Analog vs digital

[Zept]

Here's a weird thing...

Usually I prefer analog displays to digital ones (had the obligatory digital watch in the '70s, never wanted another one). I find it hard to understand why anyone would want a digital pressure gauge.

However... I find the digital depth readout on my dive computer much easier to work with than the analog one on my depth gauge. Maybe it's because the scale on the depth gauge (Suunto SM-16) is so weird, with tick marks at 3, 6, 9, 12, 15, 20, 30, 40, 50, 60 and 70m (not that I've ever wound the needle past 30m).

Anyone know why they're made that way?

Zept

 

[donacheson]

Originally posted by Zept
Here's a weird thing...

Usually I prefer analog displays to digital ones (had the obligatory digital watch in the '70s, never wanted another one). I find it hard to understand why anyone would want a digital pressure gauge.

However... I find the digital depth readout on my dive computer much easier to work with than the analog one on my depth gauge. Maybe it's because the scale on the depth gauge (Suunto SM-16) is so weird, with tick marks at 3, 6, 9, 12, 15, 20, 30, 40, 50, 60 and 70m (not that I've ever wound the needle past 30m).

Anyone know why they're made that way?

Zept

Only my guess: Obviously, it's metric, but the first 5 depths are very close to the metric equivalents 10, 20, 30, 40, and 50 feet and probably were intended for use with standard decompression tables.

 

[chris_b]

Originally posted by Zept
...Anyone know why they're made that way?...
Zept

As I understand it, the pressure gauge is filled with oil. As ambient pressure increases, the oil is compressed (linearly with pressure, due to Boyle's law?). The first ten meters down, the pressure doubles, so the volume of oil in the gauge is halved and the needle moves quite a bit. The next ten meters down, the pressure is increased by only 50%, so the volume of the oil is compressed only half as much as it was during the first ten meters. Thus the needle moves only half as much as it did for those first ten.

I think that's why the depth marker spacings get closer as one gets deeper.

Chris

[Edit: as you probably noticed, I was answering a question you didn't even ask :bonk: but I still wonder if the unusual marking scale has something to do with the nonlinear changes in the physical pressure-measuring mechanism?]

 

[Zept]

Originally posted by chris_b


As I understand it, the pressure gauge is filled with oil. As ambient pressure increases, the oil is compressed (linearly with pressure, due to Boyle's law?).

That doesn't sound quite right. Boyle's law applies to gases, whereas oil is a liquid and incompressible. I thought the oil was used under the glass, to prevent it from caving in at depth, but, hmm... I could be wrong. Anyone know for sure?

Z

 

[Ari]

this is due to the fact that pressure changess are greater at shalower depths. That is, going from 0 to 10 meters UW will double the pressure (100% difference) while adding 10 meters between 40 and 50 meters (for example) will only increase the pressure by 20%.

Ari