This all started by Michael asking me 'how do I make one at home?'. Such a question required (my assumption) something else than a set of drawings and a few tips on how to machine the parts. It was my intention - should my fragmented inputs spur any interest for the instrument - that we all, collectively, could come up with something useful for the PSUBS website in the end (as a result of a final rewriting of my own fragments by including other people's opinions, questions, proposals, advice, comments, etc., etc.). I believe that we could get some very interesting web site material using this approach. But it requires that the originator of a topic take the responsibility upon himself to do the final editing (re-writing), then issue this draft for comments prior to have a document of "instructional" format officially published on the PSUBS website. ** For what its worth, I found the posts up until #4, to be ** lacking enough detail to comment on. ** It probably would have been better if you started with the ** overall sketch and then showed the details in subsequent posts. I'll try to introduce my next topic in a more inspirational manner, and also be very clear (up front) on why I think it would be useful for other people, and not only myself. ** The unit you are describing is very similar to what we call a ** "water level" in the States. I agree that the Papenberger could be misinterpreted to be nothing more than a "water level". However, it's far from it! It is a very precise depth gauge used at shallow depths. Together with it's operational companion, the trim indicator, this unity (see attached image) provides you with an excellent tool that you can use to bring your boat to a perfect diving trim. Besides, it will even tell you how close you are at neutral buoyancy as soon as you dive. ** It is the curvature of the water surface within this thin tube ** that gives an early indication of direction the boat will travel. I could not say it any better than what David just did [in the above quote], but I would still recommend everyone to check up the fourteen text lines and two images available at Wikipedia on the same subject, ref http://en.wikipedia.org/wiki/Meniscus ** internal PSI rating of the glass tube and where do you find that information. I would simply determine its pressure rating by performing realistic bench tests. And I would probably use an "unbreakable" plexiglass instead of glass tubing to avoid splinters all over the place should the gauge burst open due to excessive depth with the instrument open to sea (ignore to shut down the sea cock valve). Maybe even furnish the assembly with a small rupture disk at the very top of the air chamber, this to prevent any view glass bursting at all. ** Also, is the depth indication (graduations) as shown by the water ** column linear, or does it vary as the air bubble is compressed? As Frank just pointed out, the variations will gradually get smaller as the pressure increases. To avoid a lot of tedious, repetitive calculations, I made a program that generate a table required to make a proper graduated (non-linear) depth scale. The same software is also a great help to determine the optimal volume (length of scale versus air-cushion volume) for a final design of such an instrument. Finally, please note, that the attached image can be found in full size, at my ftp server, ref ftp://ftp.artematrix.org/instrumentation/papenberg_and_trimmzeiger/S302.jpg Best regards, Jens Laland
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