Help with gravity discrepancy

[Jtvann]

I'm trying to figure out my system losses. Lately my calculations have been off, and I haven't been able to figure out why.

I'm using the brew boss system. I just filled with 7 gallons of cold water (50 degrees) and used the system to being it to a boil, just like I would on a regular brew day. I boiled for 1 hour, then cooled back down to 50 degrees. My measured boil off amount is about .6 gallons per hour.

I figured I've done everything correctly to this point. My problem is with my specific gravity measurements. I've been dead on with what my expected pre boil gravity is ... I think. My problem is that my OG can some time be 5 to 6 points low from what I'm expecting.

I trust that I'm boiling off .6 gallons per hour, unless I did something wrong in my test above. I trust my OG reading, as it is cooled and measured by hydrometer. I'm not trusting my pre boil gravity sample. Typically I stir while bringing to a boil, then collect a sample with a dropper after a boil is reached. My refractometer is calibrated and reads 1.000 with water.

Not sure where I'm missing a measurement. Any ideas?

 

[Tobor_8thMan]

OG is an estimate based on the ingredients. Unless the ingredients are 100% accurate, then the OG is probably off. IMO, don't sweat it.

 

[beermanpete]

Does your pre-boil gravity and post-boil (OG) gravity agree when you take the boil-off into account?

 

[Jtvann]

Does your pre-boil gravity and post-boil (OG) gravity agree when you take the boil-off into account?

No, it doesn't. That's the reason for this thread.

 

[CascadesBrewer]

I trust that I'm boiling off .6 gallons per hour, unless I did something wrong in my test above.

Are you not able to test your boil off rate (your pre-boil vol vs your post-boil vol) on an actual batch? Pre-boil to post-boil gravity is a simple linear scale based on the reduction of water...10 gal of a 1.040 wort will equal 5 gal of a 1.080 wort. Have you tried taking a sample pre-boil and measured it with your hydrometer (either by cooling or with a temp correction)?

 

[beermanpete]

Maybe your refractometer and hydrometer do not agree. Take reading using the same instrument at both pre-boil and post-boil. Sample temperature is important so cool both samples to the same temp before taking the readings.

 

[rburrelli]

I'm not trusting my pre boil gravity sample. Typically I stir while bringing to a boil, then collect a sample with a dropper after a boil is reached. My refractometer is calibrated and reads 1.000 with water.

Not sure where I'm missing a measurement. Any ideas?

Why are you taking your pre boil sample after you started a boil? I take mine when I have drained my wort to the kettle. Don’t know if that makes any difference as I have never taken gravity at that boil point.