Pre & Post Boil Gravity readings nearly identical

First post....

I just made my third all grain batch yesterday (using rubbermaid coolers for the HLT & Mash Tun and a converted keg for the kettle). The OG pre boil and the OG post boil readings have been nearly identical on each separate batch. I know it is impossible since there is evaporation, added hops, etc. for the post to be the same or less than the pre. Kind of like 2+2=4 but I'm not getting anywhere near 4 in this case.

I have made it a point to thoroughly stir my pre boil and post boil before taking gravity readings. I have used a refractometer and hydrometer for comparison and get the same results (maybe a 2 point difference between the two instruments). When using the hydrometer I have cooled the sample down to about 80 or 85 and do the temperature correction.

The first two batches turned out great (though the final gravity on both was about 4 points lower than target).

I have searched and searched this topic and all I can find is "did you apply a temp. correction factor; did you mix the wort up well enough; etc.?"

What in the heck is going on? Thanks for any help!!!

If your volume is going down during the boil and you're not doing things like adding water or extra sugars, then, quite simply, one (or both) of your gravity readings is wrong.

Try letting your wort come to a boil, wait for the hot break, then, before adding your hops, turn off the heat and pull a hydrometer sample. Turn the heat back on and continue your boil as normal. Seal up the sample and let it sit in a cool water bath until it is cool enough to measure. Then measure it. That'll make it more likely that your runnings are well mixed.

Are you topping off after your boil?

I suppose your hydrometer could also be broken in some sort of weird way.

FWIW 80-85 is pretty high. When you get that high the temp corrections aren't to be trusted. Let your samples cool to the 60° they are calibrated for and test them properly.

when you say 'almost'... what are the actual readings?

The amount you're losing to evaporation might be alot less than you expect. What's your setup like? Are you measuring how much volume you lose to evaporation and seeing if that agrees w/ your hydrometer readings?

Either gravity reading is wrong, as those above have suggested....or your volume readings before and after are inaccurate.

Thanks for all the replies.

I am able to accurately measure my pre and post boil volumes. My pre volume is either 12 to 13 gallons depending on if I'm boiling for 60 or 90 minutes. My post volume boil is always just over 10 gallons so there is definitely evaporation loss (right around 15%).

I have double checked the hydrometer with distilled water at exactly 60 degrees and adjusted the refractometer to 0 with distilled water.

The hydrometer and refractometer readings were taken when the post boil temperature was at 85, 72.5, and again at 60.4 degrees. With the temp. correction all temps. resulted in the same gravity reading.

The pre boil gravity readings were taken at a temp. of 80 degrees after stirring the wort for about 20 seconds.

With this last batch (IPA), the pre boil gravity was 1.058 (refractometer) and 1.054 (hydrometer) for an average of 1.056. The post boil gravity was 1.058 (refractometer) and 1.054 (hydrometer) --- exactly the same readings!!

When this happened after my 2nd all grain batch, I bought a refractometer just for comparison (plus I like toys )

I feel like I have taken the readings as accurately as I can and both the hydrometer and refractometer readings have been close to each other.

My first batch (IPA) had a pre boil of 1.053 and a post of 1.054 and my second batch (APA) had a pre and post of 1.048.

All of my pre boil gravities have been almost nut on with the estimated gravities (at 75% efficiency) but for some reason my post boil gravities are out of whack. Based on my pre boil readings my actual efficiency is 80% but I have still been using 75% for recipe design.

I will be brewing a stout this weekend so we'll see what happens. At least my batches are consistent, right?

FYI, the refractometer readings need to be corrected using a hydrometer. A refractometer isn't built to account for all the things that affect the gravity of wort so it is always off a bit. You therefore correct it by comparing its reading to a hydrometer reading. So, assuming you read the hydrometer correctly, your gravity on your IPA was 1.054 - not 1.056.

Something is simply wrong here, though. If you reduce your volume by 15%, your gravity points will increase by 15%. Your pot isn't some sort of magical place that defies all laws of nature. In the case of your IPA, the post-boil gravity should have been 1.062 assuming your 1.054 reading was correct. Are you topping off after your boil? You didn't say. If you aren't, you must be getting a proportionally higher amount of your first runnings for your hydrometer sample pre-boil. To rule that out, try what I said above about letting it boil a bit before drawing your sample. The boil will mix stuff up real good.

Good point on calibrating the refractometer to the hydrometer. I'll try taking the pre boil reading when it starts boiling when brewing my stout this weekend.

I'll report back to when I take an FG on this current batch in a couple of weeks.

Thanks.

This wiki has a good explanation of calibrating refractometers for wort. Just look under the heading "Wort Calibration."

mojotele said:

This wiki has a good explanation of calibrating refractometers for wort. Just look under the heading "Wort Calibration."

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Thanks for the link! I'll see what readings I get for my stout on Sunday...

No problemo! Come back after you brew your stout on Sunday and let us know your readings. I'll be interested to see what your measurements are!

Like mojo said... something just doesn't add up.

Are you by any chance taking your first gravity reading before adding top-off water to achieve your pre-boil volume? Because if you are, that would likely cause you to see the same numbers pre & post boil.

tunoffun said:

Like mojo said... something just doesn't add up.

Are you by any chance taking your first gravity reading before adding top-off water to achieve your pre-boil volume? Because if you are, that would likely cause you to see the same numbers pre & post boil.

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I'm not adding any top off water since I'm collecting the entire volume by sparging.

I brewed an Oatmeal Coffee Chocolate Stout yesterday and had way better results. The pre-boil gravity was 1.059 and the post was 1.082. Both #'s were within 3 points of their targets. I took the pre-boil measurement about two minutes after the boil started and let the sample cool down and took several measurements until it got down to 60 degrees. I took the post-boil reading just before flameout and let it cool.

I guess letting it boil for a few minutes for the pre-boil reading makes a huge difference over just stirring it agressively before boiling.

Thanks again for the replies

cheez said:

I brewed an Oatmeal Coffee Chocolate Stout yesterday and had way better results. The pre-boil gravity was 1.059 and the post was 1.082. Both #'s were within 3 points of their targets. I took the pre-boil measurement about two minutes after the boil started and let the sample cool down and took several measurements until it got down to 60 degrees. I took the post-boil reading just before flameout and let it cool.

I guess letting it boil for a few minutes for the pre-boil reading makes a huge difference over just stirring it agressively before boiling.

Thanks again for the replies

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I'm glad you got better results. But it still doesn't sound like you've figured out the problem you were having to me. There's no reason you should have to boil the 'pre-boil' before getting a meaningful hydrometer reading. I can assure you that most people who do get expected / accurate readings do not.

Don't mean to rain on your parade But if you're looking for answers, I'm not convinced you've found them.

I think my "new" readings were better for a couple of reasons... The pre-boil was definitely mixed up better than previous batches and I was more anal about taking readings at 60 degrees with my hydrometer. As a side note, I also updated my brix to SG calculation to include a 4% (I think that's what I used) correction factor for wort so my hydro and refracto were nut on with each other.

Thinking back to my previous batches I only stirred the pre-boil runoff for mabye 3 seconds even though it seemed like 20 seconds. I am fairly confident that I wasn't mixing it up enough before taking the pre-boil reading. I guess I will need to brew a few more batches for comparison and to get a good baseline going Damn the luck!

tunoffun said:

There's no reason you should have to boil the 'pre-boil' before getting a meaningful hydrometer reading.

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If you're not adequately mixing your run-offs there is, and that's why I suggested it in the first place. Boiling the wort for a bit is a sure-fire way to remove that problem from consideration. Boiling was just a convenient way to remove "user error," so to speak. He does not necessarily have to boil his wort every time to get a pre-boil reading from now on.

I am trying to keep in mind that I have only done 4 AG batches so far and to not get too worked up over stuff. Overall I am pretty happy with the results. I know I will get the kinks worked out. Bottom line - I have loved the batches I have made so far and this stout is looking promising

mojotele said:

If you're not adequately mixing your run-offs there is, and that's why I suggested it in the first place. Boiling the wort for a bit is a sure-fire way to remove that problem from consideration. Boiling was just a convenient way to remove "user error," so to speak. He does not necessarily have to boil his wort every time to get a pre-boil reading from now on.

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And a great suggestion it was, obviously.

Unfortunately, I made the mistake of taking the OPs comments at face value. I see now 20 seconds worth of vigorous stirring wasn't exactly what was going on.

Kind of hard to hit the nail on the head when the nail keeps moving.

tunoffun said:

Kind of hard to hit the nail on the head when the nail keeps moving.

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My bad! Thanks again for the suggestions. They did help me out on this last batch.