Correct Sg Temperature

[sosman]

So nothing to do with the viscosity of the wort?

[post="100831"][/post]​

Darren, it has everything to do with the density (Viscosity) of the wort. As the temperature increases the density decreases, therefore more wort needs to be displaced to hold up the hydrometer (Archimedes). Thus the specific gravity decreases.

[post="100873"][/post]​

Density and viscosity are two quite different fluid properties and measured by completely different methods. Mercury for example has a very high density and I am guessing its viscosity is quite low. Motor oil, is more viscous than water but less dense.

 

[sosman]

Sosman, the other physical parameters are changing as well and that I think that is the problem.

[post="100851"][/post]​

I am curious to know which other parameters are changing. IIRC density is a measure of mass per unit volume. Other physical properties can have an indirect effect. Heating things increases the volume (water below 4 C is an exception) of a given mass.

 

[sosman]

I have a smallish equation that can be used to calculate the SG at certain temperatures if anyone wants it. However it should be realised that the SG calculation is based on an attempt to do a regression of a curve. The problem is that the SG versus Temperature curve is not nice and smooth.

Brewsta uses this python function to calculate the density of water as a function of temperature:

def calcDensity(t):

    '''Calculate density from curve fit at temperature t (C)'''

    return 1/pow((0.9992 + t*t*0.3706e-4), 0.1352)

I would classify the curve as quite smooth, at least for water in the liquid phase.

 

[Darren]

Sos,
I don't even begin to understand the fluid properties of sucrose or oil.
One way to check it would be to drop you hydrometer into some thin oil see where it stops and some thick oil and see where it stops. It has to do with the resistence to gravity given the weight of the immersed object.

cheers
Darren

 

[hupnupnee]

So nothing to do with the viscosity of the wort?

[post="100831"][/post]​

Darren, it has everything to do with the density (Viscosity) of the wort. As the temperature increases the density decreases, therefore more wort needs to be displaced to hold up the hydrometer (Archimedes). Thus the specific gravity decreases.

[post="100873"][/post]​

Density and viscosity are two quite different fluid properties and measured by completely different methods. Mercury for example has a very high density and I am guessing its viscosity is quite low. Motor oil, is more viscous than water but less dense.

[post="100888"][/post]​

i agree Sosman, however, I think the original concern was how SG changes with temp. Darren is correct that the viscosity does decrease with increasing temperature, runny honey, SG is not a measure of viscosity but density, it just happens that density increases as viscosity decreases with increasing temperature in the case of wort. There is actually no correlation between viscosity and SG. Thin oil and think oil can have identical SGs. More Blah!

Tim
Tim

 

[dreamboat]

Sos,
I don't even begin to understand the fluid properties of sucrose or oil.
One way to check it would be to drop you hydrometer into some thin oil see where it stops and some thick oil and see where it stops. It has to do with the resistence to gravity given the weight of the immersed object.

cheers
Darren

[post="100894"][/post]​

I would think that all this experiment will tell you is the density of the oil (sg). Theoretically, of course, given enough time for the hydrometer to sink to the equilibrium level, particularly in the thick sample... and there may be some doubt as to whether that level has been reached.
Completely irrelevent for brewing where the densities and viscosities change by bugger all for all stages of the process.


dreamboat

 

[hupnupnee]

Check out for viscosity and density (SG) data for a whole range of things, most completely unrelated to brewing, unless you like your beer oily.

http://www.csgnetwork.com/sgvisc.html

Cheers

 

[sosman]

it just happens that density increases as viscosity decreases with increasing temperature in the case of wort.

Man - someone is trying to do my head in here. I had to read that a few times and I still think it is arse about.

 

[Justin]

Slightly off topic but related none the less to SG readings and temperature-but not the above stuff.

Does anyone using a refractometer lower their sample temperature a bit before adding your drop to the refractometer? I've heard that you need to be careful when adding a drop of hot liquid to your refractometer because of the evapouration that can occur in those few moments throwing your readings out a little (ie. giving you a higher SG than you actually have). ATC has nothing to do with this by the way. Given the small volume and large surface area the evapouration could be quite a bit.

Easy to test. Take some wort at 66C, cool some and read it, read the hot one-is there a difference? I'd be curious. I don't have one but have used one of the ones with the blue line/field in it and it was sweet. Nice bits of kit. Now I'm not fly sparging I don't really feel the need for one. I just cool a sample for the hydrometer.

Cheers, Justin

 

[Ross]

Slightly off topic but related none the less to SG readings and temperature-but not the above stuff.

Does anyone using a refractometer lower their sample temperature a bit before adding your drop to the refractometer? I've heard that you need to be careful when adding a drop of hot liquid to your refractometer because of the evapouration that can occur in those few moments throwing your readings out a little (ie. giving you a higher SG than you actually have). ATC has nothing to do with this by the way. Given the small volume and large surface area the evapouration could be quite a bit.

Easy to test. Take some wort at 66C, cool some and read it, read the hot one-is there a difference? I'd be curious. I don't have one but have used one of the ones with the blue line/field in it and it was sweet. Nice bits of kit. Now I'm not fly sparging I don't really feel the need for one. I just cool a sample for the hydrometer.

Cheers, Justin

[post="100956"][/post]​

Justin,

I've found that adding very hot liquid to the refrac can give odd results, especially if you add a few too many drops. i now draw my sample with a metal teaspoon & leave it for approx 30 secs before testing - results are now consistant...

cheers Ross

 

[Doc]

Does anyone using a refractometer lower their sample temperature a bit before adding your drop to the refractometer?

Justin,

I've found that adding very hot liquid to the refrac can give odd results, especially if you add a few too many drops. i now draw my sample with a metal teaspoon & leave it for approx 30 secs before testing - results are now consistant...

cheers Ross

[post="100961"][/post]​

When sparging (near the end when I'm checking the runoff gravity) I collect some liquid in a plastic jug, then pick a little up in an eye dropper and place on the refractometer. I find the contact with the jug, then the eye dropper with the amount of liquid taken drops the temp enough to not cause a problem.

Beers,
Doc

 

[Darren]

I just plunge my refract into the hot wort. It is such a small sample that ends up under the cover that it cools very quickly. Using a pipette is likely to cause inaccuracies unless it is very clean and dry each time you use it. Remember you are working with very small volumes. Only needs a drop to really skew the results.
cheers
Darren

 

[Ross]

I just plunge my refract into the hot wort.
cheers
Darren

[post="100969"][/post]​

i'm not going to bite ...

 

[pint of lager]

The correction tables are inaccurate. If you are trying to stop sparging at 1.010 and are out by .005, all of a sudden you are in danger of extracting tannins into your kettle.

You could try making a table for your very own hydrometer, make up different samples of sg, heat them to different temps and take readings. Even a sample of 1.020 heated to 70, allow it to cool and take readings at different temps. The most important reading would be 1.010 wort at 70 deg or whatever your wort temp is at runoff.

For a hydrometer sample, collect it ina plastic bag and crash cool in an ice bath, doesn't take long.

When taking a refractometer reading when sparging, I use a clean test tube dropper, collect straight from the run off tube, squeeze a few times so it is rinsed in wort at the wanted sg, fill with a few drops and transfer to the refractometer. The dropper is rinsed in water, ready for the next reading. It is not dry at this stage which is why I squeeze it a few times when taking the next sample.

A good brewing experiment is to taste the wort as the sparge progresses. Collect some in a coffee cup (thermal mass drops the temp) and try it. You will notice alot of changes in flavours as well as sweetness. Also the aroma changes as the sparge progresses. I can usually smell when it is time to stop sparging, but also use the refractometer.

 

[Kai]

Drawing wort into and out of your pipette several times should compensate for any inaccuracies in your refrac reading from residue in the pipette, assuming the pipette wasn't full of water to begin with.

I was always under the impression the reason hydrometer reasons have to be taken at a standard temperature is one of density, and nothing to do with viscosity. Pure water is most dense at 4 degrees, above that as it heats up the water molecules get more excited and sit further apart. Below 4, I can't remember so well but I suspect it's something to do with how the molecules behave when they're preparing to become ice.

 

[Darren]

The correction tables are inaccurate. If you are trying to stop sparging at 1.010 and are out by .005, all of a sudden you are in danger of extracting tannins into your kettle.

You could try making a table for your very own hydrometer, make up different samples of sg, heat them to different temps and take readings. Even a sample of 1.020 heated to 70, allow it to cool and take readings at different temps. The most important reading would be 1.010 wort at 70 deg or whatever your wort temp is at runoff.

For a hydrometer sample, collect it ina plastic bag and crash cool in an ice bath, doesn't take long.

When taking a refractometer reading when sparging, I use a clean test tube dropper, collect straight from the run off tube, squeeze a few times so it is rinsed in wort at the wanted sg, fill with a few drops and transfer to the refractometer. The dropper is rinsed in water, ready for the next reading. It is not dry at this stage which is why I squeeze it a few times when taking the next sample.

A good brewing experiment is to taste the wort as the sparge progresses. Collect some in a coffee cup (thermal mass drops the temp) and try it. You will notice alot of changes in flavours as well as sweetness. Also the aroma changes as the sparge progresses. I can usually smell when it is time to stop sparging, but also use the refractometer.

[post="100985"][/post]​

All good advice except I wouldn't put my hydrometer in 70 degree wort or it will crack.

cheers
Darren

 

[Darren]

Drawing wort into and out of your pipette several times should compensate for any inaccuracies in your refrac reading from residue in the pipette, assuming the pipette wasn't full of water to begin with.

I was always under the impression the reason hydrometer reasons have to be taken at a standard temperature is one of density, and nothing to do with viscosity. Pure water is most dense at 4 degrees, above that as it heats up the water molecules get more excited and sit further apart. Below 4, I can't remember so well but I suspect it's something to do with how the molecules behave when they're preparing to become ice.

[post="100986"][/post]​

I thought water had a density of 1.000 no matter what temp. Maybe I am getting confused with SG.

cheers
Darren

 

[Kai]

Water only has a density of 1 at 4C. Wikipedia has a density table under their specific gravity entry. I also had a quick google and found a dodgy chart.

 

[Darren]

Water only has a density of 1 at 4C. Wikipedia has a density table under their specific gravity entry. I also had a quick google and found a dodgy chart.

[post="101004"][/post]​

Nice chart kai,

Those figures are down to 4 decimal places. Hydrometer correction is much higher than that.

 

[Kai]

My hydrometer's correction chart is approximately 0.001 for every 5 degrees, which seems to correlate with the values in the wikipedia entry. The chart I posted only covers up to 15, and my hydro's table only covers down to 15, and judging from the curve I think it's reasonable to assume the slope grows stronger.

Since I'm supposed to be writing rather than posting here, I charted the wikipedia data. It's more illuminating than the other chart I found: