Specific Gravity Increasing??

[Clint Yeastwood]

My imperial stout came in at 1.027 yesterday. Tonight it's at 1.029. More like 1.0285, but I can't enter than in a calculator. I used the same equipment to measure it, and I tried to be careful both times.

Has this happened to anyone else? Measuring gravity is pretty simple, so I can't see how I caused this. I put beer in a little food processor and ran it for a little bit to get rid of the bubbles. I added one drop of Fermcap to each sample to reduce the bubbles further. I used the same Thermapen to check the temperature.

The beer is on a Flotit, so it comes from the top of the keg. I'm wondering if yeast could be accumulating up there and throwing the gravity off. The beer has a strong yeast taste today. I brewed it 48 hours ago.

I'm going to let the sample sit for a while to see if anything changes.

 

[hotbeer]

Did you add a lot of sugar at the last moment? Perhaps like one of my stouts, the dense layer of dissolved sugar was underneath a layer that it hadn't fully mixed with. As fermentation got started, the bubbling yeast mixed the two layers and I saw the SG increase by quite a few points.

 

[Clint Yeastwood]

Didn't do anything. I also rinsed the graduated cylinder with a little beer in case I somehow failed to see water in it.

I guess I should quit pestering it and wait.

 

[Clint Yeastwood]

I couldn't leave it alone, so I did another test today. Degassed the sample. Measured. It was very clearly 1.026 after temperature correction. No doubt about it.

Left it on the counter for a while. Took a look at it later on, and it was sitting at 1.028 plus a hair, before correction. The correction tool calls it 1.030.

 

[Clint Yeastwood]

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.

 

[VikeMan]

I know what's happening. The heavy solids in the beer settle and push the hydrometer up.

As heavier particulates settle out, the density seen by the float part of the hydrometer will be the same for a while, but eventually will decrease. That's because the stuff at the top is falling at the same rate as the stuff just below it and so on. The gravity won't increase.

If you're seeing a real gravity increase after sitting on the counter for a while, it's temperature change and/or evaporation.

 

[Clint Yeastwood]

That's backward. As particles sink, the fluid around the bulb gets heavier, pushing the hydrometer up.

It's neither evaporation nor temperature. Temperature is accounted for in a calculation. The surface area exposed to the air is too tiny, and the time involved too short, to allow enough evaporation to move the reading this much.

 

[Clint Yeastwood]

I should add that the high reading stays stable for hours, even though evaporation is allowed during that time.

 

[VikeMan]

That's backward. As particles sink, the fluid around the bulb gets heavier, pushing the hydrometer up.

No. The particles that started off around the float are also falling, at the same rate as the ones that are moving in. So there's no change in density seen by the float at first. Then, the particles continue to fall, and you reach a point where there are no more particles left to move down to the float, but the ones now at the float keep falling. At that point, the density seen by the float decreases. When particles reach the very bottom, they are indeed very dense, at the bottom. But that doesn't affect the hydrometer, unless it's actually touching the mass at the bottom.

 

[Peebee]

That's backward. As particles sink, the fluid around the bulb gets heavier, pushing the hydrometer up.

It's not backwards. The particles sink because they are a higher density relative to the surrounding medium. So once the particles have sunk you are left only with the lighter surrounding medium. The lighter medium is less able to support the bulb, so it sinks. By a tiny, possibly immeasurable (with a hydrometer), amount, after all the particles were suspended 'cos they had a density close to the surrounding medium.

It's confusing 'cos we insist on measuring a weight-volume thing with a floaty buoyancy thing. Flippin' nonsense. And I've just turned this thread into an example of why hydrometers are stupid things to measure density of liquids (yeah ... but I also know it's a cheap way of getting the answer you want, so long as you don't have to try and think about it too much).

 

[Clint Yeastwood]

That's wrong. The particles don't go to the bottom. They can't all sink that far.

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. 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.

 

[VikeMan]

Well, I tried. I'm out.

ETA: OK I'm back. Not to debate, because I wouldn't say anything different than I have, and the repetition is pointless. But I'll leave this link here. I'd recommend anyone read the message at the link and the one right before it. I have conducted a similar experiment, with similar results.

Suspended Flour

 

[Clint Yeastwood]

Ditto!

 

[Peebee]

Damned if I'm getting out. I smell blood, so I'm giving it some.

Now we're defining the depth of medium? Guess it wasn't defined early on? I knew these "hydrometer" things were flippin' daft! Pascal's principles? Two thousand years too late: Archimedes sorted this out jumping into a bath one night. Stick with volumes and weights, it's so much easier to picture in yer 'ead ... and you don't get so wet splashing all that liquid about.

(I shouldn't be so rabidly anti-hydrometer, I'd probably use one if they hadn't gotten to be so impossible to read).

 

[jddevinn]

Use a taller container. OR Sedimentation Analysis - Hydrometer Method

 

[Peebee]

Use a talker container. OR Sedimentation Analysis - Hydrometer Method

Flippin' Heck!

Cracking stuff that! I've got that book-marked. The start of ... of ... well, I've no idea what, but it's got to be fun.

 

[Peebee]

@VikeMan chucked some stuff into the debate (which he's refusing to debate) (an edit to earlier post):

... I'll leave this link here. I'd recommend anyone read the message at the link and the one right before it. I have conducted a similar experiment, with similar results.

Suspended Flour

A thread discussing poodles "suspended" in a swimming pool ... this obviously needs long hard consideration, but I'm completely absorbed by "Sedimentation Analysis" (with hydrometers) at this moment, so I'll have to come back to it later ...