No, this is not too old to ask a question, and all questions are good. Not to worry about that.
The capillary gauge will read deep at altitude in fresh water. The capillary gauge is the simplest gauge there is, as it is simply a tube with one end plugged. As the pressure increases, water enters the tube. According to the gas laws, the pressure becomes twice as great at one atmosphere (34 feet in fresh water at sea level). So at one atmosphere, the water will have gone half-way up the tube. At 68 feet, or three atmospheres absolute, the water will be 2/3 up this tube, etc. But at altitude, the air is less dense, while the water pressure is the same, and so the water will compress the air further, making the scale on the gauge read deeper. That is why the number read needs to be multiplied by the correction factor (0.887 at 4,000 feet altitude), to get the accurate depth. But we divers of ol' never did that. We simply used the reading, deeper than the real depth, as a safety factor for our dive when calculating repetitive dives.
Oh, and there is no internal sensing mechanism in a capillary guage, though a temperature sensor would be helpful. . The reason is that on a hot day, submerging a capillary depth gauge in cold water causes the air inside the capillary tube to contract, thereby increasing the depth reading in a way which the pressure compensation number doesn't incorporate.
John C. Ratliff Diving since 1959, at age 13. Haven't stopped, and still enjoy getting wet.
John, Thank you for that detailed answer. I did not think about the density of the air being compressed in the capillary. Honestly I hadn't put much thought into depth gauges and their readings at different altitudes. So far I have never done any diving greater than 2,000 ft. Rick
Of all the things I've lost, I miss my mind the most.
Not a lot of divers are now even aware of the capillary depth guage. Here is a photo of one from Dacor in use:
IMG_2725 by John Ratliff, on Flickr Note the asymetric numbering, which is the way that these guages show the depth. At 33 feet, the water will have come up the tube half way. And it decreases acconding to the gas laws as the depth increases. Eventually, it becomes hard to determine accurately the depth. This one shows almost twenty feet of depth (easier to see when viewing it underwater, rather than from this flash photo).
But this is the simpliest depth guage ever, and actually can be home-made using a small diameter PVC tube and wrapping it around, then putting a numbering index with 33 feet at half way down the tube, 66 feet 2/3 down, etc.