I'm using 3/16th inch ID 7/16th OD beer line from Superprod in my keezer. This is your common garden-variety 3/16th tubing with 1/8th inch wall thickness. The tubing goes on pretty tight over the 1/4 inch tailpiece. I'm securing them with a #145 single-ear pinch type Oetiker clamp. At anything over 10 PSI, I get a leak at the tailpiece barb. The clamp isn't tight enough.
I bought one-ear non-stepless Oetiker clamps from McMaster Carr by mistake. It seems that the stepless ones are preferred for this application. The single ear pinch type that aren't stepless apparently don't exert even pressure 360 degrees around. There's a gap where a leak can develop.
I measured the circumference of the tubing where it expands over the 1/4 inch tailpiece, and it was 39mm. That gives me a diameter of 12.4mm which is .488 inches, which is just over 31/64ths.
I'm about to order these 52545k51
clamps from MMC, which I think are the same size as these Oetiker Clamp - #13.3 - 1/2"
from CHI Company. But before I do, I thought I'd ask what size Oetiker clamp for 3/16th ID 7/16th OD beer line over a 1/4 inch barb do you use?