[PSUBS-MAILIST] Depth Gage

[Alec Smyth via Personal_Submersibles]

Oh my, I have so thoroughly forgotten this stuff even though it was used in
every project back in college. The gauge comes in various ranges, and I got
the 500 psi model which has a minimum graduation of 1 foot of water (no
decimals). To summarize what you are saying, would this correct? At any
depth, I could tell if I were rising or sinking with a 1 foot change in
depth since that is the minimum graduation. At full design depth of 1,000
feet, the accuracy of the depth reading would be 2.5 feet. At shallower
depths, the accuracy would be better but we're not sure exactly by how much
- I'd say its immaterial since 2.5 feet is already more than accurate
enough. There is also the issue of fresh versus salt water having a 3%
density differential, a way larger "error" than the accuracy of the
instrument. BTW the next model up (DPG-200) has both fresh and salt water
display units.


Best,

Alec

On Tue, Sep 15, 2015 at 8:23 AM, Jon Wallace via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

>
> Alec, graduations of distance measurement is going to be limited by
> processing hardware not sensor accuracy.  With a sensitive enough ADC (i.e.
> 16 bit) any sensor will show small graduations of vertical movement.  The
> higher accuracy unit will only provide a better representation of actual
> depth, but you also have to remember that the delta between measured and
> actual with a %FS unit will be greater at the higher end of the scale than
> the lower end.  So unless you are consistently diving deep a 1%FS is going
> to be more than accurate enough in most cases.
>
> Jon
>
>
> On 9/14/2015 9:02 AM, Alec Smyth via Personal_Submersibles wrote:
>
>> Coincidentally I just spent yesterday afternoon selecting one of those. I
>> would normally prefer an analog gauge because I try to minimize electronics
>> to maximize reliability. However, on a 1,000 foot sub the needle would
>> barely move on shallower dives, so digital display is the only way to go.
>> The accuracy of these instruments is measured as a percent of full scale,
>> and a cheap one is accurate to 1% FS. For a thousand foot sub, that means
>> the smallest depth change it could measure is 10 feet - not good! Well, let
>> me rephrase that. I'm fine knowing my depth to within 10 feet, but what I
>> really want to know if whether my depth is increasing or decreasing, and
>> I'd like to know that before I've traveled 10 feet.
>>
>> I settled on the Dwyer DPG-100, because it has this:
>>
>> - 0.25% FS accuracy
>> - Displays pressure directly in feet of water
>> - Wetted elements are 316 SS
>> - IP66 enclosure (waterproof to "hose-down" standard)
>> - Lighted display
>> - Battery powered, so no need to wire it into the sub power (battery life
>> 2000 hrs)
>> - Reasonably priced ($185)
>>
>> It also records the max depth, hardly a necessity but cool for unmanned
>> depth tests. For testing my K250 I just strapped a dive computer outside,
>> but that wouldn't work for a test to over 1,000 feet.
>>
>> Here is a link: https://www.dwyer-inst.com/PDF_files/A_34.pdf
>>
>> If you want to go the Arduino or PLC route with a touch screen and all
>> that, you can step up to the Dwyer DPG-200 because it transmits a process
>> signal. The nice thing compared to the normal pressure transducers is if
>> your Arduino, PLC, or display screen failed, you could still see the depth
>> directly on the gauge. The DPG-200 also has high and low programmable
>> alarms with NC and NO switches, so you could use it for example to
>> automatically turn on your scrubber when going past ten feet, or to wake
>> you up with a klaxon if you are going past your max depth.   However, it
>> has cables coming out the back that make it a little less compact, and it
>> needs an external power feed. Dwyer also offers low-cost screens that you
>> can plug the DPG-200 depth gauge into directly without any processor.
>>
>> https://www.dwyer-inst.com/PDF_files/A34A_low.pdf
>>
>>
>> I opted for the DPG-100 because my focus is on simplicity, but depending
>> on your priorities one or the other of these two should make a good
>> instrument.
>>
>>
>>
>> Best,
>>
>> Alec
>>
>>
>>
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