Graduating Carboys

Perhaps one of the most popular thread topics here is "help me fix my low efficiency!!!". While there are usually many factors which contribute to low efficiency, one which is very frequently overlooked is a lack of precision in measuring volume. Most of us are able to measure grains to fractional ounces if not grams and measure the original gravity of the wort in the fermenter to the nearest gravity point, but a great many homebrewers cannot measure volume in the fermenter very accurately.

As an example take a simple SMaSH brewed with 12 pounds of marris otter and an original gravity of 1.074. Further suppose the because of an imprecisely marked (or unmarked!) carboy the best one can do is get within a quart of the true volume in the carboy. At 21 quarts we get a brewhouse efficiency of 85.20%, at 20 quarts we get a brewhouse efficiency of 81.14%, and at 19 quarts we get a brewhouse efficiency of 77.08%. 8% range in brewhouse efficiency is a lot just because of imprecise volume measurements.

In addition, there are those who choose to mark their carboys by using gallon jugs or measuring cups to add water. My own Pryex 2 cup glass measuring cup is wildly inaccurate as can be seen in the photos below. Luckily, we have all the tools at hand to accurately graduate our carboys. All that is needed is this simple but admittedly tedious technique, a carboy, a calibrated thermometer (optional but nice), a gram scale, and a means of marking the carboy. Sharpie markers work beautifully on plastic. On glass one can use vinyl stickers as masks and obtain glass etching gel used to frost glass from art supply stores. Stickers and tape don't usually provide good long term solutions and given the tedium of this method, I recommend doing it once right and being done forever.

The first step is to decide how precisely you want to graduate the carboy; I elected for pint increments. A quart is 946 mL, a pint is 473 mL, and a cup (for you super patient) is 237 mL. For reasonable water temperatures we can neglect the temperature of water. My tap water the day I did this was 60°F which corresponds to a density of 0.9991026 g/mL. To take this into account multiply your desired volume in mL by the density of your water to arrive at the needed mass of water to obtain that volume. In my case 473 mL*0.9991026 g/mL = 473.14 g. Had my tap water been 86°F, I would have needed 473 mL*0.9956502 g/mL = 471 grams of water to yield one pint. A small difference and most will probably neglect it but I offer it here for the sake of completeness.

Now the method. I placed my measuring cup on my gram scale and tared it.
Tared measuring cup


Next add tap water until you are close to your desired mass. Note how inaccurate my Pyrex measuring cup is!
Ever so slightly overfilled measuring cup being adjusted with a pipette


Here I measured the temperature of my tap water to ensure that my calculations were accurate.
Measuring the temperature of the tap water


Once the net weight in the cup is as calculated above, carefully transfer it to the carboy being very careful not to spill any and draining the measuring cup completely. Place the measuring cup back on the scale to ensure that the weight has returned to zero. This helps assure us that we transferred all the grams of water to the carboy that we intended. Tap the sides of the carboy or carefully tip and roll it to encourage all the water clinging to the sides to move into the bulk fluid. Fill your next measuring cup to the correct mass and allow the fluid in the carboy to become still. Once it is still, carefully mark your carboy at the meniscus.

Carboy ready for its next pint graduation


Continue with this process until your carboy is full and then enjoy using it forever after. You never need to worry again about your ability to precisely measure sweet wort in the fermenter which will help rule out a common and significant source of uncertainty when calculating and troubleshooting brewhouse efficiency.

Completely graduated carboy


It is customary to read an instrument to half of a scale unit, so in my case I can call a volume 20 quarts or 20.25 quarts or 19.75 quarts. In my original example this would tighten up the variation in brewhouse efficiency to ± 1% brewhouse efficiency for a fairly big beer. The bigger the beer the larger the variation in brewhouse efficiency. Had I been patient enough to graduate in cups, I could measure ± half a cup and in the same example and so ± 0.5% brewhouse efficiency.

+1
Awesome write up. Should be a sticky (ahem, mods)

to be super proper, you need to use DI water....tap water won't weight the same as DI water...If we're getting that picky.

DI=Distilled?

SaltyTX said:

to be super proper, you need to use DI water....tap water won't weight the same as DI water...If we're getting that picky.

Click to expand...

I won't argue that point. It would take just a little over 1,000 ppm of dissolved solids (w/w) to increase my example of 473 grams per pint to 473.5 grams per pint. That would be very hard water and we're still talking about fractional grams. Most will neglect dissolved solids with no effect but it certainly can be corrected for if one were so inclined.

djsethall said:

DI=Distilled?

Click to expand...

DI means deionized. Meaning the mineral ions have been filtered out through a physical process using an ion exchange resin filter. Distilled is boiling and condensing the pure water vapor afterwards.

Deionization is faster than distillation and doesn't require heat. DI water is used in chem labs (such as were I work) since we can generate it faster and for most of our purposes, not having ions in the water is OK, although we do have a nanopure system to capture the various shat that the DI system doesn't pick up. If you want close to 100% pure water, distillation is the way to go. DI water doesn't filter out organics (that are uncharged), bacteria, viruses, etc...

RO filters like the DOW filmtec TW30-1812-75 that I use and recommend (it is cheap people!) is considered hyperfiltration and removes particles from 0.1 nm up. Not much is going to get past this and what does tends to be ionic and thus entrapped in the DI resin bed. Any homebrewer wanting to move to this level of filtration need only spend $150 believe it or not.

Are all carboys exactly the same size? Doesnt matter if its made in Mexico or Italy? I would think that they would be very close in dimensions?

I wonder if what you did can be transferred into a universal sticker. If you can calculate the distance from each mark and average it out to get distance of each mark point.

Is something like this doable?

It'd be doable, but I don't know how accurate it'd be.

Although, it'd be extremely doable for Better Bottles considering they're all the same. Hmmm... I smell a product idea...


I graduated mine using a large measuring bowl which I first checked with a bunch of different cup and pint measures. Makes everything so much easier.

I would recommend against trusting any measuring cup. Cooks Illustrated (they rigorously apply science to cooking) does measuring cup reviews from time to time and routinely find 10% variation on liquid measures. If you have a gram scale or better a decigram scale, double check any liquid measure prior to using it to graduate a carboy.

So did you calibrate your scale and your thermometer first? Ooooh, darn... better erase those graduations and try again.

pauljmccain said:

It'd be doable, but I don't know how accurate it'd be.

Although, it'd be extremely doable for Better Bottles considering they're all the same. Hmmm... I smell a product idea...


I graduated mine using a large measuring bowl which I first checked with a bunch of different cup and pint measures. Makes everything so much easier.

Click to expand...

I already sell the product you're thinking of but if you can do it cheaper go for it. While better bottles are standard, the problem with a direct application sticker where all graduations are at their appropriate place is the amount of wasted material. I suppose I'd be willing to do it if customers were willing to absorb that cost to save the calibration time.

All this is nice if you want to be accurate. I know I am not, and I still end up with beer

MazdaMatt said:

So did you calibrate your scale and your thermometer first? Ooooh, darn... better erase those graduations and try again.

Click to expand...

I realize that you're poking fun but yes and yes. I'm lucky enough to have a milligram scale at work and my modest little gram scale agrees time after time with the milligram scale although not to the same precision obviously. My thermometers are always two point calibrated every few months in boiling water and an ice water slurry.

My biggest source of error remains marking and reading the graduations themselves.

My biggest source of error remains marking and reading the graduations themselves.

Click to expand...

You could apply masking tape at precisely the meniscus, and above. Then use fingernail polish (or even a wide-tipped permanent marker) to make a clean, precise, permanent line. Just be consistent so that you always read the TOP of the graduation mark.

Very helpful post ChemE. Went through and marked up my carboy and drew some more precise lines on the pitcher I use to measure out water on brew day as well. Thanks!

Great post, ChemE. One followup Q, though. How do you precisely measure the volume when the liquid gets to the carboy shoulder? I understand we read at the base of the meniscus at eye level (90 degrees) with the liquid in the flat portions of the carboy. However, when it curves at the shoulder, is the reading still at eye level or is it at 90 degrees from the glass surface? There's a pretty big discrepancy in those two readings. I realize I can just be consistent, but I'd like to know the correct answer to satisfy my own curiosity. Thanks!

But what about the differential of the water temperature once placed within the carboy? I do like the squigly markings on the side (I do the same thing) to help with the absolutely precise measurements. I figure ball parking with a bold marker and hand written lines are good enough for a normal measuring cup.

Just kidding on the water differential ... it's a great how to in precision!

I work in the sign industry, so I cut my own vinyl numbers and lines for marking all my carboys. You could easily have a local sign shop cut you the vinyl strips and numbers. Here, our shop minimum is $15. For that $15 you could get all the lines and numbers needed to do many carboys, buckets, etc. Just a thought for those thinking of doing this. It's clean, easy to do and doesn't come off real easily. It will come off if you want it to, but not on it's own.

wbtenor said:

Great post, ChemE. One followup Q, though. How do you precisely measure the volume when the liquid gets to the carboy shoulder? I understand we read at the base of the meniscus at eye level (90 degrees) with the liquid in the flat portions of the carboy. However, when it curves at the shoulder, is the reading still at eye level or is it at 90 degrees from the glass surface? There's a pretty big discrepancy in those two readings. I realize I can just be consistent, but I'd like to know the correct answer to satisfy my own curiosity. Thanks!

Click to expand...

You would still read the meniscus parallel to the fluid's surface, don't worry about the angle of the container's wall. That said, if you fill a carboy that full with wort you better place a blow off tube since you're going to have a LOT of blow off. Everyone chasing efficiency should not that blow off obviously results in a decrease in efficiency!

Can't you just use a more precisely marked graduated cylinder or Erlenmeyer, make a temp adjustment and call it a day? If you use identical carboys, I don't see why you should have to go through the procedure more than once.

You're assuming that there is not manufacturing variation from one Better Bottle to the next. I considered that as well but decided that the risk/reward didn't make sense for me. I only have three primary fermentors so it isn't a huge time investment. Plus I'm an engineer (as are many of us here) and we tend to be pretty enamored with precision.

I have 3 five-gallon glass carboys that I use for secondary. The 5 gal mark is dramatically different in each. I tend to fill as full as they will allow to avoid headspace since fermentation is pretty much done. Prior to precise measurement as per this post, I assumed the 5 gal mark was at the shoulder as they were in my friend's. In one carboy, the precise mark is actually right at the base of the neck, with only 4 oz of extra space to overflow. For me, dialing in my equipment correct once and for all was definitely worth it.

Bobby_M said:

Click to expand...

Just looking at this picture is helpful, maybe someone can publish the measurements in mm for each marker, I would do it but my boy is occupied at the moment