Hydrometer Question

[Rob2010SS]

Every brew, I pull a sample out of the BK once I've collected all my pre-boil volume to do a pre-boil gravity check. Because I have the heating element running the entire time I'm lautering to the kettle, by the time I've collected pre-boil volume, it's usually right around 205-206F. So, I pull the sample out and let it sit in my hydrometer tube while I baby sit the boil for the first period of time, to add hops and make sure I don't have any boil overs. Once it's under control and I don't have to worry about it, I go and check pre-boil gravity.

Now obviously, the hydrometer will read differently at ~130-140F than it will at room temp, so I know to use the calculator to figure out the real reading.

Two times now, I've checked pre boil gravity, used the calculator to figure out the real reading at that temperature, and then used the dilution and boil off calculator to figure out what my expected OG will be after the boil is complete and the number has been way off. For example, on my brew this past sunday, I was targeting a final OG post boil of 1.042...

- Pre-boil sample was at 132*F at time of checking
- Hydro reading was 1.038
- Corrected value based on Brewers Friend calculator was 1.049
- Knowing that I boil off ~1 gallon during the boil, Brewers Friend boil off calculator told me I'd be at 1.052 OG.

I confirmed I did boil off ~1 gallon (based on etched gallon markers) and OG came out at 1.045.

So, here's my question: I know hydrometers aren't accurate at those temps, but can they REALLY be that far off on a hot sample?

I am running a Spike Brewing steam condenser lid and my first thought was that water from the steam condenser was running back into the kettle. However, if this was true, 2 things would be happening...
- I'd be able to see the water flowing back in when I pull the 4" clear cover off and look in there with a flashlight during the boil. I always check just because I'm paranoid and there's never any water coming back into the kettle.
- My post boil volume would be higher than my pre-boil volume, assuming the hydometer reading was accurate. The only way the OG could be lower than my corrected pre-boil boil reading would be more volume, and that is definitely not the case.

So in my head, that leaves one option - the hydrometer at that temperature is just really unreliable.

Would you agree?

 

[apache_brew]

Ya I wouldn’t trust a hot sample and conversion. A friend of mine gave me a nifty sample thief he made from a roughly 18” piece of 1/2” copper. The bottom has a soldered end cap, and the top has a nylon loop for handling. Whenever you want a gravity sample on the hot side, dip it into the boil kettle, then use a garden hose to spray the copper tube and cool the sample to 70F. Works great.

 

[Immocles]

I take most of my pre boil hydro reading at about 100F and find it pretty close with the correction formulas. I do find pre boil readings to be sketchy at best as far as the sugars being equal throughout the wort. You mention having this happen twice, but out of how many brews? Are they normally pretty close, and theres just two outlier reading that are way off? Cuz I might chalk that up to stratification

 

[Immocles]

Ya I wouldn’t trust a hot sample and conversion. A friend of mine gave me a nifty sample thief he made from a roughly 18” piece of 1/2” copper. The bottom has a soldered end cap, and the top has a nylon loop for handling. Whenever you want a gravity sample on the hot side, dip it into the boil kettle, then use a garden hose to spray the copper tube and cool the sample to 70F. Works great.

Thats a clever idea. I end up dipping a pyrex measuring cup into the wort to collect the sample and letting that sit into an ice bath or my freezer for a bit.

 

[Vale71]

Do you give the wort a good stir before collecting the sample? Sparging is where you have the highest chance of stratification as you're running ever lighter liquid into a vessel at a low rate with little turbulence. Until the rolling boil sets in there is only limited mixing of the wort so that the sample's OG could be quite off depending on whether you take the sample near the surface or near the bottom. Plus it's always best to actually cool the sample to room temperature and not rely on formulas to compensate for more than a few degrees.

 

[Rob2010SS]

I take most of my pre boil hydro reading at about 100F and find it pretty close with the correction formulas. I do find pre boil readings to be sketchy at best as far as the sugars being equal throughout the wort. You mention having this happen twice, but out of how many brews? Are they normally pretty close, and theres just two outlier reading that are way off? Cuz I might chalk that up to stratification

I want to say maybe 20 brews or so on this electric system. I check these numbers on every brew and usually they line up fairly well, but it's been these past couple brews where it changed and not sure what the difference is.

Do you give the wort a good stir before collecting the sample? Sparging is where you have the highest chance of stratification as you're running ever lighter liquid into a vessel at a low rate with little turbulence. Until the rolling boil sets in there is only limited mixing of the wort so that the sample's OG could be quite off depending on whether you take the sample near the surface or near the bottom. Plus it's always best to actually cool the sample to room temperature and not rely on formulas to compensate for more than a few degrees.

It is possible that I'm not stirring enough. I do pull the lid off and give it what I considered a decent stir but in hindsight, when you're looking at almost 20 gallons of wort, it's possible that the stir I've been giving it isn't enough.

I'll try on the next one to give it a better, more thorough stir and see how it goes.

 

[Jeremy W]

Do you give the wort a good stir before collecting the sample? Sparging is where you have the highest chance of stratification as you're running ever lighter liquid into a vessel at a low rate with little turbulence. Until the rolling boil sets in there is only limited mixing of the wort so that the sample's OG could be quite off depending on whether you take the sample near the surface or near the bottom. Plus it's always best to actually cool the sample to room temperature and not rely on formulas to compensate for more than a few degrees.

I had very high pre-boil numbers on a couple brews and this is what I determined was the problem. Now I typically take a pre-boil reading once I reach rolling boil to take advantage of those convection currents to ensure it is mixed together.