nit picking? depth ratings on watches

[sandanbob]

Good morning!

This is a small thing, I know, but something I have thought about for some time.

What might I be missing?

Here it is- some watches are labeled that they are rated to 200 meters/660 feet, and 20 atm. Wouldn't 200 meters/660 feet be 21 atm? And, the inverse- wouldn't 20 atm be more like 627 feet/190 meters?

I know in a practical sense that it won't make a difference, but it's just something that seems not quite right.

Thanks!

Bob

 

[L13]

The difference between atmospheres gauge and atmospheres absolute. 20 atm gauge is 21 ATA (atm absolute).

 

[sandanbob]

The difference between atmospheres gauge and atmospheres absolute. 20 atm gauge is 21 ATA (atm absolute).

You think that's what they are doing? Using gauge instead of absolute? Could be.

 

[jborg]

That is what the watch experiences. It’s manufactured and sealed at one ATA, so the experienced pressure differential at one ATA is zero.

 

[sandanbob]

Ah, so it is gauge/relative pressure.

Thanks for the responses!