All grain gravity is it 'real'

[Northern Brewer]

When I next brew I am going to use a sinle sample and take readings (both gravity and brix) over a period of time. Hopefully this will enable us to work out whether the readings we take are accurate straight away or not!

unless of course somebody brews before me and decides to work this out and post the results themselves

I always fill my boiler to the same 40 pint calibration mark and take a gravity reading as the boil begins to roll. I cool the sample to 20C first and return it to the boiler afterwards. At the end of the boil I remove and drain my bagged hops and top up the boiler to the 40 pint mark with boiling water. I then fit the lid on my boiler and seal it in place with cling film, and I leave it to cool overnight outdoors. Never had a protein haze yet

Some 12 hours later I transfer the cooled wort to my fermenter, leaving the trub behind and I take another gravity reading, which usually comes out at 2-3 points lower than the original reading. The half-pint of wort that was lost with the hop bags and replaced with water isn't enough to cause such a drop in gravity, so I can only assume that the protein that's been left behind in the boiler impacted upon the original gravity.

[Mashman]

I have always understood that hydrometers measure the density of a liquid. The less dense the liquid the deeper they float.

[Andy]

I have always understood that hydrometers measure the density of a liquid. The less dense the liquid the deeper they float.

Indeed, archie meedies and all that

[mr.c]

Good read http://byo.com/mrwizard/1002.html that go well with your posts

[PieOPah]

A good article but it basically says that while solids shouldn't effect your reading you should let them settle out anyway.

[delboy]

The article is working on the what i believe to be the incorrect assumption that hydrometers only measure dissolved compounds. If this is the case why do soil scientists use hydrometers to measure soil texture (ie particle size). These are not dissolved particles but suspended particles.
Suspended particles change the density of the liquid they are present in AFAIK.

Having read that article i wouldn't haven't reached the same conclusion as the author about density stratification (he admits this shouldn't be a problem for all grain).
To my mind the more plausible explaination is that the suspended particles are falling out of solution therefore the top of the wort which has cleared will give a lower reading than the bottom of the wort which still has suspended particles falling through it.

I think the whole principle of the hydrometer has been 'dumbed down' for the homebrewing community. Probably some historical text handed down from one homebrewing text book to another is at fault.

[delboy]

I tend to take a lot of those BYO articles with a grain of salt, I reckon your theory holds water DB

Like the pun!

[PieOPah]

I think that the problem being experienced is that a sample is being read and the hydrometer being left in the sample for a number of hours is slowly sinking.

The more sugar a solution has, the thicker it is so the longer it takes for the hydrometer to sink (hence why it floats more)

If you took a jug of thick gravy and a jub if water and placed a stone in each, the stone would sink to the bottom of the gravy much much slower, but it would eventually sink.

Now this is just me assuming (so I could be totally wrong) but I think that when you first put your hydrometer in a fresh sample then the reading is accurate (on the assumption that your hydrometer is accurate) but over time as it sinks the reading becomes false.

As I have said in earlier posts, the only way to know this would be to take numerous readings over a period of time - but removing the hydrometer between readings!!!!

[Andy]

think that the problem being experienced is that a sample is being read and the hydrometer being left in the sample for a number of hours is slowly sinking.

The more sugar a solution has, the thicker it is so the longer it takes for the hydrometer to sink (hence why it floats more)

....

Now this is just me assuming (so I could be totally wrong) but I think that when you first put your hydrometer in a fresh sample then the reading is accurate (on the assumption that your hydrometer is accurate) but over time as it sinks the reading becomes false.

I think you're wrong there PoP.

Edit: As in your theory of the hydrometer sinking over time. If the density of the solution it's immersed in changes over time then that's a different story and that's probably what's actually happening.

[delboy]

POP this is exactly what is happening IMO. But the falling of the hydrometer into the liquid is not a gradual process its the sedimention of the suspended particles that is IMO.

If you have distilled water at 20C it will read 0.00 (hopefully) if you left this for a week (presuming no evaporation) i would bet my house on it still reading 0.00. The liquid has the ability to resist a force based on its density if the density doesn't change and the exerted force (weight of hydrometer) doesn't change it will remain exactly the same.

[PieOPah]

Now this is just me assuming (so I could be totally wrong) but I think that when you first put your hydrometer in a fresh sample then the reading is accurate (on the assumption that your hydrometer is accurate) but over time as it sinks the reading becomes false.

I think you're wrong there PoP.

It is very possible that I am wrong but we will never know until the theory is tested - which is something I intend doing when I next brew.

[delboy]

Quick clarification on my opinion of the BYO article. I actually agree that there is a density stratification (in AG) but that its caused by the sedimenting particles not the sugars.
To get density stratification of dissolved sugars from a homogeneous solution (well mixed like a full boil AG) requires enormous amounts of centrifugal forces (does anyone have an industrial scale ultracentrifuge for 5 gallon buckets maybe the BYO author does ).

[delboy]

Before i take this thread into new depths of home brew nerdiness. I'd like to add that the brew on which i saw the largest difference on the overnight readings was only a 60min boil (was pushed for time).
I feel that this may have excerbated the problem ie less of the smaller proteins had clumped together and therefore more was left in suspension.
If this hunch is right the difference in readings should be smaller for those doing 90-120 boils since more proteins are likely to have clumped together into large masses.
Moral of the story - do 90-120 min boils if you want more accurate hydrometer readings.

[PieOPah]

Ah but if you are consistently wrong without knowing it?

You could be measuring a OG of 1.050 when in reality it is 1.040

[delboy]

Does it really matter though as long as you are consistent.

Guess not, just making the point that in future im going to leave my boils consistently for 90 mins (thats what i get listening to american forums and their advocation of 60 min boils )