Hydrometer problems

[schwartzr33]

I've done several all grain batches. Per Ray Daniels (Designing Great Beers), I've been taking hydrometer readings after I collect my wort, but before I boil, and adjusting with DME to hit my target gravity. I boil for 60 min., cool, and measure again to get my OG. The last couple of batches my OG does not seem to vary very much from the pre-boil gravity. Of course, my gravity measurement should increase due to evaporation. For example, today my pre-boil, temperature adjusted, gravity was 1.057. After boiling and cooling, my OG into the fermenter was 1.059! Beersmith says I should be at 1.067 after boiling. I am hitting all of my volumes for collection and boil-off, meaning my boil-off rate was as expected, 12%. All I can think of is that I'm getting inaccurate pre-boil gravity measurements. I've tested my hydrometer with plain water and a known amount of boiled DME and the hydrometer is accurate. Any idea what is going on?

 

[Yooper]

First, a question- are you cooling your hydrometer sample to below 100 degrees (preferably even lower, to 90 degrees) before reading it and then adjusting with software? If not, a reading over 100 degrees is so inaccurate as to be worthless even with correction tables and software.

 

[schwartzr33]

Yes. Today my pre-boil reading was done at 77 degrees and OG reading was done at 67.

 

[schwartzr33]

And yes, I'm adjusting with software.

 

[stratslinger]

How thoroughly are you mixing your worst, pre boil, before you draw a sample to test for gravity? Depending on your sparging method, your first runnings may not be adequately mixed with your spared volume, and you could be getting a heavier concentration of your first runnings, making your gravity read higher than it should.

 

[JLem]

Can you explain your process and recipe from today's brew session?

 

[schwartzr33]

I'm doing BIAB with a 2 gal. rinse at 170. I dump the rinse wort into the main kettle and mix a bit. I don't go crazy with the mixing due to concern about hot-side aeration, but I do mix a bit, but perhaps not enough?

 

[schwartzr33]

Recipe:
7.7 lbs Pale ale malt
4.8 Red Wheat
3.1 Caracrystal Wheat (Briess)
0.8 Midnight Wheat (Briess)
1 oz. Columbus at 60 min.
1 oz. Palisade at 15 min
1 oz Centennial and 1 oz. Willamette at 5.0
6.5 gal BIAB mash for 90 minutes
2 gal mashout/rinse of the bag
Mix original mash and rinse wort together in kettle, Total wort collected 7.4 gal
Steal 2 cups of wort from kettle, boil 1 minute on stove top, cool in snow bank to 77 degrees, measure gravity=1.055 (1.057 adjusted to 60 degrees)
60 minute boil, 0.88 gallons boiled-off
Immersion chiller
Drain kettle into fermenter.
Measure gravity from sample near the end of what went into the fermenter (lots of trub in sample). Measured gravity=1.058 at 67 degrees.

 

[stratslinger]

Alright - first off, don't boil your pre boil gravity sample. Boiling for only a minute, you're not gonna lose a lot of volume, but you will lose some, and that will throw off your number. You want to know the gravity of what's in your kettle, and if you modify your sample in any way then you're tainting your results.

 

[schwartzr33]

Here is a new theory. What if the problem is the measurement of the OG? After chilling we let the pot sit for 15 minutes, tilted away from the valve at the bottom, so that the trub could settle out and away from the valve a little. We drained the pot from the bottom using the valve, then measure the OG on the last bit of wort left in the pot. Since we were draining from the bottom, is it possible that we drained off heavier, higher gravity wort out from under the lower gravity wort (which we then measured)??