Gravity Readings Off based on Math

[Thetasters]

Hey There!

Great and informative site. It's quite obvious there is a ton of knowledgeable folks here! Quick question - I have been playing around with making apple wine using apple juice. When I do the math on how much sugar I should use based on the idea that 1 pound of sugar = .046 gravity (I tested this with my own hydrometer), my gravity readings are invariably lower than what the math dictates after I have added all of the sugar, and not by a small amount either (one reading was at .090 when it should have been closer to .106). I should also mention that I am vigorously stirring and shaking the solution, for 5 - 10 minutes and the sugar appears to be dissolved. This has happened twice and I am a bit confused as to why.

I am suspecting that either my calculations are off, (I simply used the above equation until I got to .106 and then broke down how much sugar to use) the sugar is not fully dissolving, or maybe my expectations for accuracy are too high? I would be grateful for any advice on this. Thanks!

 

[BobBailey]

Suspended sugar and dissolved sugars will yield different gravity readings. I doubt the sugars are fully dissolved. Also, if you're pouring a sample into a hydrometer tube you're likely getting a less dilute liquid than that in the bottom of your primary vessel.

 

[RPh_Guy]

^ that's probably part of it.

The other part is that you need to take the volume of sugar into consideration.

For example, if you add a pound of sugar to one gallon of juice, the density will be different than if you add juice to a pound of sugar to reach one gallon. ... The final volumes will be different, affecting the concentration of sugar.

Hope this makes sense.

 

[BobBailey]

^ that's probably part of it.

The other part is that you need to take the volume of sugar into consideration.

For example, if you add a pound of sugar to one gallon of juice, the density will be different than if you add juice to a pound of sugar to reach one gallon. ... The final volumes will be different, affecting the concentration of sugar.

Hope this makes sense.

Absolutely. Adding 1 lb. of sugar to 1 gallon will raise the gravity by .046. Making a gallon with 1 lb. of sugar will produce a higher gravity solution, which is the opposite of what the OP's result was.

 

[RPh_Guy]

Density of sucrose: 1.59 g/cm³
1 lb of sucrose = 454g = 286mL

3785mL in a gallon ...
A gallon of cider with 1 lb added is 4071mL (1.076gal)

46 ppg / 1.076 gal = 43 points per pound

The expected gravity increase by adding 1lb of 46ppg sugar to one gallon is actually 43 points, because it's now larger than one gallon.

...whether this applies to the OP, I'm not sure without knowing the process used.