Undercarbonation on keg with priming sugar

Brewing Processes
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Although I normally force carbonate my kegs with CO2, sometimes I like to use priming sugar to carbonate in the keg (esp. for styles that are OK sitting at room temp for a few weeks, and/or in cases where I open transfer and want to scavenge oxygen).

I use the carbonation calculator in BeerSmith, and as expected the amounts for corn sugar (and other sugars) are less for kegging than bottling. I’ve generally followed this, but also note that I consistently end up undercarbonated in the keg. This is no huge deal, because I can just force carbonate for the last little bit, but it is annoying.

As an example (consistent with my general experience). I kegged ~5.25 gallons of ale into my 5 gallon keg, noting that it was pretty close to full to the top. To hit ~2.8 volumes of CO2, at my fermentation/beer temperature of 66 degrees max (no cold crash, etc.), BeerSmith calculated ~2.6 oz of corn sugar required (bottling would be ~5.3 oz.). Knowing that my kegs usually undercarb, I rounded up to 3 oz. I let the keg sit ~2.5 weeks (sometimes I go longer - results are always the same). I measured the keg pressure (by depressurizing my CO2 tank regulator and then attaching it to my keg - the backpressure should give an estimate of keg pressure), and it was 16 psi at 68 degrees, or ~1.8 volumes of CO2.

So…what’s up? What’s causing this mismatch between the calculations and reality?

I posed this question during last night’s Homebrew Happy Hour presentation on packaging, and it was suggested (by Drew?) that I might be getting CO2 leakage if the lid is not firmly seated, esp. if I don’t pressurize the keg with an initial shot of CO2. I can say that I pressurize the keg after kegging, to this end. The only possibility I can think of is that I’m just not pressurizing enough, so the CO2 is dissolving into the beer, which reduces head pressure, and then gets a bit of leakage during early stages of the priming sugar getting fermented. (I unfortunately don’t remember what I pressurized to most recently, although it’s usually in the 15 to 25 psi range, maybe over 30, I think).

Any thoughts on this? Have others had problems getting full carbonation in keg priming? Should I just be sure to super pressurize the keg to 30 psi or more? Are the calculators just flat-out wrong, or at least wrong in my use case?

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I replaced all my O-rings with the softer yellow versions from MoreBeer. I have not had any issues with lids seating even before replacement under lower pressure. I lift up on the spring ring and then secure the lid. Check for leaks like you would a gas leak. Light soapy water and look for bubbles

As I replied for my beers I use 1/4 to 1/3 cup table sugar. Google says this is about 2.3 oz. I remember way back in the day the recommendation was to use 1/2 cup sugar for a 5 gal keg. That is 3.5 oz according to Google.

did you mix with water (and boil)?

So I would think you simple underprimed. I do not crash chill or purge with CO2 In fact I want a bit of yeast to come over.

My beers are ready to serve in 1.5 weeks.

edit. I roll my kegs around after priming and sealing to ensure the sugar is well mixed

No I do not have oxidized beer!!!

Well it does not oxidize within 1-2 months of serving which is as long as my beers last

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I mix the sugar with water and boil, dump that solution into the bottom of the keg, and rack the beer onto that. I’m not too worried about mixing (unlike in bottling), especially because it all should eventually get mixed to whatever extent is needed.

Kegs seem to hold pressure just fine otherwise…no leaks, including with a bubble check.

Yeah, so perhaps it is underprimed…or too much headspace in some cases…or something else!

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Physics works the same everywhere I can’t imagine it would be something else. My procedure is what you describe. Even before my beer engines I underprimed for the softer CO2. I would think that if you use more priming sugar allow more time for the yeast to eat through the sugar and condition properly. suggest 2 weeks then condition in your fridge for 3 days.

Edit again. After rereading your initial post, it would seem you are probably getting the correct CO2 amount based on the amount of priming used

I fill just to the lower end of the gas in short dip tube as a point of reference

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I have never heard that priming in a keg requires less sugar than priming the same volume in multiple bottles. I am curious about the logic/science behind this. Keg or bottles, in both cases you are packaging the same volume of the same beer. Same number of yeast cells. Why would a keg require nearly half the amount of sugar as bottles?

You are using 57% (3oz / 5.3oz) of the sugar indicated for bottling, and you are achieving 64% (1.8 vol / 2.8 vol) of the target CO2 volumes. I think the answer is to use the bottling amount.

Also, how does the carbonation seem to your palate?

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I’m curious on the science behind this, too. Pretty much every resource says to use less sugar for the same amount of beer in a keg. The explanations I have seen suggest it’s either something to do with the proportion of headspace in a keg (not sure that makes sense), or how people expect bottled beers to be more carbonated, so kegs should use less priming sugar to pour equivalently.

I’m beginning to wonder if it’s not just one of those homebrew myths. But perhaps there is some explanation out there? It’s clearly ingrained in our hobby.

Definitely undercarbonated! (as expected)

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I’ve primed kegs about 10 times in the last few years and it hasn’t been a problem at all as long as I hit it with a short burst of CO2 to ensure that there’s a good seal. Just a very small puff is all it takes. You might have bad O-rings or a leaking relief. I did have a leaky relief once. I always prime at the standard rate for kegs, traditionally the equivalent of 1/3 cup per 5 gallons. After about 10 days, it’s enough to push out a couple pints of perfectly carbonated beer.

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Cool to hear you’ve had good luck…another data point! I haven’t had any sign of leaky o-rings (the kegs hold pressure in all circumstances, and are quite pressurized even if disconnected from gas for a month or two or three, when force carbonating); I did have a bad relief once also, but back to normal once I swapped out the lid. All of this is why it’s so puzzling to me…ah well; maybe I’ll just have to adopt a “house standard” for keg priming.