Help me calculate grain absorption, please.

[slayer021175666]

So, I figured that I could wait until my whole brewing process was done and the wort was in the buckets. I weighed the mash tun with the spent wet grain inside and it came to around 52 lb. I then, dumped the mash tun and weighed it again. It weighed around 13 lb. From 52 lb total, I subtracted 25 lb for the original grain weight and then subtracted 13 lb for the empty mash tun weight. That left me with 14 lb. When I divided that 14 lb by 8.33 for the weight of 1 gallon of water, I got .067.
So, to me that would be, 067 gallons of water for every pound of grain. Is that correct? When I punch it into my brewing software it tells me to use about a gallon less water than what I know the recipe actually takes. Is there something I'm not getting? Is this the correct way to calculate it from the weight of the whole mash tun after I brew? Or, am I doing this all wrong? Now, I've thought about it so much I've just confused the hell out of myself! Looking for some insight. Thank you.

 

[3 Dawg Night]

Your math checks out, but that does seem low. I've never measured mine, but a couple of online calculators recommend 0.125 gal/lb for grain absorption. That's what I use, and it's in the ballpark.

What do you get if you work by volume? Subtract the volume of your collected wort from the total water you added, then multiply by 8.33 to see if you get the same 14 lb of water left in your grain.

 

[slayer021175666]

Your math checks out, but that does seem low. I've never measured mine, but a couple of online calculators recommend 0.125 gal/lb for grain absorption. That's what I use, and it's in the ballpark.

What do you get if you work by volume? Subtract the volume of your collected wort from the total water you added, then multiply by 8.33 to see if you get the same 14 lb of water left in your grain.

You got me thinking here. The wort that I extracted after putting the full 15 gallons through it. That is the original strike water and sparge water. Was, 12.5 or 12.75 gallons. For easy math, I just now figured it at 12.5. That would leave 2.5 gallons in the mash tun. On 25 lb of grain, that would be .1 gallons absorbed by every pound of grain. When I punched .1 into my brewing software, it gave me 14.82 total water needed which, I know is about the right number because, I actually use 15 gallons of water in the recipe.
Now, I'm wondering: How could my math by weighing it be so far off from figuring it by volume of water instead of weight? Why would I get .067 if I calculate it by weight but get right near .1 if I calculate it by volume?
What the hell?!

 

[friarsmith]

Grain absorption is usually 0.12 to 0.18 gallons of water per pound of grain. Grists with a lot of wheat and rye seem to land at the high end for me. I usually get 0.13 to 0.15 for all barley grists. Remember to factor in loss under a false bottom, if that is in use

 

[doug293cz]

Your method for measuring/calculating grain absorption rate is not correct. The correct way is to use volumes. The formula is as follows:

Grain Absorption Rate = (Strike Volume - First Runnings Volume) / Grain Weight​

​

Since you are taking the difference of two almost equal numbers, it is important to measure both volumes accurately if you want an accurate result. You should also correct the measured volumes for thermal expansion (~2% shrinkage from mash temp to room temp.)

If batch sparging, you can also use the following alternate formula:

Grain Absorption Rate = (Total Brewing Water Volume - Pre-Boil Volume) / Grain Weight​

Brew on

 

[slayer021175666]

Grain absorption is usually 0.12 to 0.18 gallons of water per pound of grain. Grists with a lot of wheat and rye seem to land at the high end for me. I usually get 0.13 to 0.15 for all barley grists. Remember to factor in loss under a false bottom, if that is in use

Thank you. I'm quite aware of mash tun loss. Or at least, that's what I call the amount left from the dead space under the false bottom. What I'm really trying to do though is, figure the exact grain absorption amount for this particular recipe. I realized that there are averages and rules of thumb. I'm going for an exact amount, though.

 

[doug293cz]

You got me thinking here. The wort that I extracted after putting the full 15 gallons through it. That is the original strike water and sparge water. Was, 12.5 or 12.75 gallons. For easy math, I just now figured it at 12.5. That would leave 2.5 gallons in the mash tun. On 25 lb of grain, that would be .1 gallons absorbed by every pound of grain. When I punched .1 into my brewing software, it gave me 14.82 total water needed which, I know is about the right number because, I actually use 15 gallons of water in the recipe.
Now, I'm wondering: How could my math by weighing it be so far off from figuring it by volume of water instead of weight? Why would I get .067 if I calculate it by weight but get right near .1 if I calculate it by volume?
What the hell?!

The problem with using weight, is that somewhere between 60 - 80% (80% if you get 100% conversion efficiency) of your grain weight gets converted to sugar, which is dissolved in the wort, and is drained into the BK. You can do the math, but it actually gives you an unusable number (the true grain absorption), when what you actually need is the "apparent" grain absorption, as defined in by previous post above. What you want to know for your recipe is how much water to use to get a specific target value of wort into the BK, and that's what determines the formula used.

Brew on

 

[slayer021175666]

Your method for measuring/calculating grain absorption rate is not correct. The correct way is to use volumes. The formula is as follows:

Grain Absorption Rate = (Strike Volume - First Runnings Volume) / Grain Weight​

​

Since you are taking the difference of two almost equal numbers, it is important to measure both volumes accurately if you want an accurate result. You should also correct the measured volumes for thermal expansion (~2% shrinkage from mash temp to room temp.)

If batch sparging, you can also use the following alternate formula:

Grain Absorption Rate = (Total Brewing Water Volume - Pre-Boil Volume) / Grain Weight​

Brew on

Maybe I'm confused but, it seems to me that it would be the other way around. You would batch sparge to get a first runnings amount but, you would fly sparge to get the full runnings amount.
Maybe I'm wrong about that but, anyway. I knew what the full runnings amount was so I calculated by volume from that. Please read post #3 and tell me if my way of calculating is now, correct.

 

[doug293cz]

Grain absorption is usually 0.12 to 0.18 gallons of water per pound of grain. Grists with a lot of wheat and rye seem to land at the high end for me. I usually get 0.13 to 0.15 for all barley grists. Remember to factor in loss under a false bottom, if that is in use

The volume under a false bottom affects the effective mash thickness, because the grain cannot "swim around in" the volume under the false bottom (or the volume between a grain basket and the MLT sidewall.) The amount of wort collected is affected by the grain absorption, and any undrainable MLT volume - which should only be a fraction of the volume under a false bottom.

Brew on

 

[slayer021175666]

The volume under a false bottom affects the effective mash thickness, because the grain cannot "swim around in" the volume under the false bottom (or the volume between a grain basket and the MLT sidewall.) The amount of wort collected is affected by the grain absorption, and any undrainable MLT volume - which should only be a fraction of the volume under a false bottom.

Brew on

Now, this part, I do understand.

 

[doug293cz]

Maybe I'm confused but, it seems to me that it would be the other way around. You would batch sparge to get a first runnings amount but, you would fly sparge to get the full runnings amount.
Maybe I'm wrong about that but, anyway. I knew what the full runnings amount was so I calculated by volume from that. Please read post #3 and tell me if my way of calculating is now, correct.

Ok, fly sparging can be ambiguous, because many brewers continue sparging until they have collected their target pre-boil volume in the BK (or the SG of the runnings starts to get too low), leaving an MLT full of very dilute wort. With this version of fly sparging, grain absorption is meaningless.

You, on the other hand, use a fixed amount of brewing water, split between strike and sparge. You then run all of the allotted sparge water thru the grain, and drain the MLT completely into the BK at the end of the sparge. For grain absorption this is mathematically identical to batch sparging.

The math you used Post #3 is the second formula that I gave. If batch sparging, the first formula I gave is preferred, as the accuracy is better, assuming the error in volume measurements is proportional to the volume being measured. If the errors in volume measurement are independent of the volume measured, then both formulas have the same accuracy.

Brew on

 

[slayer021175666]

Ok, fly sparging can be ambiguous, because many brewers continue sparging until they have collected their target pre-boil volume in the BK (or the SG of the runnings starts to get too low), leaving an MLT full of very dilute wort. With this version of fly sparging, grain absorption is meaningless.

You, on the other hand, use a fixed amount of brewing water, split between strike and sparge. You then run all of the allotted sparge water thru the grain, and drain the MLT completely into the BK at the end of the sparge. For grain absorption this is mathematically identical to batch sparging.

The math you used Post #3 is the second formula that I gave. If batch sparging, the first formula I gave is preferred, as the accuracy is better, assuming the error in volume measurements is proportional to the volume being measured. If the errors in volume measurement are independent of the volume measured, then both formulas have the same accuracy.

Brew on

Okay. I understood your first and second paragraph but, your third paragraph, I'm a little confused. The part about errors being independent or proportional. You lost me there. Maybe you could explain it a different way or maybe you could just decide whether I really need to know that or not and just tell me if you think I'm correct in assuming that right around 2.5 gallons of water is the amount absorbed by the grain and lost under the false bottom.

 

[3 Dawg Night]

The problem with using weight, is that somewhere between 60 - 80% (80% if you get 100% conversion efficiency) of your grain weight gets converted to sugar, which is dissolved in the wort, and is drained into the BK. You can do the math, but it actually gives you an unusable number (the true grain absorption), when what you actually need is the "apparent" grain absorption, as defined in by previous post above. What you want to know for you recipe is how much water to use to get a specific target value of wort into the BK, and that's what determines the formula used.

Brew on

I think this probably solves the Case of the Missing Weight. Theoretically, you could get there by weight by the following process:

1) Weigh the dry grain.
2) Weigh (or calculate weight of) strike water & infusions
3) Weigh the full MLT (at the end of the mash before draining).
4) Weigh the empty MLT (after draining wort and dumping grain).

(Weight of full MLT - weight of empty MLT - weight of strike water - weight of dry grain) / weight of dry grain = grain absorption

Somebody check my logic on that. In the end, it's much easier to do it by volume.

 

[doug293cz]

I think this probably solves the Case of the Missing Weight. Theoretically, you could get there by weight by the following process:

1) Weigh the dry grain.
2) Weigh (or calculate weight of) strike water & infusions
3) Weigh the full MLT (at the end of the mash before draining).
4) Weigh the empty MLT (after draining wort and dumping grain).

(Weight of full MLT - weight of empty MLT - weight of strike water - weight of dry grain) / weight of dry grain = grain absorption

Somebody check my logic on that. In the end, it's much easier to do it by volume.

Weight of full MLT = weight of empty MLT + weight of strike water + weight of dry grain, thus the numerator in your formula will always be 0.

Brew on

 

[3 Dawg Night]

Weight of full MLT = weight of empty MLT + weight of strike water + weight of dry grain, thus the numerator in your formula will always be 0.

Brew on

Like I said, it's easier to just use volumes!

 

[bwible]

Shouldn’t it be 14 lbs / 8.33 lbs per gallon = 1.68 gallons? Then if you go 1.68 gallons was absorbed by 25 lbs of grain, then it’s .067 gallons per pound or 128 x .067 = about 8 ounces of water or a half pint absorbed per pound. Thats pretty much standard.

 

[hottpeper13]

When I first went AG I used .14 as my constant and would end up with 1 gal of 1.020 wort in my pitcher by the end of the boil. My current constant is .11 for the mash tun and .06 for squeezing the bag.

 

[Rob2010SS]

I'm going to put my process here in regards to post #11.

I fly sparge 100% of the time. I've done some trial and error on what number to use for grain absorption and I now use .11 gal/lb as my number. Previously, I've always used .15 but I was ending up with a lot of left over wort in the MT.

My goal when sparging and transferring to the boil kettle has always been to run the MT dry and end up with my desired pre-boil volume.

The calculation I run through looks like this, using my most recent recipe with 30.80 lbs of grain.

30.80 lbs of grain X .11 gal per lb of grain absorbed = 3.388 gallons of water absorbed
10 gal initial mash volume - 3.388 gal water absorbed = 6.612 gallons left after mash
10.75 desired pre boil volume - 6.612 gallons left post mash = 4.138 gallons of sparge water needed

So when I sparge, I kill my sparge once I've hit my desired sparge volume. In this case, I sparged until I used 4.25 gallons of water. I'm usually within 1-3 points of my estimated OG by following this math.

Only throwing this out there as an example where fly sparging is also using a fixed amount of water, instead of continuously sparging.