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Author Topic: SG Going Up and Down  (Read 826 times)

Offline goose

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Re: SG Going Up and Down
« Reply #15 on: January 31, 2024, 02:34:05 pm »
Did you de-gas the beer when you did your measurements?  If there is CO2 coming out of solution of the beer in the test jar, it will make the hydrometer float higher when the bubbles stick to the sides of it.  You need to pour the beer back and forth into two beakers/glasses or whatever vessels you have before you take a measurement to get the CO2 out of solution.
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Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #16 on: January 31, 2024, 05:49:47 pm »
It was definitely degassed.

Settling is what caused the measurement problem. Maybe this is a bigger problem with quick yeasts like kveik.
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Offline Skeeter686

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Re: SG Going Up and Down
« Reply #17 on: January 31, 2024, 06:26:07 pm »
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.

I'm trying to understand this.  I can see where "the poodles" (i.e., the denser particulates) could increase the density of the liquid in which they're floating.

However, when they settle to the bottom, then wouldn't you have a denser solution at the bottom and a less dense solution near the top?  If the hydrometer is floating, it would be in the less dense liquid near the top, wouldn't it?  So, shouldn't the density decrease with time if material is settling?

Offline Richard

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Re: SG Going Up and Down
« Reply #18 on: January 31, 2024, 07:40:01 pm »
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.

I'm trying to understand this.  I can see where "the poodles" (i.e., the denser particulates) could increase the density of the liquid in which they're floating.

However, when they settle to the bottom, then wouldn't you have a denser solution at the bottom and a less dense solution near the top?  If the hydrometer is floating, it would be in the less dense liquid near the top, wouldn't it?  So, shouldn't the density decrease with time if material is settling?

See my answer above. The average density of the whole liquid volume is irrelevant. The only thing that matters is the weight of the volume displaced by the hydrometer.  Anything that settles to the bottom and isn't displaced by the hydrometer will not affect the reading.
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Offline denny

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Re: SG Going Up and Down
« Reply #19 on: February 01, 2024, 09:18:39 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.

I'm trying to understand this.  I can see where "the poodles" (i.e., the denser particulates) could increase the density of the liquid in which they're floating.

However, when they settle to the bottom, then wouldn't you have a denser solution at the bottom and a less dense solution near the top?  If the hydrometer is floating, it would be in the less dense liquid near the top, wouldn't it?  So, shouldn't the density decrease with time if material is settling?

How would the poodles increase the density of the water if they're not dissolved in the water?
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Offline BrewBama

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SG Going Up and Down
« Reply #20 on: February 01, 2024, 10:03:44 am »
It’s easy to prove. Put water in a hydro flask and take a reading.  Put a tick mark at the low point of the hydrometer. This is your baseline.

Put enough marbles in the flask to come to a point just under the tick and take a reading. $10 to a doughnut the reading doesn’t deviate from baseline.

Now, take a cup of water soluble material (salt) and dissolve it in a cup or two of water and pour that in your hydro flask. $10 to a doughnut the reading deviates from baseline.

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.  Not the density of objects below the solution.

Water alone (aka the swimming pool):



Bottom sitting at ‘4’:



Marbles just below ’4’ (aka poodles):





Salt water (aka a solution):

« Last Edit: February 01, 2024, 11:29:20 am by BrewBama »

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #21 on: February 01, 2024, 10:31:07 am »
Picture a tube filled with oil and water. They separate. You put a hydrometer in the tube. The bulb is down in the water. The skinny part of the hydrometer is in the oil.

You're measuring the density of the water, not the oil. The oil makes some contribution, because part of the hydrometer is in it, but the fat part of the hydrometer makes the biggest contribution to the reading, and it's down in the water.

In case anyone doesn't know, oil is much less dense than water.
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Offline BrewBama

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SG Going Up and Down
« Reply #22 on: February 01, 2024, 10:45:50 am »
Oil floating on water is not a solution. A solution is a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).

Oil doesn’t dissolve in water. IOW it’s not water soluble. When you mix oil and water you get an emulsion (before they separate). An emulsion is a fine dispersion of minute droplets of one liquid in another in which it is not soluble.

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.  For the instrument to be accurate the liquid should be in solution.

Wort is a solution. There are water soluble components dissolved in the water.  Not all of these components are sugar.
« Last Edit: February 01, 2024, 11:25:59 am by BrewBama »

Offline Skeeter686

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Re: SG Going Up and Down
« Reply #23 on: February 01, 2024, 07:07:12 pm »
How would the poodles increase the density of the water if they're not dissolved in the water?

Poodles is probably a bad example.  :)

If the insoluble particulates are sufficiently small and similar enough in density to the water, they could stay in suspension for a while as they slowly drop to the bottom.  In that case, it seems that while they're still suspended, the hydrometer would be displacing both the water and the slightly denser particulates.

Then, once the stuff settles out, the hydrometer would be displacing only the less dense water. 

But if this actually works as described, it should make the SG go down over time.  I thought OP was talking about it increasing.

Offline Clint Yeastwood

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Re: SG Going Up and Down
« Reply #24 on: February 01, 2024, 07:56:02 pm »
I think it really helps if you have a physics background to help you understand analogies that are totally valid but not exactly like the things they represent. It's not the kind of thing high school grads or people with liberal arts degrees can grasp right away.

Physicists use exaggerated examples to understand and explain how things work. I'm sure engineers do it, too. For example, if you want to understand how a system behaves when one item's mass is changed, but you need a quick answer without much math, about principle, not exact numbers, you may pretend the mass is infinite or zero, even though these examples could never happen, and this answers your question instantly. That would be called using "limiting cases." I have forgotten most of the physics I learned before leaving grad school many years ago, but I recall how exaggeration is used to make systems easily understandable, and I have a pretty good understanding of buoyancy, which is very simple as physics go. It's the kind of thing doctors learn in their brief and very limited exposure to basic physics, which is all most of them learn. I used to teach them.

The oil example I gave is exaggerated in order to help people who don't know much about science understand it, but it is valid for illustrating a principle that indisputably exists. It is easier for most people to understand an extreme example of oil and water than a fluid which has a smooth density gradient, like stout that has settled.

In the case of beer with heavy ingredients concentrated toward the bottom, and the case of oil and water, you have a liquid sample which is denser at the bottom than the top, so a hydrometer will behave similarly, not identically, in both cases. The principle is the same. Same kind of error.

The bulb of the hydrometer displaces more fluid per unit of length than the skinny part, and therefore more mass, even when a fluid is homogeneous. When a fluid is heavier toward the bottom than the top, the bulb's contribution to the reading is even greater than it is in a homogeneous fluid.

When heavy ingredients move down in a beer, you get a heavier fluid toward the bottom, where the bulb is. This pushes the hydrometer up and gives you a reading that is not representative of the fluid as a whole. To get an average reading, you have to shake the fluid so the density is uniform. This would be true even with a refractometer, because heavier wort has a higher index of refraction.

If you don't like oil and water, think of a HYPOTHETICAL fluid with a density gradient that is extreme. Say the density increases with the cube of the distance from the meniscus. Up high, it may be like water. Down deep, it may be more like mercury, which is denser than lead. The heavy fluid at the bottom will exert a disproportionate amount of upward force on the hydrometer because it's down where the bulb is. That gives a high reading that isn't the average density of the fluid.

If it works in an imaginary extreme case, it will work in ordinary wort.

The business with poodles is not helpful because poodles are large and discrete. Beer is full of tiny suspended items and things that are sort of like, but not really, liquids. Think of the milk example Larry Shepley mentioned. Milk is full of tiny globules of fat until it separates. These globules aren't actually dissolved, but they are so small, the shaken milk acts like a homogeneous solution (hence "homogenized") until it separates. At first, you have a uniform sample, and a hydrometer works perfectly. Then you have a sample with a density gradient that changes with time. Then you end up with two liquids with a defined border. During the last two stages, a hydrometer will not work, and by "work," before someone says something that isn't right, I mean work to give you the average density of the entire sample.

If you don't believe I understand how this works, believe Larry did. He was a pretty smart guy. He was a protege of John Wheeler, and he went from a master's to a Ph.D. in two years. Easy in history, literature, and sociology. Not easy in physics. It's actually more impressive than it sounds, because Princeton used to give all grad students master's degrees as soon as they were accepted, so he went from undergrad to Ph.D. in two years.

Poodles float, by the way, so not really useful at all.

Here's something interesting, at least to me. Remember the scene in "Aliens" where Ripley dropped herself in molten lead and sank in? Couldn't happen in real life. She would have rolled around on the hot surface in agony because she couldn't sink more than a couple of centimeters, and she would have broken a lot of bones on impact because the mass of the lead would have resisted accelerating away from her body. She was not dense enough to sink.
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Offline denny

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Re: SG Going Up and Down
« Reply #25 on: February 02, 2024, 11:34:43 am »
Everybody can easily test this for themselves. Measure a sample with trub. Pour it through a coffee filter and measure it again. I have.
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