Pre-boil gravity?

[curtw]

The last time I brewed, I took a sample of my post-mash wort (BIAB), before starting the boil, to test the gravity. I wanted to do the measurement before I started the boil, to see if I needed to add some DME, but was in a hurry and didn't have time to cool it properly.

When I went to measure an hour or two later, though, the 8 oz glass I had it in had an inch of trub on the bottom.

So my stupid question is: What do I use to measure the gravity? Jut the clear wort sitting above the trub? Or the whole contents, mixed together? What if I don't wait long enough for the trub to settle out -- will I get a useless reading?

I'm guessing that if I mixed it together the "gravity" would be a lot higher, right?

Thanks!

 

[brewkinger]

No.

Specific gravity is a measure of density of liquid when compared to water (1.00)
The suspended particles play no role in the density of the wort as they are not dissolved into it.
The dissolved sugars on the other hand, do change the density.

So as long as the sample is cooled to around 68 degrees ( which most hydrometers are calibrated at) you will get an accurate reading.

 

[BackAlley]

No.

Specific gravity is a measure of density of liquid when compared to water (1.00)
The suspended particles play no role in the density of the wort as they are not dissolved into it.
The dissolved sugars on the other hand, do change the density.

So as long as the sample is cooled to around 68 degrees ( which most hydrometers are calibrated at) you will get an accurate reading.

I don't think that's correct. You're right, the density of the wort does not reflect the undissolved particles but the hydrometer is measuring the SG of the suspension, not just the liquid.

 

[Yooper]

I don't think that's correct. You're right, the density of the wort does not reflect the undissolved particles but the hydrometer is measuring the SG of the suspension, not just the liquid.

No, the particles on the bottom aren't "heavier" or "lighter" than the wort. The SG is reading the density of the liquid.

 

[brewkinger]

The sugars dissolve into the water.

The dissolved sugars cause the density of the water to increase.
The floating hop and grains particles have no influence on the gravity of the wort.
A 1.040 wort is 1.04 times as dense as water. The floating hop and grain bits have no effect.

 

[BackAlley]

Folks in soil mechanics use hydrometers to measure the density of water/clay suspensions. So while the effect may be negligible, the presence of suspended particles in the wort must have some effect in the gravity reading doesn't it?

 

[brewkinger]

The inquisitive part of me had to research this a little bit, so I called a friend of mine who is a civil engineer.

Asked him about soil mechanics and the use of hydrometers and now I have more questions.

It seems that they do indeed use hydrometers for just this purpose.
He explained that they take a soil sample and add something called sodium metahexonate which causes particles to separate.
It is tested after 40 seconds which I the time it takes for sand to settle to the bottom. This gives them the clay/silt ratio.
Then it is tested again, although I forget how long he said they wait until the clay settles and this gives them a silt ratio.
His explanation went along the lines of the suspended particles, especially the finer particles, essentially are small enough to alter the density because they are very small.
He also mentioned that the range of values for gravities was different than we use for brewing.
Huh... Who knew??
Learn something new every day!

 

[BackAlley]

The silt actually settles first and the clay last. As the particles are falling they are actually changing the pressure distribution around them. Get enough of them and you change the pressure profile enough to effect the apparent density of the suspension. In other words, while the little clay particles don't actually push in the hydrometer, the effect is the same. Now clay particles have a density about twice that of water. Obviously hops and coagulated proteins etc aren't nearly as dense so again I'm not sure clear vs suspension is really a practical matter. Might be an interesting experiment.

Thanks for digging into this a little, I was starting to doubt myself. Hope I wasn't a jerk about it.

 

[FuzzeWuzze]

Use A Refractometer.

Problem solved

 

[brewkinger]

Use A Refractometer.

Problem solved

This^