Consistently getting incorrect preboil gravity

[Jtvann]

I'm having trouble on a regular basis getting an accurate reading on my preboil gravity.

Example: preboil gravity is 1.050 at 13 gallons. Then Post boil or original gravity is also 1.050, but at 12 gallons.

I know for sure that my gravity is not constant when losing a gallon of water. I trust my OG readings as it is cooled wort measured by hydrometer, refractometer and tilt. All 3 are measuring the same within .001.

I feel like I'm having stratification problems. Typically I will suspend my basket and let it drip, or squeeze the grains until a boil starts. Once its boiling, I take a sample via refractometer.

I figured that the boiling would sufficiently mix my sample.

Only thing I can figure is that
1- The sample temp is still too hot
2- Stratification
3- Unknown

Can someone suggest a better process. I'd like to be able to get a fairly accurate reading to be able to make up large deficiencies, if any, with DME in correcting low gravities.

 

[kh54s10]

I'm having trouble on a regular basis getting an accurate reading on my preboil gravity.

Example: preboil gravity is 1.050 at 13 gallons. Then Post boil or original gravity is also 1.050, but at 12 gallons.

I know for sure that my gravity is not constant when losing a gallon of water. I trust my OG readings as it is cooled wort measured by hydrometer, refractometer and tilt. All 3 are measuring the same within .001.

I feel like I'm having stratification problems. Typically I will suspend my basket and let it drip, or squeeze the grains until a boil starts. Once its boiling, I take a sample via refractometer.

I figured that the boiling would sufficiently mix my sample.

Only thing I can figure is that
1- The sample temp is still too hot
2- Stratification
3- Unknown

Can someone suggest a better process. I'd like to be able to get a fairly accurate reading to be able to make up large deficiencies, if any, with DME in correcting low gravities.

In red is impossible. It may be temperature differences of the samples.
Too hot could be. But the readings should still be different.
Stratification could be. Stir it.

Unknown - got me???

 

[Jtvann]

Yeah, what you highlighted in red is why I said the reading is inaccurate. I know it's not possible. That's what the reading says though ... thus the subject of the thread. Inaccurate readings.

I've stirred in the past, and figured that boiling water could stir better than I do.

 

[doug293cz]

If you are not taking care to eliminate evaporation from your small, hot, pre-boil refract sample, you will get a reading that is too high. I take about a 1/4 cup sample, cover, let cool, and then take the sample from there with a dropper.

Brew on

 

[Jtvann]

That would possibly explain how my preboil and post boil gravities match.

I have not been collecting a larger sample and cooling. I am going to start up my whirlpool while draining from the basket to stir and try collecting a larger (maybe 25 ml sample) and test it. I should have enough time to add DME as needed within the last half hour of the boil.

 

[Qhrumphf]

Let it boil, cut the heat, wear some heat-resistant gloves and draw a sample into a pipette. Rotate it away from you upside down (likes to spit some near boiling wort so best to spit away from you) and then run the outside of the pipette bulb under cold tap water. Agitate it a bit. Will cool it in a few seconds with little to no evaporation, much faster than a large sample (though a metal cup in an ice bath is how I cool larger samples rapidly).

 

[kh54s10]

The evaporation still doesn't explain the same numbers. The preboil should be at about 145 degrees and the OG would be at 70 -80 degrees. The evaporation of the preboil should make the difference GREATER than being experienced.

 

[Jtvann]

The evaporation still doesn't explain the same numbers. The preboil should be at about 145 degrees and the OG would be at 70 -80 degrees. The evaporation of the preboil should make the difference GREATER than being experienced.

The difference is greater than expected. Preboil should always be (not should ... it is always) lower gravity than post boil final gravity.

If the slightest bit of water is evaporating from my small sample on the refractometer, it is in effect concentrating the sample. The number would read artificially higher than it should.

In my example above both preboil and post boil were 1.050. I'm betting a more accurate preboil was 1.045 plus or minus a point.

 

[kh54s10]

The difference is greater than expected. Preboil should always be (not should ... it is always) lower gravity than post boil final gravity.

If the slightest bit of water is evaporating from my small sample on the refractometer, it is in effect concentrating the sample. The number would read artificially higher than it should.

In my example above both preboil and post boil were 1.050. I'm betting a more accurate preboil was 1.045 plus or minus a point.

I should have left the word should out of the sentence... Preboil will always be less than OG.... But I don't know what I was thinking.... If you did have evaporation with the hydrometer your readings could be the way you experienced...

My experience with my refractometer is that evaporation differences if there are any at all are very minimal. My preboil readings are within a couple points of the Beersmith predictions and my OG is also within a couple of points.

This with taking a pipette straight out of the boil kettle and putting it on the refractometer.

 

[tofuguy]

Only additional thing I do is stir the wort before taking a sample to ensure closer to a homogeneous solution.

 

[TheMadKing]

I should have left the word should out of the sentence... Preboil will always be less than OG.... But I don't know what I was thinking.... If you did have evaporation with the hydrometer your readings could be the way you experienced...

My experience with my refractometer is that evaporation differences if there are any at all are very minimal. My preboil readings are within a couple points of the Beersmith predictions and my OG is also within a couple of points.

This with taking a pipette straight out of the boil kettle and putting it on the refractometer.

This is exactly what I do as well, boiling wort in a pipette straight on the refractometer, slide cover down, and then let it finish cooling like that for a minute or two. I'll usually take 3 samples and average them if they are different but they usually aren't.

OP have you double checked your volumes?

The one time I had something like that happen, it was because my immersion chiller had sprung a leak and I gained a bunch of water after the boil was over.

 

[Carolina_Matt]

I had similar issues in the past, and evaporation makes perfect sense as an answer. I was just stirring the wort with a mash paddle, then letting a few drops drip off of the paddle and onto the refractometer. I would sometimes get misleading gravity readings (105% conversion efficiency, etc) and the OG would be lower than the pre-boil gravity. I've started stirring the wort and then scooping an ounce or two into a stainless steel cup, letting it cool, then taking a reading from that. It seems to be more accurate.

For those of you that take a pipette sample directly from the boil kettle, what kind of pipette are you using? I've just been reusing the cheap plastic one that came with my refractometer, but that would melt in boiling wort. I guess I should look for a better one that can withstand higher temperatures.

 

[kh54s10]

I bought a big bag of plastic pipettes, they are a little larger than the one that came with the hydrometer. Neither melted when sampling wort. Come to think of it. I am looking for OG so I don't know if I ever sampled straight from boiling.

Because I almost always get within a couple points of target OG, I don't take a reading to see if I am there during the boil. But I am sure I take readings in the 90s because my tap water will only take the wort down to the temperature of the air and I have brewed on 90 degree days.

 

[jack13]

Does the pre-boil gravity lead you to make any adjustments during the boil, or do people measure it mainly for record keeping to better work out volumes and such for subsequent brews?

I'm thinking if it's the latter, might it be more accurate to calculate what the pre-boil gravity must have been given 1) the post-boil gravity (which may be more straightforward to measure) and 2) the change in wort volume (also more straightforward to measure)?

It's counterintuitive to do a calculation to get a measurement rather than just taking the measurement, but I'm thinking of it like this: There are 3 measurments involved (the change in volume, the pre-boil sg, and the post-boil sg). If you know any two, you can calculate the 3rd, so why not use the two reliable ones (volume change and post-boil sg) to get the 3rd, less-reliable one (pre-boil sg)?

 

[kh54s10]

I measure and record my preboil gravity. I have my boil off rate down pretty well so I know what volume I will end up with (pretty close) So, if the preboil gravity is way off from the prediction I can make adjustments. I have only done so on one occasion. It was a big beer and my preboil was low, so I added some DME.

Most times I am close enough that I just accept what I get.

 

[seilenos]

Here is the process I use:

After mashing, I raise the bag, and squeeze.
After squeeze, but before lighting the burner, I give the pot a good stir and use a cheap-o plastic pipette to take a sample from 2-3" below the surface. (I'd picked up a lifetime supply of the pipettes from Amazon for like $10).
I drip a few drops on the glass of the refractometer and on the underside of the cover plate, flip it closed, and let it sit for a minute while I deal with getting the bag off the pulley, adding Fermcap, and lighting the burner. Comparisons of the value from right out of the tun to waiting a minute is that the reading rises by a "heavy" 1 point.

If I am short it is just a few calculations to figure out how much DME to add, though I haven't had to do that in a while.
I take this pre-boil number and water volume and use a boil-off calc with my target volume to estimate OG.
If my estimated OG is too low and either I don't have DME or don't want to use it, an alternative is increasing boil-off to further concentrate the wort.

I don't take another reading until towards the end of cooling. This is more to confirm the volume than anything else; if I know the volume then I know what the OG is going to be and vice-versa. Same ritual as post-mash: stir, use a pipette that's been sitting in sanitizer, empty the sanitizer from the pipette, pull a sample from below the surface, drip a few drops on the glass of the refractometer and on the underside of the cover plate, flip it closed, and let it sit for a minute.

That all being said, that doesn't answer your original question.

A boil-off from 13 to 12 gallons would see a 1.050 become 1.054. That's only 4 points. For a hydrometer, a 40 deg F swing would give the same difference. I'd be curious if you did the measurements right after mash and at the end of cooling with a refractometer if you'd see more reliable values. The lower temps limit evaporation and the relative mass of the drops to the body of the instrument mean that the temperature effects are limited pretty quickly.

 

[eric19312]

can someone provide a link to pipettes that will handle boiling wort?

 

[TheMadKing]

can someone provide a link to pipettes that will handle boiling wort?

Basic Essential Bottles 100pcs 3ml Disposable Graduated Plastic Transfer Pipettes https://www.amazon.com/dp/B073WLCQWD/ref=cm_sw_r_cp_api_i_bJYZDbDYB84EW

These are the ones I use

They don't "melt" but the do get a little more floppy when there's boiling wort in them

I compress the bulb and suck up only 1-2mL of boiling wort and keep the pipette tip pointing toward the ground. If you attempt to turn it over it'll allow in an air bubble, spit boiling wort out the tip, and burn your fingers when the wort enters the bulb.

Plus if you keep it tip down it has a higher surface area to volume ratio while it's stuck in the tip and cools faster.

From there I put a couple drops on my refractometer for a reading and wait a minute before tasting the rest.

They do soften but don't melt, and I reuse the same one for several batches before tossing it.

 

[CodeSection]

Basic Essential Bottles 100pcs 3ml Disposable Graduated Plastic Transfer Pipettes https://www.amazon.com/dp/B073WLCQWD/ref=cm_sw_r_cp_api_i_bJYZDbDYB84EW

These are the ones I use

They don't "melt" but the do get a little more floppy when there's boiling wort in them

I compress the bulb and suck up only 1-2mL of boiling wort and keep the pipette tip pointing toward the ground. If you attempt to turn it over it'll allow in an air bubble, spit boiling wort out the tip, and burn your fingers when the wort enters the bulb.

Plus if you keep it tip down it has a higher surface area to volume ratio while it's stuck in the tip and cools faster.

From there I put a couple drops on my refractometer for a reading and wait a minute before tasting the rest.

They do soften but don't melt, and I reuse the same one for several batches before tossing it.

I use the same pipettes. The only thing I do differently, once I use it once, I discard it. They are so cheap and I do not want to potentially have any readings compromised from any remaining solution from previous samples.