Author Topic: Evaporation and preboil gravity readings  (Read 754 times)

Offline archstanton

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Evaporation and preboil gravity readings
« on: November 21, 2014, 08:32:00 PM »
Saw this mentioned a few times in another thread so I decided to see what happens in my kitchen. I use a refractometer and collect a few tablespoons in a bowl then use  spoon to stir and dose. These are the results:

8:50 pulled 3 tablespoons from kettle
8:54  reading 12b at 73 degrees
9:08              12.2b at 67
9:40              12.4b at 67
9:56              12.4 b at 67
10:15            12.6b at 67

point 2b is about .85 gravity points so over the course of 1 hour I would have seen about a 1.7 increase in the gravity of the solution due to evaporation. About 1/3 of a tablespoon was used per reading. Since I was able to obtain a suitable temperature in less than 5 minutes, I will continue be ignorant of evaporation.  While writing this I realize I should have put an open hydrometer size glass in the freezer to compare, next time.

Offline quattlebaum

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Re: Evaporation and preboil gravity readings
« Reply #1 on: November 22, 2014, 12:41:48 PM »
I wish i could say i use my refractometer but it just collects dust. It was just to inconsistent for me. On the evaporation observation mine varies batch to batch i believe its related to relative Humidity in my region ( or at least i think thats what it is). I havent charted it per say in that relation but it sure seems that way.  I just take a cooled hydrometer reading every 15 min and adjust my boil as needed to get my FG.  Works pretty much every time.

Offline a10t2

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Re: Evaporation and preboil gravity readings
« Reply #2 on: November 22, 2014, 07:03:53 PM »
Did you chill a sample in a sealed container in order to have a control? If you're cooling it in the open air for those 4 min, 12°Bx probably wasn't the baseline reading.

FWIW, I use a small syringe to pull ~0.5 mL for refractometer samples. It cools to room temperature in less than a minute and I can then cover with my finger and shake to homogenize the sample.
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Offline archstanton

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Re: Evaporation and preboil gravity readings
« Reply #3 on: November 22, 2014, 07:37:25 PM »
Did you chill a sample in a sealed container in order to have a control? If you're cooling it in the open air for those 4 min, 12°Bx probably wasn't the baseline reading.

FWIW, I use a small syringe to pull ~0.5 mL for refractometer samples. It cools to room temperature in less than a minute and I can then cover with my finger and shake to homogenize the sample.

I did not do that.  It was a spur of the moment thing. I will certainly do it next brew to compare vs my initial reading. I was interested in seeing how much evaporation would change the reading over time, as I take such a small sample it seemed it would be more susceptible to evaporation vs a hydrometer sized sample in a freezer-which I used to do.  I believe the evaporation will occur faster during rapid cooling and I will need that control to determine that.  While my wort decreased in size after each sample the temp was essentially stationary.  How big a role humidity plays is something else to consider.

Offline erockrph

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Re: Evaporation and preboil gravity readings
« Reply #4 on: November 23, 2014, 05:16:33 AM »
I spoon a small sample into an old white labs vial then put the cap on. Once it cools I give it a shake then take my refractometer sample. That takes care of the evaporation concern.
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Offline Stevie

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Re: Evaporation and preboil gravity readings
« Reply #5 on: November 23, 2014, 05:38:15 AM »
3ml disposable pipettes for me. Take a sample, turn upside down, and swirl bulb in a cup of water.

Offline HoosierBrew

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Re: Evaporation and preboil gravity readings
« Reply #6 on: November 23, 2014, 03:29:52 PM »
I put my preoboil sample in a small Mason jar with a lid, into the freezer for a couple minutes and then take a reading.
Jon H.

Offline reverseapachemaster

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Re: Evaporation and preboil gravity readings
« Reply #7 on: November 23, 2014, 04:27:07 PM »
I use my cat's frigid personality to cool samples.
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Offline morticaixavier

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Re: Evaporation and preboil gravity readings
« Reply #8 on: November 24, 2014, 07:39:34 PM »
I use my cat's frigid personality to cool samples.

I have to try this.
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Offline archstanton

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Re: Evaporation and preboil gravity readings
« Reply #9 on: December 04, 2014, 08:54:15 PM »
Was able to brew again today, so I took some readings.

3 tsp sample from top of kettle cooled in 3 minutes to 68 reading                13.6b
3 tsp sample from ball valve at bottom of kettle cooled to 68 in 3 minutes     13.6b
3 oz sample sealed and chiiled then shaken and sampled about 10 mins at 67         13.4b (was really close to 13.5)

.2 brix is less than one gravity point. Evaporation is a non issue for me, stratification as well.