Donovan, Since you ask, my dive computer wound up with a custom molded enclosure made from UHMWPE, or 'Hypact' This was used as a balance between durability/impact resistance & rigidity. More to the point, unless your sub is of extraordinary structural design, it too will be compressed some degree, even in 10 ft of water. If the 5050 is also an integrated silicon chip type sensor, consider that the hard silicon needs to deform in order for the sensor to function, and that it will deform in inches of water. I am inclined to think that a structure that is fairly strong and well designed will not compress enough to inject significant error, however, since you are working in such shallow water, it is more likely that you want fairly high depth resolution. If you want to be able to measure depth to say, =/- .5 in., you may find yourself needing to compensate. Of course, you could always cheat and use values verified at 10 ft, and nobody would ever know, unless they were actually checking your readings. Anyway, you originally asked about converting DP to Depth. I am not certain of what you mean, but see if this helps, if you don't already have this info: Pascals to psi = 1.45 X 10^4. For ATM to psi, use 1.47 X 10^1. For fresh water use 34 ft per additional ATM (of course, that's assuming 1 ATM at surface). This gives you something like .432 lbs./ft. of depth. If your water has lots of suspended or dissolved solids, such as salt water, adjust toward 33 ft./ATM, as is commonly used for salt water. At 10 ft. depth in fresh water you should see something like an absolute pressure of 19.02 psi. If you assume your sub is perfectly unchanged at that depth, you are really working with gauge pressure, since your enclosed rigid sub actually converting your sensor from a differential type to one with a fixed back-side pressure of 1 ATM. So you should see about a 4.32 psi increase (psig) at 10 ft. The sensor doesn't care if the medium is water or air - it should work the same way in both cases. Boyle's law says that the volume of air in your balloon will be inversely proportional to the absolute pressure exerted upon it - as long as the temperature remains constant. Personally, I would ditch the balloon for fresh water use.....and I would be thinking about how I was going to compensate for temperature changes. At least that's how I would do it if I were relying on success in exchange for a paycheck. Chris Chris, even -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Donovan Parks Sent: Sunday, February 03, 2002 10:29 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [OT]: Measuring Depth of Submarine using Differential Pressure Hello Chris, Thanks for the info. I'm only looking at a depth of only 10 feet. Right now I am using the Motorola MPX5050 (a differential pressure sensor) and am *hoping* that the sub will not change volumetrically at shallow depths so I can get an accurate differential pressure. Was your scuba computer is a rigid vessel? Why was it deforming in shallow water? Regards, Donovan > This is similar to a scuba computer that I did recently. The vessel will > deform and change volumetrically at fairly shallow depths - this is > something one learns > during the earliest scuba cert training. I wouldn't want to even try to > calibrate a system > with a differential sensor inside the vessel. > What max depth will your sub run at? for shallow depths (<100 ft.)I think > the most convenient, > reliable, and simplest solution for this may be a > Motorola gauge type pressure sensor. According to the data sheets they are > only for use in air, > but since they have a fluorosilicone gel covering the chip, I have found > that they have a > pretty long life in fresh water, even in chlorinated pool water. These are > very small devices, > approx 20mm round by 6 thick. 5V analog output models are very easily > scaled. The device can be > inside the sub with it's barbed fitting through the outer hull with a small > o-ring. > They are fairly linear. The hardest part of working with them is > themperature compensation. > f your water temp is pretty regular, say within =/- 10C, it will be no > problem getting accurate readings > to within about 1/2 ft, down to about 100 ft. > You might want to check out the MPS5700 family. > > Chris > > Alternately > > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Donovan Parks > Sent: Saturday, February 02, 2002 1:13 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: [OT]: Measuring Depth of Submarine using Differential Pressure > > > Hello, > > Here is the setup. I have a submarine with a differential pressure sensor. > The high pressure end of the sensor will be placed in a 'ballon' that is in > the water and the low pressure end will be in the interior of the sub. For > technical reasons I can't run a tub up to the surface of the water. > > How do I can this differential pressure into a measurement of the subs > depth? Since the submarnie was sealed at the water surface pressure (i.e. 1 > atm) and is rigid I am assuming the pressure in the submarnie will remain 1 > atm regardless of the depth of the submarnie. Now, the pressure of the > 'ballon' will change will change will depth as it is non-rigid, but what is > the conversion factor from differential pressure to depth? I have a book > (which I don't have access to right now) that gives the relationship, but I > don't really trust the source and it is to 0 decimal places of accuracy! I > will be using the sub in FRESH water. (A search of the Internet has been > unfruitful - likely because I am not sure what I should be searching for.) > > Regards, > Donovan > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.