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Author Topic: SG Going Up and Down  (Read 778 times)

Offline Clint Yeastwood

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SG Going Up and Down
« on: January 30, 2024, 04:00:11 pm »
I brewed a stout at 1.085, according to a Smartref. Pitched one packet of hydrated Lutra. This was three days ago.

One day later, I measured it with my old hydrometer, and I got 1.027.

Yesterday, i measured again and I got 1.029

Today, I got 1.026. Then I walked away for a while, leaving the sample on the counter. Came back later, and I had 1.028 plus a hair at 73. That's 1.030.

Can't figure out what's happening.

I'm degassing the beer. I'm making sure there is no water or anything but beer in the graduated cylinder. I'm letting the beer settle.

Any ideas?
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Offline Richard

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Re: SG Going Up and Down
« Reply #1 on: January 30, 2024, 04:24:55 pm »
If repeated measurements on the same sample give different results then your measuring technique is at fault (unless you have quantum beer). Without being there and seeing what you are doing, I can't go any further in diagnosis.
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Offline chinaski

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Re: SG Going Up and Down
« Reply #2 on: January 30, 2024, 05:21:06 pm »
Not sure what a Smartref is or how it measures, but regardless there is always some degree of error in measuring anything.  Your measurements are clustering around 1.028 or so with not a ton of variation.  If I had to guess, a Smartref gives you some sort of digital readout that makes you think that its more accurate than it really is.  I'd settle for the average reading and use that for whatever you need it for.  You could always take another reading with another device, say a hydrometer, and compare.  Without careful calibration, you cannot know which device is accurate.

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #3 on: January 30, 2024, 05:49:46 pm »
Thanks for the replies.

The Smartref is a digital machine I only use for OG.

There is probably someone on this forum who has had the same problem and will know what's happening.
Go ahead. Make my IPA.

Eccentricity is its own reward.

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #4 on: January 31, 2024, 06:01:56 am »
I know what's happening. The heavy solids in the beer settle and push the hydrometer up.


Hydrometers don't work in solutions that aren't homogeneous.


The low readings are more correct than the high ones.
Go ahead. Make my IPA.

Eccentricity is its own reward.

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #5 on: January 31, 2024, 07:17:01 am »
Someone else suggested evaporation and temperature change are what caused the change. That's wrong.

Temperature change is accounted for in a calculation, obviously.

The exposed area of the beer is way too small for evaporation to matter. As proof, let me point out that the gravity readings of my beer stabilized with time, in spite of continued evaporation.

I haven't been able to think of anything that would cause a false low reading. Maybe someone else knows. Doesn't matter in this case, but something to wonder about.
Go ahead. Make my IPA.

Eccentricity is its own reward.

Offline denny

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Re: SG Going Up and Down
« Reply #6 on: January 31, 2024, 08:25:13 am »
I know what's happening. The heavy solids in the beer settle and push the hydrometer up.


Hydrometers don't work in solutions that aren't homogeneous.


The low readings are more correct than the high ones.

If you throw poodles into a swimming pool does the water get more dense? Solids in the beer don't matter unless there is so much the hydrometer bottoms out on them.
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Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #7 on: January 31, 2024, 08:44:13 am »
Sorry, but totally wrong. Poodles DO make the pool more dense. Obvious if you think about it. Weigh the water without poodles. Weigh it with poodles in it. Compensate for the difference in volume. The density is higher.

Imagine putting a condensed neutron star the size of a fist into a pool and then weighing the contents of the pool. Denser as a sample, even if not uniform.

Would putting a poodle in a bathtub affect a hydrometer a foot away? No. But put a million poodles in a tub the size of a ship,let them sink, and use a giant hydrometer (lighter than poodles) that rested among them, and the density difference would show. It would sit on top of a layer of poodles.

This is not about huge particles like poodles. It's about tiny,slick particles than blend in with the fluid. And unlike poodles, proteins and yeast globs and so on don't go all the way to the bottom.

This is hard to explain to people who don't have physics backgrounds, but it's not a binary thing.

Think of a can of tomato juice that hasn't been shaken. The solids do not go to the bottom. They form a thick layer next to the bottom, just like the solids in my beer. If you were to measure the gravity of that layer, you would find it higher than the density of the clear liquid above it. There is more mass in it per unit of volume.

The hydrometer bulb rests in a deep layer of liquid full of heavy solids that have fallen lower in the column, so it measures the density of that layer.

This reminds me of a funny story one of my profs told me when I was in grad school in physics. He told it to show how people who think they know more than they do have to be careful. His name was Larry Shepley. He came out of Princeton's Institute for Advanced Studies. Larry said he had seen this question asked in a room full of top physicists, and every single one got it wrong.

Anyway, he said you take unhomogenized milk and shake it. Then you put it down. How do you measure the pressure at a given point? Knowing he asked so I could say something wrong, I said what the other guys said. I didn't try to figure it out. I said you measure the depth at that point. He said, "That is, of course, wrong."

Because the milk isn't homogeneous, the density varies with the depth, so it's not linear. You don't know how much mass is above the point of measurement. You have to know that in order to know the pressure.

Pressure is what drives a hydrometer. Pascal's principle. The molecules in the fluid push in every direction, including up.

My beer is like the settling milk. You have to measure it right away, not after the cream begins to rise.

There were no bubbles. Evaporation over hours didn't matter. Everything was clean. I compensated for temperature every time. It's the solids.
« Last Edit: January 31, 2024, 08:47:15 am by Clint Yeastwood »
Go ahead. Make my IPA.

Eccentricity is its own reward.

Offline ynotbrusum

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Re: SG Going Up and Down
« Reply #8 on: January 31, 2024, 08:45:21 am »
Stratification, perhaps?
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Offline Richard

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Re: SG Going Up and Down
« Reply #9 on: January 31, 2024, 08:54:44 am »
The principle of buoyancy is that the upward force is equal to the weight of the material displaced. If there are solids in the liquid but they aren't displaced by the hydrometer, as in a poodle in a swimming pool, they won't affect the reading of a hydrometer. If the solids settle to the bottom and are not displaced by the hydrometer they will not affect the reading. If there are solids suspended that settle out very slowly then they will affect the reading, but the fact that they settle very slowly means that their density is not much different than the surrounding liquid so they won't affect the reading very much.
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Offline neuse

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Re: SG Going Up and Down
« Reply #10 on: January 31, 2024, 09:18:58 am »
I'm a retired mining engineer. It's not something that's helpful in most life situations, but it helps in this case. Heavy media separation is sometimes used to separate minerals by specific gravity. From https://www.911metallurgist.com/blog/dense-heavy-medium-separation-hms-dms: "Since most of the liquids used in the laboratory are expensive or toxic, the dense medium used in industrial separations is a thick suspension, or pulp, of some heavy solid in water, which behaves as a heavy liquid."

For this method to work, the solids must be very fine so that it stays in suspension long enough for the separation to take place. This is the same effect as floating a hydrometer in wort that has very fine yeast or other solids suspended - the water with solids acts as a heavy liquid.

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #11 on: January 31, 2024, 09:22:00 am »
Quote
Stratification, perhaps?

Yes.

Quote
The principle of buoyancy is that the upward force is equal to the weight of the material displaced. If there are solids in the liquid but they aren't displaced by the hydrometer, as in a poodle in a swimming pool, they won't affect the reading of a hydrometer.

Yes.

Quote
If there are solids suspended that settle out very slowly then they will affect the reading

Yes.

Quote
the fact that they settle very slowly means that their density is not much different than the surrounding liquid so they won't affect the reading very much.

Depends on what you mean by "very much." A density discrepancy of a few points is very small. Also, the separation occurred over a few minutes, so not that slowly. Everyone here knows what it's like to see stuff at the bottom of an empty beer glass.
Go ahead. Make my IPA.

Eccentricity is its own reward.

Offline Richard

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Re: SG Going Up and Down
« Reply #12 on: January 31, 2024, 09:48:34 am »
I agree that 1.026 - 1.028 is a small difference, and could be due to particles settling out. I personally wouldn't worry about it. Most SG measurements are not highly accurate in the first place.
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Offline denny

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Re: SG Going Up and Down
« Reply #13 on: January 31, 2024, 09:56:32 am »
I agree that 1.026 - 1.028 is a small difference, and could be due to particles settling out. I personally wouldn't worry about it. Most SG measurements are not highly accurate in the first place.

This is the correct answer.
Life begins at 60.....1.060, that is!

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The best, sharpest, funniest, weirdest and most knowledgable minds in home brewing contribute on the AHA forum. - Alewyfe

"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell

Offline neuse

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Re: SG Going Up and Down
« Reply #14 on: January 31, 2024, 11:13:02 am »
After it's finished the solids should settle. I imagine the gravity readings will stabilize.