I’ve been naturally priming my kegs with a boiled water (about a pint) and 1/3 cup of corn sugar mixture. I generally hit about 12 psi via this method. I leave the kegs inside at ambient temperatures (72F’ish) for about a week which seems to work well. However, after reaching it’s 12 psi, I’ve been taking the kegs to my outdoor porch (around 32F) to crash cool. However, when I hook up my psi device, it shows the psi level to be around 5. I’m not expert on carbonation, but do you lose psi at colder temps?
Two things happen. You pressure decreases with temperature and more CO2 in the headspace dissolves into the beer.
+1
The colder the beer, the more soluble CO2 becomes in the beer. Which means you have to either add more priming sugar or apply more pressure to increase the carbonation level.
Try warming the beer back up to 70ish and adding a couple more ounces of priming sugar. That should get you to a better CO2 volume.