Refractometer readings before and after boil

[fendersrule]

Hi all,

I just ordered a refractometer for my upcoming all-grain efforts.

It just dawned on me while thinking about it, that your OG should be lower pre boil vs after boil. The reason is simple: boil off.

We know that boil-off is the same no matter how much gallons you are boiling.

The reason I got a refractometer, is to better (and easily) safe guard myself from a lower-than-expect OG reading. Because the easiest time to add DME for boosting purposes would be pre-boil.

So my question: what is the general expected gain in OG pre vs post boil given that you have a decent kettle (a more taller than wider kettle).

For example, let's say my recipe says 1.056 OG.

Pre-boil, I take a reading and come out to 1.046. At what point do I freak out (or just R.H.A.H.B) and add DME.

A general ball-park number is OK with me, I'm not looking for exact science.

 

[fendersrule]

From what I can tell, losing about 16% is pretty normal. If you lose 16%, you should expect a 1.010 difference. So in my example above, I would be directly on target.

 

[govner1]

You are correct that your preboil SG will be lower than your post boil SG. one of the purposes of the boil is to concentrate the wort as well as sterilization.
If you use a good brew software program such as BeerSmith there is a calculator that will tell you the amount of DME or LME to add. You might consider using the latter since it is sterile and easier to deal with around boiling liquid. If you use DME consider dissolving it in water you’ve boiled or distilled water before adding.
You should also determine the boil off amount of your BK before brewing. It is simple to do. Just add the average preboil volume you normally have for an average recipe. Say 6+/- gal. Boil for an hour and measure your ending volume. You can also setup your system in any software so that it will automatically calculate the volumes you need for mash, sparge, and preboil.

 

[fendersrule]

Good idea.

For now, I'm going to count on a general rule of thumb...expect a .010 lower reading, since 16% boil-off is pretty reasonable.

The reason I like DME is that It stores very well and I consider it pretty easy to handle. I'm not sure how long LME would last if I rarely, if ever choose/need to use it anymore...and since I am measuring pre-boil, I could easily add the DME without worry about dumping it in boiling water!

If I wanted to get more exact, you're definitely correct, I could go through that effort. I boil double batches, so my pre-boil is going to be around 12 gallons (with my old pot, still looking for a new pot). My old pot is a wider pot, so it's not as efficient. With that pot, I probably lost closer to 20%. With a modern taller pot, my guess is that 15% would be reasonable to expect.

Since boil-off rate is the same in theory, I could only just put 5 gallons in there, boil for an hour, then measure. I don't think there's a need to use 12 gallons of water IMO. In that way, I can have an exact percentage.

 

[balrog]

calc your points

points pre boil * gal pre boil / gal post boil = points post boil
Say you measure 1.047 with 6.5 gal and you *KNOW* you boil a gal/hr and your boil goes an hour then you will have 5.5 gal post boil and can expect 47 * 5.5/6.5 = 1.0555 post boil. Unless you have spills, boil overs, add water, or add sugar during boil.

[PS my boil is right at 1 gal/hr. Not %. If I do 6gal batch then % would bite me, while 1gal/hr boil off is the metric that works]

 

[JimRausch]

Fairly simple math:
Let's say preboil volume is 7G and SG is 1.046. 46x7=322 gravity points, or amount of sugar you recovered from your mash.
You know from prior experience that you consistently boil off 1.5 G, so your postboil volume =5.5G.
322 divided by 5.5=58.5. Postboil SG=1.059 (I tend to round up).
If you wanted an OG of 1.067, then I'd add 1 lb. DME or LME, or sugar, or honey. They all will give you roughly 8 points in a 5G batch.
edit- I see balrog beat me to it by a couple minutes

 

[mongoose33]

You have to find out what your boiloff is. It's not the same for all circumstances, depends on how much heat you're applying. A simmer won't produce as much boiloff as a rolling boil.

 

[fendersrule]

Thanks guys. When I did partial mashes (double batch only) after the first couple brews I was able to nail down exactly how much pre-boil I need, without having to do math, to fill up 2x 5 gallon carboys completely and leave JUST ENOUGH head-space for krausin. Basically I took an unfolded steel clothes hanger and marked it each time and have it perfectly nailed. That's all I've been using for my past 6 brew days, and it's pretty dead simple...just fill up to the mark on the clothes hanger and R.H.A.H.B.

But since I'm getting a new pot, and I plan to take refractometer readings, I'm going to have to get more....scientific. That means more drunken math on brew days!

When I get my 20G pot (coming from a 15G kettle) I will pour 5 gallons into it and let it do a rolling boil for an hour. Then I'll disperse it back into a container and measure it out. I wanted to confirm that 5 gallons, 10 gallons, 15 gallons all will equal the exact same boil off number, OR is it the same boil off %?

For example, which is true. Let's say I lose 1 gallon from a test batch of 5 gallons. That should mean I would lose 1 gallon from a 15 gallon test batch as well.

Or is it true that it's a percentage. Let's say I loose 1 gallon from test batch of 5 gallons. That's 20 percent. That means if I would have done this test with 10 gallons, I would have lost 2 gallons.

I believe the former is true that the boil of amount is constant, and it isn't a percentage....

 

[balrog]

It should be fixed volume per hour loss due to boiling. It matters not whether you have 5, 10, or 10000 gallons in the pot. A boil will boil off a fixed volume during an hour. That's why I do not use %. If the preboil volume changes, that "boil off is 14%" is not the same for different preboil volumes.

 

[balrog]

People will claim they see different rates in winter or summer, high or low relative humidity, high or low barometric pressure. I personally do not measure to a gnat's a$$ to see that sort of difference in boil off for different weather.

I have run out of propane during a boil and that changed things a bit, but there was a pretty good reason.

If you get the same rolling (volcanic boil is not required) boil, you can expect the same general boil off number and that can be used over and over.

 

[govner1]

A simpler way to measure your boil off and add a simple but useful item to your kit would be to mark a wooden dowel in 1 gal increments.
Generally, after a few gallons the distance on the dowel for each gallon will be the same so you can then just add marks up to whatever max volume you want.
First, you won’t have to pour your post boil water into another container and you’ll have an easy to use tool for measuring pre boil volume and post boil volume.

 

[fendersrule]

That's a good idea govner! Then I can just leave the water in the pot and cover it until brew-day happens.

 

[govner1]

Depending on how long that might be!

 

[fendersrule]

Yea...I have enough bottles to cover 25 gallons of beer/cider...and right now it's all full.

There's only one thing to do about that though...

 

[Qhrumphf]

It should be fixed volume per hour loss due to boiling. It matters not whether you have 5, 10, or 10000 gallons in the pot. A boil will boil off a fixed volume during an hour. That's why I do not use %. If the preboil volume changes, that "boil off is 14%" is not the same for different preboil volumes.

With the same surface area into the same pot, if you're putting 20k BTUs or whatever in either way, you'll see a higher percentage boiloff from 4 gals than you will from 8 gals. That energy has to go somewhere.

The point is that for a given volume with repeatable process on a given system, it should be repeatable. Air temp, pressure, humidity all impact but if you're not boiling one day in Death Valley and the next in Denver (extreme rhetorical example) and brewing in the same place those should be minor enough to discount. May see a summer vs winter difference depending on where you live, assuming you don't brew indoors.

Edit: I misread you, so don't take that as a response, but a general statement.

In reality I don't find %/hour or volume/hr to be scaleable.

 

[fendersrule]

Well, then If I'm going to put 12 gallons in the pot to be "extra" accurate, then I'll have to add this onto brew day. Since for BIAB you start at full pre-boil volume, it makes sense to wait until brew day to do this....and definitely will need to make a marked dowel since that amount of liquid is going to stay in the kettle!

 

[TechFanMD]

People will claim they see different rates in winter or summer, high or low relative humidity, high or low barometric pressure. I personally do not measure to a gnat's a$$ to see that sort of difference in boil off for different weather.

I have run out of propane during a boil and that changed things a bit, but there was a pretty good reason.

If you get the same rolling (volcanic boil is not required) boil, you can expect the same general boil off number and that can be used over and over.

Well, I don't need to measure to a gnat's backside to know that I get 1.5GHP boil-off when it is dry (usually due to winter cold December-Feb) vs when it is warm and humid when I get a rate of 1GPH. That's enough of a difference to matter to me.

 

[balrog]

Hm. I brew in the garage year round. I get a roiling boil going. I get ~1gal/hr regardless of -12°F winter or 90°F summer ambient. I use more/less propane to do that, of course, but for the (admittedly unscientifically measured) same looking boil, I get the same general boil off.

 

[mongoose33]

Hm. I brew in the garage year round. I get a roiling boil going. I get ~1gal/hr regardless of -12°F winter or 90°F summer ambient. I use more/less propane to do that, of course, but for the (admittedly unscientifically measured) same looking boil, I get the same general boil off.

That makes sense. Once the boil is going, to use the phrase @Qhrumphf used above, that energy has to go somewhere. I don't think the water knows what the humidity is. Inside the kettle, once that energy produces a steam bubble, it has to exit.

 

[TechFanMD]

That makes sense. Once the boil is going, to use the phrase @Qhrumphf used above, that energy has to go somewhere. I don't think the water knows what the humidity is. Inside the kettle, once that energy produces a steam bubble, it has to exit.

Except that dry air is 'thirsty' air......seriously - water evaporates more quickly in the presence of dry air, and when heating water to the level of creating steam it can happen even faster. My guess is that it does depend on the gravity of the beer to some extent, and to a larger extent the conditions around the kettle. In a garage (so no wind) the temp and humidity directly above the kettle are high. Outdoors, even with a light breeze, the humidity stays low directly above the wort.

With drier are the evaporation rate is higher (due to vapor pressure). Vapor comes off and dry air absorbs moisture seeking equilibrium, slight breeze carries it away.

With warm, humid air the evaporation rate is lower (due to condensation rate being higher - it immediately cools, higher humidity air doesn't absorb moisture as easily and the condensate falls back). This is also the garage scenario since the air above the kettle is warm and humid and not replaced as quickly by new, dry air from a breeze.

Water doesn't know what the humidity is.....science does.

When you sweat (or are wet) it feels much cooler when the air is dry than when it is very humid - regardless of temp for the same reason. Water evaporates (at 33*F or 212*F) more quickly in the presence of drier air.

 

[doug293cz]

Post-Boil SG = 1 + (Pre-Boil SG - 1) * Pre-Boil Volume / Post-Boil Volume

If the above is not true, then one, or more, of the measurements is wrong. Or, you added sugar after pre-boil SG measurement, or lost significant wort to a boil over.

Brew on

 

[Qhrumphf]

Post-Boil SG = 1 + (Pre-Boil SG - 1) * Pre-Boil Volume / Post-Boil Volume

If the above is not true, then one, or more, of the measurements is wrong. Or, you added sugar after pre-boil SG measurement, or lost significant wort to a boil over.

Brew on

Important note: correct those volumes for temperature, or read the volume at the same temperature. If that immediately before and after boil (as in, pre chill when both are at like 210 degrees), that'll do. But there's enough thermal expansion/cooling shrinkage (that your gravity readings when cooled DO account for) to throw that math off by a couple percent otherwise if you're comparing 65 vs 165 vs 212.

 

[mongoose33]

Except that dry air is 'thirsty' air......seriously - water evaporates more quickly in the presence of dry air, and when heating water to the level of creating steam it can happen even faster. My guess is that it does depend on the gravity of the beer to some extent, and to a larger extent the conditions around the kettle. In a garage (so no wind) the temp and humidity directly above the kettle are high. Outdoors, even with a light breeze, the humidity stays low directly above the wort.

With drier are the evaporation rate is higher (due to vapor pressure). Vapor comes off and dry air absorbs moisture seeking equilibrium, slight breeze carries it away.

With warm, humid air the evaporation rate is lower (due to condensation rate being higher - it immediately cools, higher humidity air doesn't absorb moisture as easily and the condensate falls back). This is also the garage scenario since the air above the kettle is warm and humid and not replaced as quickly by new, dry air from a breeze.

Water doesn't know what the humidity is.....science does.

When you sweat (or are wet) it feels much cooler when the air is dry than when it is very humid - regardless of temp for the same reason. Water evaporates (at 33*F or 212*F) more quickly in the presence of drier air.

Perspiration isn't boiling water. It's just evaporating. I'm not sure that this scenario you outline is correct.

Yes, in dry air water evaporates more rapidly, but that difference isn't likely to be noticeable in an hour of just evaporating. We're talking boiling water, and the steam that escapes is being generated inside, not at the surface. Further, and perhaps more damning (as I see this), the air right above the boiling wort is NOT dry. It's full of steam which is...high humidity.

I may be wrong on this, but I'd have to see some pretty serious physics explained by a pretty serious physicist to see this otherwise.

 

[TechFanMD]

Perspiration isn't boiling water. It's just evaporating. I'm not sure that this scenario you outline is correct.

Yes, in dry air water evaporates more rapidly, but that difference isn't likely to be noticeable in an hour of just evaporating. We're talking boiling water, and the steam that escapes is being generated inside, not at the surface. Further, and perhaps more damning (as I see this), the air right above the boiling wort is NOT dry. It's full of steam which is...high humidity.

I may be wrong on this, but I'd have to see some pretty serious physics explained by a pretty serious physicist to see this otherwise.

"Perspiration isn't boiling water. It's just evaporating." ....it is exactly the same physics....different temperatures, humidity, and pressure.....but the same at different scales (add heat/low to liquid and lower humidity to the air and it speeds up)

Evaporation off your skin at 'room temp' is due to the same physics as evaporation at near-boiling -> temperature, humidity, and partial pressure. It is exactly the same either way.

Yes, the area at the near-surface is not dry due to steam escaping....BUT when outside there is likely to be at least some breeze (even 1-2 mph, a non-vertical steam plume) which is carrying humid air away and replacing it with dry air. It is exactly why a fan blowing on a bowl of water evaporates faster than one without.....and you apply heat to the bowl and the symptoms are more pronounced. More heat and more air blowing mean more evaporation.

Again - the steam that escapes may be generated 'inside' as you say, but if the air immediately above is warm and moist it will immediately condense and fall back (precipitate). If it is dry, it will evaporate ....and with minimal breeze that area stays relatively dry in winter/dry weather. In humid weather the steam in that zone may precipitate within 1/4-1/2" of the surface and be unseen.

"Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%. If the partial pressure is greater than the vapor pressure, condensation takes place. The capacity of air to “hold” water vapor is determined by the vapor pressure of water and has nothing to do with the properties of air." - https://courses.lumenlearning.com/physics/chapter/13-6-humidity-evaporation-and-boiling/

Add a tiny breeze and you have fresh, dry air washing over the top of your wort the entire time.

Easy way to think about it: really dry air at a full boil with the lid partially on is similar in boil-off rate at high humidity with the lid completely off. So if you take the lid off in really dry air, the boil off rate is higher.

 

[mongoose33]

"Perspiration isn't boiling water. It's just evaporating." ....it is exactly the same physics....different temperatures, humidity, and pressure.....but the same at different scales (add heat/low to liquid and lower humidity to the air and it speeds up)

Evaporation off your skin at 'room temp' is due to the same physics as evaporation at near-boiling -> temperature, humidity, and partial pressure. It is exactly the same either way.

I understand humidity, dew point, and relative humidity. I don't agree that there's going to be an apprciable difference in boiloff whether it's humid or dry outside.

Yes, the area at the near-surface is not dry due to steam escaping....BUT when outside there is likely to be at least some breeze (even 1-2 mph, a non-vertical steam plume) which is carrying humid air away and replacing it with dry air. It is exactly why a fan blowing on a bowl of water evaporates faster than one without.....and you apply heat to the bowl and the symptoms are more pronounced. More heat and more air blowing mean more evaporation.

I don't brew outside, and I think you're altering the ideas here a bit.

Again - the steam that escapes may be generated 'inside' as you say, but if the air immediately above is warm and moist it will immediately condense and fall back (precipitate).

I do not understand this. Steam will condense when it's cooled, and having it go into warm and moist air and then condense....this does not make sense. It's not a closed system.

I'll grant that if it's very dry outside it might possibly be that boiloff will be slightly greater, but only that. Maybe.

If it is dry, it will evaporate ....and with minimal breeze that area stays relatively dry in winter/dry weather. In humid weather the steam in that zone may precipitate within 1/4-1/2" of the surface and be unseen.

So it's raining back into the kettle? You have positive vapor pressure and it's exiting the kettle. I'm always willing to be shown that I'm wrong, and I'm not an expert in everything, but this doesn't make any sense to me.

"Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%. If the partial pressure is greater than the vapor pressure, condensation takes place. The capacity of air to “hold” water vapor is determined by the vapor pressure of water and has nothing to do with the properties of air." - https://courses.lumenlearning.com/physics/chapter/13-6-humidity-evaporation-and-boiling/

But it's not a closed system. The kettle is open to the atmosphere, so this doesn't seem to me to be applicable to our problem.

 

[TechFanMD]

"Perspiration isn't boiling water. It's just evaporating." ....it is exactly the same physics....different temperatures, humidity, and pressure.....but the same at different scales (add heat/low to liquid and lower humidity to the air and it speeds up)

Evaporation off your skin at 'room temp' is due to the same physics as evaporation at near-boiling -> temperature, humidity, and partial pressure. It is exactly the same either way.

Yes, the area at the near-surface is not dry due to steam escaping....BUT when outside there is likely to be at least some breeze (even 1-2 mph, a non-vertical steam plume) which is carrying humid air away and replacing it with dry air. It is exactly why a fan blowing on a bowl of water evaporates faster than one without.....and you apply heat to the bowl and the symptoms are more pronounced. More heat and more air blowing mean more evaporation.

Again - the steam that escapes may be generated 'inside' as you say, but if the air immediately above is warm and moist it will immediately condense and fall back (precipitate). If it is dry, it will evaporate ....and with minimal breeze that area stays relatively dry in winter/dry weather. In humid weather the steam in that zone may precipitate within 1/4-1/2" of the surface and be unseen.

"Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%. If the partial pressure is greater than the vapor pressure, condensation takes place. The capacity of air to “hold” water vapor is determined by the vapor pressure of water and has nothing to do with the properties of air." - https://courses.lumenlearning.com/physics/chapter/13-6-humidity-evaporation-and-boiling/

Add a tiny breeze and you have fresh, dry air washing over the top of your wort the entire time.

I understand humidity, dew point, and relative humidity. I don't agree that there's going to be an apprciable difference in boiloff whether it's humid or dry outside.



I don't brew outside, and I think you're altering the ideas here a bit.



I do not understand this. Steam will condense when it's cooled, and having it go into warm and moist air and then condense....this does not make sense. It's not a closed system.

I'll grant that if it's very dry outside it might possibly be that boiloff will be slightly greater, but only that. Maybe.



So it's raining back into the kettle? You have positive vapor pressure and it's exiting the kettle. I'm always willing to be shown that I'm wrong, and I'm not an expert in everything, but this doesn't make any sense to me.



But it's not a closed system. The kettle is open to the atmosphere, so this doesn't seem to me to be applicable to our problem.

I don’t think I can ever convince you. I deal with these principles daily job, I see it when I brew, and I have discussed it at length with those much smarter than myself. I’ll sleep Ok if you don’t believe me no matter what I say.

My pasta water boils faster with the lid on, I guess yours it does not. Because in my world evaporation takes heat energy to create evaporate (in this case steam) and evaporation is reduced in a humid environment. In your world, it does not. Evaporation increases as the humidity in surrounding air decreases.

Imagine light gauze over the pot - humidity increased at the surface, less evaporation.

Now imagine a cotton sheet - even more humidity, even less evaporation.

Now imagine a lid that doesn’t drip condensate back in off of the lid (so we are ignoring the ‘condensate return’) but keeps the humidity above the liquid at 100% - even less evaporation.

Now picture boiling in a room with 100% humidity and no lid - like a sauna...it’s just as if a lid were on it. Boil-off would be very low. Compare this with boiling in a room with 20% relative humidity, and a light breeze to keep 20% relative humidity air washing over the surface constantly. This is boiling outdoors in dry winter air.

It is also why people see lower boil-off with 5 gallons in a 10 gallon pot than 5 gallons in a 7 gallon pot. The larger pot creates a large vertical area of high humidity, stable air above the liquid.