Does a weak boil affect gravity?

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!

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.

So a faulty hydrometer is the most likely cause?

I would bet it's a faulty volume reading.

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.

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.

Link to the 160k btu burners please?

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.

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.

You'll find a lot of folks on this site that think hot side aeration ("oxidizing" before the boil is done) is a myth and isn't going to hurt your wort. If it is something you're worried about, you can stir like crazy... just don't splash. Just stirring it isn't going to really introduce a lot of oxygen.

It's gotta be either the reading is off for some reason or the volume is off. I can't think of any other reason why it would be happening.

Cape Brewing said:

You'll find a lot of folks on this site that think hot side aeration ("oxidizing" before the boil is done) is a myth and isn't going to hurt your wort. If it is something you're worried about, you can stir like crazy... just don't splash. Just stirring it isn't going to really introduce a lot of oxygen.

It's gotta be either the reading is off for some reason or the volume is off. I can't think of any other reason why it would be happening.

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I think it is a myth as well - I always drop my brew pot (this time of year) in a snow bank and stir constantly for 10-15 minutes. 12-15 brings me down to at least 65F or less. Towards the 10 minute mark I start to get rough with my stirring to aerate. To say that I don't have some plashing and foam build up form the first 10 mins of cool down would be a lie. i.e. I am aerating even if not a lot. I.e. I doubt the validity of aerating hot wort, unless you extreemly do so perhaps.

Arg, I just figured it out guys. As it turns out, I'm a jackass. I was figuring my batch size at 5 gallons, which is what goes into the fermenter. I forgot that I was leaving about 1/2 gallon in the kettle with the trub. 6.5*46=299 / 5.5*50=275, that's a heck of a lot closer than 5*50=250. I'm still thinking that I'm not getting a totally accurate pre boil gravity reading due to poor mixing. I'm guessing it should've been more like 6.5 gal@1042 / 5.5 gal@1050, 78% effeciency. Which is higher than expected, but I'll take it You guys are the best! Thanks for all the responses. If I can remember I'll post the results of my next brew to compare.

Actually, that's about 72% into the fermenter, even closer to the 70 I was shooting for.

I'm having a little bit of a problem with Pre-boil gravity as well and am hoping someone can't clear this up for me. I'm collecting 6.5 gal prior to the boil and then boiling off a little over a gal, and leaving a little wort behind with the trub so that I am putting 5 gal into my fermenter.

To calculate preboil volume I would be taking a gravity reading of the 6.5 collected gallons correct. Multiplying the gravity by 6.5 and then dividing by the amount of grain used? At least this is what I am doing...

Also does the temp of the preboil wort have an effect on the gravity. I know Ideally for OG you want to take your reading around 70 degrees. Does it matter for pre-boil gravity? Do I have to adjust based on temp?

matalec1984 said:

I'm having a little bit of a problem with Pre-boil gravity as well and am hoping someone can't clear this up for me. I'm collecting 6.5 gal prior to the boil and then boiling off a little over a gal, and leaving a little wort behind with the trub so that I am putting 5 gal into my fermenter.

To calculate preboil volume I would be taking a gravity reading of the 6.5 collected gallons correct. Multiplying the gravity by 6.5 and then dividing by the amount of grain used? At least this is what I am doing...

Also does the temp of the preboil wort have an effect on the gravity. I know Ideally for OG you want to take your reading around 70 degrees. Does it matter for pre-boil gravity? Do I have to adjust based on temp?

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To "calculate" preboil volume? If you already know the volume, since you measured it, why would you need to calculate it?

Temperature has an effect on the actual volume, as water close to boiling will shrink about 4% as it gets close to room temperature. No, it should not have an actual effect on the gravity. It will have a small effect on the measuring of the gravity, but not the actual gravity.

I'm not trying to calculate pre-boil volume. I'm trying to calculate pre-boil gravity, and asking if the temp of the liquid would skew my hydrometer reading since it is calibrated to 60 degrees. Which someone on another thread as confirmed. Hence if my pre-boil gravity hydrometer reading is 1.042 at 140 degrees then the temp corrected gravity reading is 1.054 which gives me a significantly different efficiency then the non temp corrected one.

matalec1984 said:

I'm not trying to calculate pre-boil volume. I'm trying to calculate pre-boil gravity, and asking if the temp of the liquid would skew my hydrometer reading since it is calibrated to 60 degrees. Which someone on another thread as confirmed. Hence if my pre-boil gravity hydrometer reading is 1.042 at 140 degrees then the temp corrected gravity reading is 1.054 which gives me a significantly different efficiency then the non temp corrected one.

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Using a 60° hydrometer in 140° wort is not going to give you anywhere close to accurate results, no matter what correction formula you try to apply to adjust for the incorrect temperature. At least, that's what I've read a goodly amount of times on here.

LandoLincoln said:

Using a 60° hydrometer in 140° wort is not going to give you anywhere close to accurate results, no matter what correction formula you try to apply to adjust for the incorrect temperature. At least, that's what I've read a goodly amount of times on here.

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Yes, that's been my experience. They are pretty good at under 100 degrees, and then converting with the table/correction software, but a hydrometer reading above 100 degrees is notoriously inaccurate, and at mash temps totally useless.

Then what are your suggestions yooper for getting an accurate gravity?

I'm interested in the reply to the last question there...

Cool the sample to 60 degrees and take your gravity reading.

yup.... it's that simple.

i take out a couple of cups or boiling wort, put it in a good sized sauce pot and swirl the pot in a sink half full of cold water. You can bring down the temp extremely quickly, take a temp (it should be well below 100F) and then take a gravity reading.

Like Yoop said earlier, if you're under 100F (the lower the better) basic correction calculations get better and better and more reliable.

I think my refractometer says 68F and the hydrometer is 66F?

I know if your wort is hot and you take a reading with the hydrometer you will read a lower than true gravity. As the wort cools the hydrometer rises.

using a refractometer has an advantage here. Hot wort in the pippet cools in a matter of seconds for an accurate reading.

matalec1984 said:

Then what are your suggestions yooper for getting an accurate gravity?

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He's probably going to say using a refractometer. I love love love mine. I take 1st runnings gravity, second runnings, pre-boil gravity, gravity during the boil, etc.. all for a fraction of the wort usage of taking a single reading with a hydrometer.

A majority (I think) of hydrometers are set to use 60F as a baseline temp but there are also a good number of ones that are set at 66F. Not sure why the difference but as long as you know which one you have, you can make the corrections easy enough.

matalec1984 said:

Then what are your suggestions yooper for getting an accurate gravity?

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If you're using a hydrometer (which is fine), pull the sample, and stick the test jar in a pitcher of ice water. Cool to under 100 degrees, and then read it and THEN use the conversion charts. It will cool pretty quickly that way, and you can depend on your reading then. You can pour the sample back into the mash/boil kettle when you're finished.

Or you can use a refractometer if you have one.

TheBeerist said:

SHe's probably going to say using a refractometer. I love love love mine. I take 1st runnings gravity, second runnings, pre-boil gravity, gravity during the boil, etc.. all for a fraction of the wort usage of taking a single reading with a hydrometer.

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Fixed!

Just be careful going from boiling to ice water... I had a bell jar es-plode in my hand doing that very thing.

(why I moved on to the whole saucepan dealy)

Thanks everyone...Not gonna lie after I posted my "what do you suggest Yooper" question common sense finally kicked in and I realized I simply need to cool the wort to get the accurate reading. Sometimes the questions come quicker then my brian actually works.

matalec1984 said:

Thanks everyone...Not gonna lie after I posted my "what do you suggest Yooper" question common sense finally kicked in and I realized I simply need to cool the wort to get the accurate reading. Sometimes the questions come quicker then my brian actually works.

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we'll leave it alone now.