Question So what is this?

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You are correct, I missed that. But it's not nearly as dramatic an increase as with the capillary gauge. The formula for any depth marking in bar (approximately 10m) on a capillary gauge (starting at infinite depth) is 1/bar absolute. So 10m is 1/2 the length, 20m is 1/3, 30 is 1/4, 40 is 1/5, etc.
Yup.
 
Interesting find! @Angelo Farina ?
I and my wife owned two, identical. Diaphragm-type depth meter with polycarbonate body and filled with air, manufactured by Mega Sport, Torino, Italy and sold with various names (Mares, Scubapro, Cressi, Modulo M, etc.). My one is marked Modulo M, and it is still working perfectly, the one of my wife was flooded. Here some photos of my one:
20230122_131350.jpg

20230122_131441.jpg

20230122_131511.jpg


After removing the back stainless-steel cover, the brass diaphragm acting on the internal mechanism is exposed (with some oxide, as you can see).
Pressing the diaphragm the depth indicator moves, as shown here:
20230122_131536.jpg

At the time (ca 1980) this was possibly one of the best depth meters you could buy, as it is demonstrated by the fact that my one is still working perfectly, after 40 years and more than 1000 dives.
I always kept my one in a console together with a brass Scubapro pressure gauge, which is also still being used.
I replaced the depth meter with a Cressi Leonardo computer a couple of years ago, because I had to dive in a site where a DC was mandatory.
And yes, the NDL limits are to be evaluated against the orange "max depth" needle, according to US Navy tables (for a single dive per day, of course).
 
It must be noted that there were two versions, the one in the original message was the version with double scale (two turns of the needle), my one is the single scale.
It is not a bourdon tube, which is remarkably linear. This is a diaphragm meter, with a pseudo-logarithmic scale.
 
It must be noted that there were two versions, the one in the original message was the version with double scale (two turns of the needle), my one is the single scale.
It is not a bourdon tube, which is remarkably linear. This is a diaphragm meter, with a pseudo-logarithmic scale.
Thanks! Your pics didn't make it through... At least on my side.
 
It is not a bourdon tube, which is remarkably linear. This is a diaphragm meter, with a pseudo-logarithmic scale.
The learning never stops :)

Diaphragm depth gauges are still being made. For example:

71lzdiC+UKL._AC_UF1000,1000_QL80_FMwebp_.jpg


 
Thanks! Your pics didn't make it through... At least on my side.
They were links to Google Photos.
Now I reposted them properly, they should be visible.
 
It's a standard Bourdon tube gauge, not a capillary gauge. Capillary gauges do not use a needle, there's just a clear tube with a colored liquid inside, the end of the liquid indicates the depth. They are easy to spot, not just because of the lack of a needle, but because the depths indications get closer as you go deeper.

This is a capillary depth gauge

il_570xN.4477298641_phku.jpg
There is no “colored water inside.” It is open and ambient water enters the tube. It has a colored area behind, which changes slightly with the water that enters. You can easily see the line which indicates the depth, which is shown by the numbers. If you’ll note, the 33 foot depth is half way around the capillary gauge, and 66 feet is 2/3s of the way around. Does that remind you of anything?

SeaRat
 
To be sure, it is only a depth gauge, not a decompression meter/computer. It does have no-decompression limits at various depths based on the US Navy dive tables and a square dive profile. It must be used in combination with a watch or bottom timer. Yes, it is a Bourdon Tube mechanism.
 
To be sure, it is only a depth gauge, not a decompression meter/computer. It does have no-decompression limits at various depths based on the US Navy dive tables and a square dive profile. It must be used in combination with a watch or bottom timer. Yes, it is a Bourdon Tube mechanism.
It is a diaphtagm mechanism, explaining the not-linear scale (more expanded at small depth).
Bourdon tube pressure meters are linear.
 
https://www.shearwater.com/products/perdix-ai/

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