OG Always Low

[Pozzi]

Looking for some advice, I keep undershooting my OG. The pre boil gravity is within 1 or 2 points but my OG is down to 6-8 points. I check my pre boil and post boil volume and it's correct for what I planned. What else am I missing?

Today's brew was supposed to be:

Planned: Pre boil: 1.045 OG 1.053
Actual: Pre boil: 1.043 OG 1.045

How does that happen when my boil off was correct?

 

[McKnuckle]

Usually this is because the pre-boil gravity is inaccurate, which is very common due to stratification of the wort while hot. In other words, the sugar concentration is not even throughout the liquid. A second factor, less dramatic but still contributing, is temperature correction. It's highly unreliable when there's a large discrepancy between the wort temp and the ideal temp for the measuring device (hydrometer or refractometer). You can also get some evaporation while wort cools, which concentrates the sugars more and gives a reading that's too high.

So, that's why it happens!

If you measure OG with a hydrometer and know your true boil-off volume, you can easily calculate the actual pre-boil gravity. You'll find that it's lower than what you measured.

 

[Pozzi]

Usually this is because the pre-boil gravity is inaccurate, which is very common due to stratification of the wort while hot. In other words, the sugar concentration is not even throughout the liquid. A second factor, less dramatic but still contributing, is temperature correction. It's highly unreliable when there's a large discrepancy between the wort temp and the ideal temp for the measuring device (hydrometer or refractometer). You can also get some evaporation while wort cools, which concentrates the sugars more and gives a reading that's too high.

So, that's why it happens!

If you measure OG with a hydrometer and know your true boil-off volume, you can easily calculate the actual pre-boil gravity. You'll find that it's lower than what you measured.

Thanks for the response, I will try calculate the per boil gravity via OG and boil off now. I did use a hydrometer and I used brewers friend to correct for temperature. I took a measuring cups worth of wort out of the kettle while heating up to the boil. I let that cool in the fridge and then poured into the tube to measure the pre boil gravity. At first it came out as 1.03 something, after that I let it sit a while, then shook it up again and took another reading with the temp correction that got me the 1.043.

 

[McKnuckle]

Sounds like you gave it a good try, but even then as you can see, you got different measurements before and after waiting.

I use a refractometer for pre-boil gravity most of the time, which has its own annoyances, but when I've used a hydrometer and feel like being obsessive, I do this:

Stir and scoop the wort repeatedly with a ladle to mix it. Then take a full cup worth and decant it into a deep steel baking dish and tilt/roll that around to expose the wort to surface area and cool it quickly below evaporation temps. Then cover it and place in the fridge, or in a cold water bath. It really has to be no warmer than ~80ºF to have any chance of decent temp correction. The discrepancy gets big quickly with warmer temps.

 

[Pozzi]

Sounds like you gave it a good try, but even then as you can see, you got different measurements before and after waiting.

I use a refractometer for pre-boil gravity most of the time, which has its own annoyances, but when I've used a hydrometer and feel like being obsessive, I do this:

Stir and scoop the wort repeatedly with a ladle to mix it. Then take a full cup worth and decant it into a deep steel baking dish and tilt/roll that around to expose the wort to surface area and cool it quickly below evaporation temps. Then cover it and place in the fridge, or in a cold water bath. It really has to be no warmer than ~80ºF to have any chance of decent temp correction. The discrepancy gets big quickly with warmer temps.

Haha that is quite a process! I'm just glad to know that there are some inconsistencies in the hydrometer readings. I think I need to get a refractometer to simplify the pre boil and OG readings. Thanks for the help!

 

[slayer021175666]

Drop the hydrometer and grab a refractometer. So much easier to use. They used to be expensive but now, you can have one for 20 or so on ebay.

 

[VikeMan]

Refractometers definitely have a place, as long as you don't need ultimate accuracy, and use a good refractometer calculator to get "close enough" accuracy.

 

[Pozzi]

Drop the hydrometer and grab a refractometer. So much easier to use. They used to be expensive but now, you can have one for 20 or so on ebay.

Refractometers definitely have a place, as long as you don't need ultimate accuracy, and use a good refractometer calculator to get "close enough" accuracy.

I'm definitely gonna get one, been reading up on them today.

 

[slayer021175666]

Cool! Ya, that's the way to go. Mine has an auto temp adjustment feature so it's closer to real gravity even with hot wort. Still, I like to let it sit with the sample in it for a while. It is always a more accurate reading when it cools to room temp and that is very fast because the sample is too small to hold the heat for very long. My flask takes about half a beer to read with a hydrometer, too. A refractometer only takes a couple drops. No more sanitizing a wine thief and washing a flask, either. Just Star San the refractometer tip and dip it in the wort or beer. Run it under water to rinse it off. Way easier!

 

[Pozzi]

Cool! Ya, that's the way to go. Mine has an auto temp adjustment feature so it's closer to real gravity even with hot wort. Still, I like to let it sit with the sample in it for a while. It is always a more accurate reading when it cools to room temp and that is very fast because the sample is too small to hold the heat for very long. My flask takes about half a beer to read with a hydrometer, too. A refractometer only takes a couple drops. No more sanitizing a wine thief and washing a flask, either. Just Star San the refractometer tip and dip it in the wort or beer. Run it under water to rinse it off. Way easier!

Yea seems much easier, I found one with both brix and SG for a good price! Gonna order today so I'm prepped for the next batch. I see Brewfather also has a correction for fermented wort/beer as well! Definitely gonna try that out as well.

 

[VikeMan]

Cool! Ya, that's the way to go. Mine has an auto temp adjustment feature so it's closer to real gravity even with hot wort. Still, I like to let it sit with the sample in it for a while. It is always a more accurate reading when it cools to room temp and that is very fast because the sample is too small to hold the heat for very long.

Right, the automatic temp adjustment adjusts for the fact that the instrument responds differently at temps other than its calibrated temp, and not for the fact that the sample itself isn't at calibration temp. And it does take a while for the sample temp to come down. Longer than a lot of people assume. It is definitely not "instant." "A couple drops of wort and all that glass. It's gotta be instant, right?"

I once participated in one of the big word wide "collaboration" brews at a local nano. They took a hot sample of wort, measured with a refractometer, and almost made a bad decision based on it. Luckily, I convinced them to let me cool a sample down to room temperature and then measure.

 

[VikeMan]

Yea seems much easier, I found one with both brix and SG for a good price! Gonna order today so I'm prepped for the next batch. I see Brewfather also has a correction for fermented wort/beer as well! Definitely gonna try that out as well.

FWIW: The SG scale on the refractometer is not really accurate for beer wort. It's for sucrose solutions, which wort mostly isn't. That's why refractometer calculators include a wort correction factor that corrects OG measurements, as well as the math needed to correct FGs for alcohol presence.

 

[Holden Caulfield]

FWIW: The SG scale on the refractometer is not really accurate for beer wort. It's for sucrose solutions, which wort mostly isn't. That's why refractometer calculators include a wort correction factor that corrects OG measurements, as well as the math needed to correct FGs for alcohol presence.

It's a great day - learned something new. I was aware of the alcohol correction factor but was not aware that the wort needed one too. The effect from alcohol on the reading is very significant, do you know how significant it is for wort and any direction on the value?

 

[VikeMan]

I was aware of the alcohol correction factor but was not aware that the wort needed one too. The effect from alcohol on the reading is very significant, do you know how significant it is for wort and any direction on the value?

The true OG is normally lower than what you'd get by a standard "brix" to SG equivalence. On the order of 4% of gravity "points." The perfect wort correction factor (WCF) would depend on the wort's carbohydrate profile. Different refractometer calculators use different default values (e.g. "1.04") for the WCF. Using the 1.04 example...

15 "Brix" (without correction) -> 1.0611
15 "Brix" (with 1.04 WCF) -> 1.0587

 

[MikeCo]

I use a refractometer for post mash and pre-boil gravity measurements using one of the cheap refractomters, and it's reasonably accurate but not always repeatable, so I take two or three measurements to verify. With such a small liquid sample, you need to make sure the wort is mixed well to get a representative reading. I always check my OG after chilling with a hydrometer, which is very repeatable. The hydrometer is also my prefered way of measuring FG, since it's more accurate than estimating with a refractometer and calculations. Bottom line for me is that both hydrometer and refractometer have a place in your tool box.

To the OP's original question:
Are you doing extract batches? If so, the OG should come out close as long as you are using the correct water volumes and ingredients. If OG is always low and it's not because of stratification, be sure you are not adding too much water for the boil or top-off.

Also make sure your sample temperature is close to the calibration temperature of the hydrometer. It's usually 60 or 68F if in imperial units. You can also use a hydrometer calculator to correct for the temperature.

 

[Pozzi]

I use a refractometer for post mash and pre-boil gravity measurements using one of the cheap refractomters, and it's reasonably accurate but not always repeatable, so I take two or three measurements to verify. With such a small liquid sample, you need to make sure the wort is mixed well to get a representative reading. I always check my OG after chilling with a hydrometer, which is very repeatable. The hydrometer is also my prefered way of measuring FG, since it's more accurate than estimating with a refractometer and calculations. Bottom line for me is that both hydrometer and refractometer have a place in your tool box.

To the OP's original question:
Are you doing extract batches? If so, the OG should come out close as long as you are using the correct water volumes and ingredients. If OG is always low and it's not because of stratification, be sure you are not adding too much water for the boil or top-off.

Also make sure your sample temperature is close to the calibration temperature of the hydrometer. It's usually 60 or 68F if in imperial units. You can also use a hydrometer calculator to correct for the temperature.

Thanks for the advice, I'm doing all grain biab. I'm gonna get the refractometer and use my hydrometer together from the next batch. I figure that way will average out the readings. I'm not too stressed about hitting the numbers exactly, I just want repeatability as much as possible.

 

[MikeCo]

I also do BIAB. Make sure your grain crush is fine enough. If the homebrew shop does it for you, have them crush it twice. I have my own mill and set the gap to 0.030 inch for a fine crush and get good mash efficiency. This is one of the biggest factors in hitting the right OG with BIAB.

Use a calculator or software to calculate your water volumes. Using too much water will result in low OG as well. This means you have to know the grain absorption as well as boil-off which was previously mentioned. Whether or not you squeeze the bag with affect absorption. Keep track of the starting and post-mash volumes to calculate actual absorption to help get it right on the next batch. If water volume is the problem you will know by how much you have at the end of the boil. If you have more than target, you will get a lower than target OG and need to adjust something for the next batch.