Gravity unit difference pre-boil vs post boil

This one has me baffled. It isn’t the first time it has happened. More common on the bigger beers. Hopefully someone has some suggestions.

I just finished a tripel. My preboil gravity was 1.055 by hydrometer. Volume at about 200F was 7.5 gal. I use 96% correction for thermal expansion giving me calculated preboil GU’s = 396.

I added 2# table sugar at 46 GU/lb with 10 min in the boil. Total is now 488 GU.

My post boil target was 6 gal at 1.081 (486 GU) so I am right on.

Post boil, I measured 6 gal at 60F. SG = 1.077. Checked this with a second hydrometer, got 1.078. Calculated GU’s = 462 which is 5.4% lower than expected.

The beer will be fine, not my concern. My concern is why the difference.

The discrepancy must be one or more of the following: bad samples?, volume measurement inaccuracy?, instrument problem?, GU of sugar?, all of the above?

Other comments: My sample post boil is clear wort. Preboil samples are typically cloudy, pre hot break. Boil kettle volume measured with a dipstick. Hydrometer checked with distilled water.

thanks

I am thinking you are giving sugar too much credit. I will use 42 per points per pound, yet you should be a point or two over the secondary results. Then are we going to assume that then the weigh or volumes are to be at fault? Sneezles61

Thanks. I’ve seen sucrose reported between 42-46 GU/lb. Not enough to throw this 5% off. Hydrometers are typically accurate to +/- .002, +/-.001 if you know what your doing.

After considering for a day, my problem is most likely the volume measurement. If I am off 1 qt high preboil, the difference would account for 4%.

Just doing the math, roughly that is, there almost 6% difference just between the sugar potentials… So, betwixt both, you may have found yer answer… Sneezles61