I was just catching up on this episode and the question about water resistance ratings was something that interests me. The explanation I was given about these rating by my old physics teacher is this; the 'depth' rating of a watch is its safe maximum depth in metres (or atmospheres) IN STILL WATER. In other words, a 30m watch could theoretically be placed under 3ATM pressure of water. However, any movement of the watch or the water briefly places higher pressures on parts of the watch.
For example, if you are snorkelling and dive down to 3m, the action of lunging your arms as you swim down can momentarily place much higher surges of pressure on parts of the watch. If you dive into a pool (or the ocean from a pier), the initial surge of pressure on the crown or crystal could be many times higher than the 3 or 4 metres you dive down. Even the action of placing a watch under a running faucet can exert several 'metres' of water pressure on the parts of the watch the water hits first, possibly exceeding a 5-10m rating, resulting in a flooded watch and an upset customer who says they simply washed their hands wearing it.
I suspect what's happening with these ratings is that if a watch is tested by the maker to be (for example) safe to 300m in static pressure, they may advertise it at 100-200m to account for additional stress from diving or swimming actions that can't be easily predicted.
I was just catching up on this episode and the question about water resistance ratings was something that interests me. The explanation I was given about these rating by my old physics teacher is this; the 'depth' rating of a watch is its safe maximum depth in metres (or atmospheres) IN STILL WATER. In other words, a 30m watch could theoretically be placed under 3ATM pressure of water. However, any movement of the watch or the water briefly places higher pressures on parts of the watch.
For example, if you are snorkelling and dive down to 3m, the action of lunging your arms as you swim down can momentarily place much higher surges of pressure on parts of the watch. If you dive into a pool (or the ocean from a pier), the initial surge of pressure on the crown or crystal could be many times higher than the 3 or 4 metres you dive down. Even the action of placing a watch under a running faucet can exert several 'metres' of water pressure on the parts of the watch the water hits first, possibly exceeding a 5-10m rating, resulting in a flooded watch and an upset customer who says they simply washed their hands wearing it.
I suspect what's happening with these ratings is that if a watch is tested by the maker to be (for example) safe to 300m in static pressure, they may advertise it at 100-200m to account for additional stress from diving or swimming actions that can't be easily predicted.