Does a weak boil affect gravity?

tullamoremike · 02-25-2011, 05:30 AM

So I just bought one of those 160k btu burners ($30 shipped, let me know if you want the link) to try to correct this but I wanted to get some input from you guys anyway. On todays brew beersmith was calling for 1040 preboil (6.5 gal)/1052 postboil (5 gal) @75% eff. Preboil gravity was 1046, got 6.5 gals and tested last runnings at 1014. Boiled for approx. 70 mins to get down to 5 gals, gravity was 1050. I took a sample before chilling, cooled the sample to 70 and tested. Just for the heck of it I tested again after chilling the batch to 68 and it was the same. All of my hydro tests were taken below 70 degrees, what the heck is going on? This has happened on 5 consecutive batches. How is it that I get 85% into the boil and 71% out? The only thing I can point my finger at is the weak burner on my stove. My boil-off rate is fine but it's not even close to a rolling boil. Could this affect effeciency? Thanks guys!

northernlad · 02-25-2011, 05:36 AM

Gravity is linear and fixed. If your gravity was 1.046 @ 6.5 it will be 1.060 @ 5 gallons regardless of the boil strength.

tullamoremike · 02-25-2011, 05:42 AM

So a faulty hydrometer is the most likely cause?

robtotten · 02-25-2011, 06:04 AM

I would bet it's a faulty volume reading.

tullamoremike · 02-25-2011, 06:10 AM

I tested my hydrometer (24g sugar/176g water = 1.048) and it's spot on. I stir well before taking samples but I suppose I could stir longer; if gravity is linear and my hydrometer isn't screwy than all I can figure is that I've somehow managed to get a sample of the higher grav runoff. I'll be more careful next session and see what I come up with.

tullamoremike · 02-25-2011, 06:16 AM

My collection bucket, kettle and fermenter all have temp adjusted marks and always coincide. I used the same gallon container for all of them so even if it's volume isn't correct, the ratio would be the same.

nilswhite · 02-25-2011, 06:45 AM

Link to the 160k btu burners please?

tullamoremike · 02-25-2011, 01:00 PM

http://www.instawares.com/range-accessories.179.3.7822.0.0.8.htm

**Cape Brewing** · 02-25-2011, 01:44 PM

Your efficiency doesn't really have anything to do with the boil. The efficiency is how "efficient" you are at converting and then washing out the sugars from the mash. Boiling is done after that process so it has no bearing on efficiency.

Also... like northern was saying... gravity is linear and fixed.

Gravity is simply the amount of sugar that is desolved in the solution. You can't get more or less sugar into a boiling pot unless you specifically add sugar, or replace the boiling wort with a lower gravity wort... or water. I am guessing you don't have a leaking pot or something wierd so it is probably safe to assume the amount of sugar in your boil pot isn't changing.

What WILL change is obviously the concentration of sugar since, by boiling, you are removing water... making the sugar more concentrated... and increasing gravity.

So, after you boil all of that down (pun intended) I think the real question is: How do you go from 6.5 gallons of 1.046 down to 5 gallons of 1.050? On the burner... the speed with which you boil has nothing to do with it. If you go from 6.5 down to 5 in ten minutes or ten hours, the effect is still the same.... all you've done is take out a gallon and a half of water. Sugar won't evaporate so you can't lose sugar that way.

... and 6.5 gallons of 1.046 CAN'T turn into 5 gallons of 1.050... unless you take out some of that boiling wort (and sugar) and add water or something.

6.5 gallons of 1.046 down to 5 gallons will give you 1.060

I can't think of anything other than the gravity readings are wrong.

First, Do you know what temp your hydro is calibrated to? Most are 60 (that might be a SMALL factor)

Second, you said you measured a 1.046 preboil and then had 1.014 second runnings... are you making sure ALL of the second runnings are run off into the kettle and then mixing it all REALLY well... then chilling your pre-boil down to 60-something degrees before taking your pre-boil gravity reading? This is my guess for the problem. My guess is that you're pre-boil wasn't actually 1.046 but after a really good mixing with the last of the second runnings and then a chilling down to room temp, your pre-boil was more like 1.039... which would give you a 1.050 post-boil.

tullamoremike · 02-26-2011, 12:18 AM

I'm obviously screwing something up somewhere, lol. My hydrometer is calibrated to 68, so I always get my grav samples to 70 before testing. 1014 was where I stopped sparging, at which point I had collected 6.5 gallons. I stirred the wort before taking a sample but I may not have stirred well enough and ended up with an abnormally high sample. What is the most accurate way to get a pre boil gravity sample? Is it ok to really stir the heck out of it at this point without worrying about oxidizing? It's either this or my volumes are off, I'll check both next time I brew.