Horological Meandering

A common topic on watch related websites is the Water Resistance ratings of various watches.

The common element in achieving WR is the humble O-ring - whether the watch is a 30m rated dress watch, a 610m Rolex Sea Dweller (see Dr Strong's recent post) or the completely over-specified 1000m+++ watches.

In the (poor quality) photo below are 5 O-rings. Scaled against a Euro 10c coin.

The challenge is: Can you guess which has the highest pressure resistance and which has the lowest??

Bonus points for those who can identify the O-ring application.

(disclaimer: for any engineers or specialists out there...yes, there are factors beyond the mere O-ring that determine the pressure rating, but let's let it roll....)

but let's say 3 highes and 5 lowest.

Best, Kari

One Highest and Four lowest.

My reasoning is that the more the O ring can be compressed the more it will adapt to its environment and seal properly.

My model behind it is that an O ring in a watch is very slim and compressable.

Regards,

Yves

1 is lowest since it's big and wobbly. 3 is highest since it's thick and small.

... I would have thought it was easy, but then I started to imagine some of the variables. Thickness, compressibility, surface area exposed to the pressure, and who knows what else a real engineer might think of.

So I will guess 3, 1, 2, 4, 5.

Look forward to the truth (and the science) ...

Andrew

..is one of those things where it's so simple, yet you get into the detail you just start doubting!

Let's take a crown o-ring for that 200m watch.

Where do we start?

Well, unless we're happy with a 15mm diameter crown, we're restricted to small <5mm OD o-rings. 

So out with a catalogue. There are literally thousands designs of these things.

Hmm, there's some with a x-section of .5mm which would make packaging easy, or there's monster 2mm x-section ones....which won't go over the stem. OK, lets go down to 4mm and use the 1mm x-section design, fits inside the crown with room for a stem.

It also has a very very low hardness (measured on a durometer). Not ideal for something that might be unscrewed once or twice a week. The o-ring might be pulled out of its groove. So lets try that other one....super hard like plastic. Will last forever and not pull out....but is so hard it won't seat until under great pressure. So no good for even a paddle pool.

So we rummage back though the catalogue and find a 3mm ID with a 1mmx-section, perfect. But it only comes in silicone...and I was planning on advertising this watch as a Super-Mechanics and Divers watch. With an ad campaign showing Oil and stuff all over the watch and arm of the wearer....Silicone get eaten by this stuff.

Round we go again. So I'm searching for an o-ring 3mm ID, 1mm x-section, that can cope with being moved once a week (semi-static), that can cope with petrol and oil....

Easy.

2 of the O-rings are from a watch.

2 of the O-rings are from Scuba equipment.

1 of the O-rings is from an Engine.

Number 5 is from the Egiziano?



A

First off, thanks to those who took the plunge on such a vague quiz.

 

This was a bit of a dishonest question. As I alluded, there are a plethora of factors that contribute to the pressure resistance of an o-ringed joint. However I tried to find as diverse a range of o-rings to demonstrate firstly, the maddening variety of o-rings and that by merely looking, it's difficult to evaluate what is intended to do what. Nothing is really as it looks.

 

So the answers.

 

Highest to Lowest: Four, Two, One=Three, Five

 

And what were their applications?

 

One: Caseback O-ring from a 200m ISO rateddive watch

Two: Scuba 1st stage regulator, High Pressure port O-ring

Three: Crown O-ring from a 200m ISO rated diving watch

Four: Scuba Tank valve O-ring

Five: Cylinder head O-ring from Honda RS125R

 

 

And what pressures do they see? (I am using Bar here as the unit, 1 Bar=approx 1 atmosphere=14.5psi)

 

Five:

Seals between the head and cylinder of a two-stroke race engine. It sees the maximum pressure of each combustion phase, a strangely low 15 Bar (218psi).

One and Three:

Are 200m rated watch o-rings. So according to ISO, they need to meet 125% of the rated pressure. Lets say, 23Bar (333psi)

Two:

Takes the pressure from the 1st stage of a Scuba regulator to the Pressure Gauge. It'll be rated much higher, but it will see up to say, 300Bar (4350psi)

Four:

Takes the pressure between the Scuba tank valve and the 1st stage. Again, it'll be rated much higher but will see similar pressures to the above, 300Bar (4350psi)

 

 

So there's an order of magnitude between the pressures those o-rings see....yet if you were to view them in cross section, they differ very little, from say 1mm diameter to 3mm.

  Would like to be able to attribute this picture….but it’s an ancient scan.

 

 

nt

No message body

Thanks for the quiz and the education. A lot of fun.

O-rings are forever associated in my mind with the Shuttle Challenger disaster. When o-rings fail, they can do so in spectacular and disastrous ways.

Andrew

...dipping an o-ring into the glass of ice-water to demonstrate the change in o-ring response at lower temps.

There's a humungous amount of info on the Challenger disaster, but as I understand it, the o-ring failure was caused by: An uneven groove depth around the 37foot (!) circumference the o-ring was trying to seal, (not to mention that the o-rings were found not to have a consistent x-section), exacerbated by the groove opening up slightly under launch loads, exacerbated by the poor response of the o-ring at low temperatures (the o-ring hardened at the lower temp and could not react to the changes in the groove depth (which should allow for a ~40% compression of the o-ring x-section) allowing the super hot gas out, which cooked the o-ring, letting out more gas.....awful.

O-ring's fault? not really. If the groove was properly designed and took into account the deflection under load when launching, it would have had fighting chance. (Or NASA could have paid attention to the o-ring company not to launch until it got warmer....)

 

honest guv'nor i did, i knew them all

dishonestly knowledgable

Graham

....it's already December...



Cheers,
pplater.

I'm getting round to it this weekend

Hope it's OK for next week in sunny London..!!

See you soon.

Thought you were being shy.

I have some other ideas that I'll make sure you get the chance to guess at...