Hydrometer question

[EdWort]

How much will evaporation impact on a hydrometer reading in 90 minutes?

I filled my hydrometer jar with the hydrometer in it and tried to take a reading but the wort was a bit bubbly. I came back 90 minutes later and was able to take a reading. My temp dropped to 75 (room temp) and it read out at 1.050 which corrected is 1.052.

Thanks

 

[Yuri_Rage]

I'll see if I can find something to back this up scientifically, but I'm guessing the answer is at or beyond the ten-thousandths place (.0001).

EDIT: I'm giving up on the scientific data part of this...there are WAY too many variables (water temp, water volume, water surface area, container volume, container geometry, air temperature, humidity, sunlight/UV absorption, etc, etc, etc) and probably some calculus (rate/rate of change) equations that I'm not willing to delve into right now. I'm sticking with my short answer.

 

[david_42]

A swimming pool in Tucson will lose about 6 inches of water a month during the summer. We can take that as the high end. So, we are looking at about 1/100 th of an inch. Assume the tube is 10" tall, that's 1,000th of the volume or for a 1.050 reading about 0.00005.

Waiting for it to completely degas is always a good idea. This can make a difference of several points.

 

[EdWort]

Yep. I picked up two points by waiting. 1.050 to 1.052 after temp correction. Thanks!

Today was a stellar brew session. Decent efficiency, I'm dialed in on my sparge amounts to get 7 gallons, I'm dialed in on my evaporation rate for my boil. 70 minutes to get down to 5.5 gallons, my pre-chiller made a big difference, and Yuri's Homebrew Stirplate kicked butt. Lag time today with Wyeast 3068 was 3 hours and I have enough left over for 3 more batches.

Life is good!

 

[chillHayze]

Yep. I picked up two points by waiting. 1.050 to 1.052 after temp correction. Thanks!

Today was a stellar brew session. Decent efficiency, I'm dialed in on my sparge amounts to get 7 gallons, I'm dialed in on my evaporation rate for my boil. 70 minutes to get down to 5.5 gallons, my pre-chiller made a big difference, and Yuri's Homebrew Stirplate kicked butt. Lag time today with Wyeast 3068 was 3 hours and I have enough left over for 3 more batches.

Life is good!

Yes that stirplate is truly awesome! I have had very similar lag times using it. My Old ale actually overfilled the blowoff tube mug of sanatizer. (I'll post pics when I get the chance) It was ugly, and gorgeous at the same time.

 

[Yuri_Rage]

Assume the tube is 10" tall, that's 1,000th of the volume or for a 1.050 reading about 0.00005.

So I made a pretty decent guess at the answer for evaporation... Glad you guys like the stir plate! I can't wait to dial in my AG setup - sounds like you've gotten yours up and running pretty quickly, Ed!