Something wrong in boil?

[0verdrive]

I recently switched to brewing all-grain but am missing my target gravity, and I'm trying to figure out where things are going wrong.

The following calculations are based on this Guinness Draught Clone. I'm not positive that I'm doing the calculations correctly, but it looks like something is going wrong during the boil, as I have an 83% mash efficiency, but only a 59.5% brewhouse efficiency. This seems like a pretty drastic drop in efficiency, so I'm wondering if my boil process is off, or if my calculations are incorrect.

Recipe:

8 lbs Canadian 2-Row (80 Extract CG => 294.4 max points)
3 lbs Flaked Barley (70 Extract CG => 96.6 max points)
1 lb Roasted Barley (70 Extract CG => 32.2 max points)

Total potential: 423.2 points

I had 8 gallons pre-boil volume, which I boiled down to 6 gallons. This means that the max mash efficiency (423.2/8) was 52.9 points, and the max brewhouse efficiency (423.2 / 6) was 70.53 points.

(If the above numbers check out, then my calculations are correct, indicating that there's something wrong with my boil process.)

Before the boil, I measured the gravity at 1.033 @ 120* (adjusted: 1.044). This gives me a mash efficiency of 83%.

After the boil, I measured the gravity at 1.041 @ 72* (adjusted: 1.042). This gives me a brewhouse efficiency of 59.5%

Is it normal for there to be this large of a discrepancy, or am I doing something wrong? Should I be cooling my pre-boil wort sample, rather than relying on a temperature conversion? Anything else to check?

Thanks for your help,
~Dean

 

[0verdrive]

One related question, which may help highlight what I'm trying to get at. The recipe calls for boiling down to 5.5 gallons. I accidentally only boiled down to 6 gallons, so I know that will effect my starting gravity. But I still shouldn't have been as low as I was.

Recipe OG: 1.053 (for 5.5 gallons)

If this were a 6 gallon recipe, the expected OG would be (53 * 5.5) / 6 = 48.58 => 1.048.

My 8 gallon pre-boil measurement was 1.044. Which theoretically means that my (6 gallon) post-boil measurement should have been (44*8)/6 = 1.058

However, my 6 gallon batch ended up at 1.042. So my pre-boil measurement indicated that I should have been .01 over the recipe, but I ended up .008 under. Any thoughts on what I could have done during the boil to cause this?

Thanks again,
~Dean

 

[naiek]

I think your gravity measurements are off; I don't think it's possible to boil off 2 gallons and have the gravity decrease by 2 points. Perhaps one (or both) of your samples were not well mixed and you pull a high/low concentration sample

 

[Yooper]

Keep in mind that any SG reading taken over about 90 degrees is useless. Cool the sample to under 100 degrees, THEN take the reading and use the conversion. Over 100 degrees, those tables are so inaccurate as to be worthless. I bet that's part of the issue- the wrong preboil OG reading.

 

[mgortel]

I take my sample and put it in my sample tube and place in a 1/2 gallon pitcher full of ice & water.....it doesnt take long to cool it down to 70 degrees or so......try that next time for your pre boil OG

Definately sounds like a bad measurement because the SG's just don't add up ....one of them is wrong

 

[0verdrive]

Thanks for all the replies!

Since this is a learning experience for me, I figured that my mistake was in my understanding of how mash or brewhouse efficiency is calculated. So I was so focused on making sure my calculations and conversions were correct, that I apparently forgot to step back and look at the fact that the numbers my calculations were based on had to be wrong. (I should have seen that there isn't any way for the amount of sugar to somehow reduce after boiling off 2 gallons, but I somehow missed it.)

But that's why I posted here - I appreciate the help. Next time I'll be sure to get a well-mixed sample, and will cool the sample before taking my reading.

Thanks again!