Dive-Computers.net

Computers will flip into Altitude Mode automatically or at least any made in the last 5 years will.
A bit on capillary gauges. The concept of it is an open tube on one end. The air trapped inside is compressed as you dive deeper, by the ambient pressure of the water acting on it, makes that little widget connected to the gizmo turn the dial They operate according to Boyle’s law (the volume of a sample of gas is inversely proportional to the pressure applied to the gas if the temperature is kept constant.)
A capillary gauge won’t be at all accurate when
diving at altitude – at least as far as actual depth readings go.That’s why you adjust for altitude on the RDP.

The gauge will read more than your actual depth. In fact, the
ratio of its reading to your actual depth will be the same as the
ratio of the pressure at sea level to the pressure at the surface
of the lake.

I double-checked the essay on altitude diving at the back of the
PADI AOW manual. In a nutshell, it recommends scaling actual depths
up by the above pressure ratio to get a "theoretical depth" which
you’d then use in the tables in place of your actual depth. As it
turns out, a capillary gauge does this same scaling automatically!

Of course, you’d still need your regular depth gauge (or computer)
if you wanted to know exactly how deep you are. You couldn’t use
its reading to determine no-decompression limits, though. But even
if you used only your computer , a capillary gauge
and tables would make a cheap backup that also adjusts.
Good luck in the AOW!