How off will your OG reading be if you pull sample from top of carboy?

[thrasher141]

I have brewed about ten batches of beer in my life, all but the first being all grain. For a while I was worrying that I was getting low efficiency on my mash process, because my OG readings were always lower than expected, to the point where my mash efficiency would be 50 or 60% instead of an expected 75% or so. A brewer friend recently suggested that my readings might be low because I draw a sample from the top of the carboy before pitching the yeast.

Here's my normal process: cool wort from kettles (I have to use two or 3 b/c they aren't big enough) and pour each into the carboy. Draw a sample with my turkey baster, then pitch yeast.

It's been suggested that depending on how long you wait to take the sample, the high density wort will sink to the bottom and you are left pulling the lighter stuff off the top. Is this true? And does anybody know how much the resulting OG reading could be off by? Like could it read a 1.04 even though overall your wort is a 1.06? That would surprise me if it could be off by that much.

 

[TyTanium]

Usually that's only the case with extract batches where you have concentrated wort + topoff water. But in your case, with 2 or 3 kettles, I suppose that could happen.

When you sparge, do you just collect until a kettle is filled, then move to the next? Meaning you have vastly different gravities in each kettle? Or do you mix them?

What I would do is pull a sample from each kettle. If the weighted average is significantly different from your aggregated sample, then you know that's your problem. If not, then you may actually have an efficiency issue.

 

[thrasher141]

Oh good point, I should have said that each kettle/pot would be a different sparge from the mash, with the last one being added to the carboy the weakest (lowest grav). But I would figure that when I pour them in to the carboy they would mix...no?

 

[dcp27]

unless there is a significantly different wort in each kettle, top or bottom shouldnt make a difference. after the boil its a uniform solution

 

[TyTanium]

I'd bet that's your issue. They'd mix, but not perfectly right away - it takes a while to equalize. Take readings from each pot to be sure.
(V1 x G1 + V2 x G2) / (V1 + V2) = OG, where V = volume, G = gravity

 

[RM-MN]

Go order a black and tan and watch it being poured. Now do you think that pouring the wort into the carboy will mix properly? I think your lowest OG being poured into the top tends to stay there until the yeast mix it up.

 

[thrasher141]

Thanks guys, that is helpful. I'm really hoping that is my issue and not efficiency issues! So what about when taking a FG reading after the brew has been sitting for a couple weeks in carboy, do you need to worry about it being mixed up before you draw a sample off the top of the carboy?

 

[TyTanium]

Nope, no need to worry for FG readings. It'd equalize completely on its own in that time, much less all the yeast activity.

 

[AKnewbrews]

Shake the carboy and aerate the wort, then take a sample before pitching the yeast, that way you have an equal solution. You may have to wait for a little oxygen to come out of the sample to get an exact reading.

I have drained a kettle into two parts and measured them separately before. The first half was about 2 points higher in gravity than the second, probably because the heavy, more sugar laden wort is on the bottom of the kettle after the boil. I had let is sit for a short period of time before draining to the fermentors as well.

 

[FarmerTed]

If the OG is off when you take from the top, it's gonna be off when you sample the bottom as well.

 

[Damoxemus]

Usually that's only the case with extract batches where you have concentrated wort + topoff water. But in your case, with 2 or 3 kettles, I suppose that could happen.

When you sparge, do you just collect until a kettle is filled, then move to the next? Meaning you have vastly different gravities in each kettle? Or do you mix them?

What I would do is pull a sample from each kettle. If the weighted average is significantly different from your aggregated sample, then you know that's your problem. If not, then you may actually have an efficiency issue.

I just did an extract/partial mash and I had to top it off with quite a bit of water (maybe 1.5 gallons). My reading at first was 1.042. Is this a pretty common thing that happens? Should I boil more LME next time?

And how much does the temperature of the wort effect the reading? I think it was 69*F.

 

[Riseabovebb]

Temps in the 60-69 degree range shouldn't affect your hydro reading to any great degree. IIRC, temps in the 70 degree range only change the reading by .002 or so. Pretty insignificant, as far as I'm concerned.

 

[Bear419]

Last week I topped off a partial boil with 2 gallons and was 20 points off, but knew this wasn't the case because I took a gravity reading of the post boil wort before diluting and it was dead on. The best way is to take a reading of all components individually, then use a dilution formula to find the OG.

 

[basilchef]

Will using the hydrometer in the primary bucket instead of test tube have a different result?

 

[dcp27]

Will using the hydrometer in the primary bucket instead of test tube have a different result?

no, just easier to read in a tube