of grains. Squeezing alone is not getting that out.

 

[RM-MN]

Welcome to AG brewing, you've entered a fascinating world. I don't do BIAB, but see 2 things you can improve on.

1a. The refractometer sample is very small so there is no need for temp correction. Placing 2 drops on the relatively cold and large crystal almost immediately brings it down to ambient temps. I'd take that reading as firm. If you're doing high gravity measurements (say over 1.060), unless you have a truly corrected refractometer (rare), a correction factor may be needed since the SG scale is off. In that case use Brix and convert. Print out a small conversion table (Excel) and read the corrected SG.

1b. That small sample, although convenient, is also a problem. It tends to evaporate a bit when transferring it from your kettle to the crystal. I use the point of a butter knife and questioned the reading on hot wort. So now I ladle a little out (couple table spoons) into a metal or glass cup and allow it to cool a bit before I take my measurement. I transfer the 2 drops quickly, and dump the wort back into the kettle.

2. Did you sparge at all? There is some good heavy wort trapped in that bag of grains. Squeezing alone is not getting that out.

This is an important point. If you got 73 to 75% efficiency without the sparge, you did good. If you added a sparge step you should have picked up another 5% or so, putting you at nearly 80 %. That would be awesome. In either case, congratulations! You're well on your way toward beer.

 

[richl025]

Thanks everyone for the replies!

The comments about how rapidly the drops of wort on the refractometer crystal cools is a great point that did not occur to me

And IslandLizard, thanks for pointing out about evaporation of a warm sample like that - another thing that didn't occur to me... next time I will use your suggestion of cooling a small sample in a glass before transferring the drops onto the crystal...

One thing that surprised me about my first BIAB today, though, was the amount of suspended sediment in the wort. I used a whirlfloc tablet (also first time I ever used one) so was hoping to see it a little clearer... I'll wait until the yeasties have had their chance to work their magic before worrying too much about clarity, though...

Thanks to everyone here for the replies, and all the help in general!

 

[RM-MN]

Many people worry about the clarity of the wort but that isn't the objective. The objective is clear beer and wort clarity doesn't have a lot of bearing on that. Give the yeast plenty of time to complete the ferment and settle out and you get clear beer.