O-ring sealing: how to know it’s not overtightened, but still tight enough?

[Rogerdd]

Maybe I’m overthinking it but when putting back the battery cap of a Perdix for example or changing the battery of dive lights, it’s usually recommended to ‘tighten until snug and do not overtighten’.

But in practice how to ensure this? I understand overtightening can cause deformation of the o-ring and compromise the sealing, but at the same time not tight enough would also not create a proper seal to make it waterproof.

So is there a method to make sure it’s tight but not too tight (at least from a dive equipment perspective)?

I was helping change the perdix’s battery of another diver (he’s been diving longer than me) and noticed his battery cap was tighter than mine, so I’m wondering if both were fine or if one was not tight enough/too tight.

 

[LFMarm]

For the Perdix overtightening doesn’t do anything to the orings because they are on the side of the screw. It may damage the thread though. Given this setup, tightening doesn’t improve the seal; you should stop tightening as soon as you reach the stop without tightening. It’s almost impossible that the screw will get loose.

 

[hedonist222]

Maybe I’m overthinking it but when putting back the battery cap of a Perdix for example or changing the battery of dive lights, it’s usually recommended to ‘tighten until snug and do not overtighten’.

But in practice how to ensure this? I understand overtightening can cause deformation of the o-ring and compromise the sealing, but at the same time not tight enough would also not create a proper seal to make it waterproof.

So is there a method to make sure it’s tight but not too tight (at least from a dive equipment perspective)?

I was helping change the perdix’s battery of another diver (he’s been diving longer than me) and noticed his battery cap was tighter than mine, so I’m wondering if both were fine or if one was not tight enough/too tight.

I finger-tighten the vast majority of things I own (unless it requires a torque wrench).

 

[Tom_Ivan]

O-rings dont use "tightness" to seal. They rely on the pressure differential causing the material to deform into the space and fill any gaps.

My rule of thumb is finger tight for everything then a slight bit extra with a tool to ensure it won't accidently come undone.

 

[happy-diver]

Your coin will jump out of the slot, before you damage anything

If you do not tighten completely by bottoming out the cap the Perdix will Not function

Most everything, requires its own particular touch of tenderness

 

[lairdb]

Just for completeness, the literal answer to “in practice how to ensure this” is “use a torque wrench, torque to spec.” To pick a scuba example, most reg manufacturers will spec the torque for everything that gets screwed together in a reg.

Torque screwdriver - Wikipedia

en.wikipedia.org

Torque wrench - Wikipedia

en.wikipedia.org

See, for example, language like “Reassemble and torque to 130 IN.-LBS.” and “Reassemble with same tool and torque to 33 FT.-LBS.” in this reg service manual: Atomic Regulator Service Manual

Although I haven’t seen one, it would be pretty easy to mount a coin blade to a torque screwdriver and use that to precisely torque the battery caps.

That said, as most have pointed out: torque does not prevent leaks on most o-ring joints in scuba:

O-rings dont use "tightness" to seal. They rely on the pressure differential causing the material to deform into the space and fill any gaps.

Generally, torque prevents unscrewing, and over-torqueing causes more issues than under. See, for example, the “do not overtighten” and “snug” language page 24 of this regulator sales brochure manual. Generally, if no spec is given in an assembly instruction, conclude that no specific torque is required, and tighten until something bottoms out and you’re confident it won’t work loose.