[PSUBS-MAILIST] Depth Gage

Tue Sep 15 14:43:33 EDT 2015

Interesting. I use the depth sounder for approaching the bottom but never
look at it thereafter. I suspect, however, that it's simply because the
sounder is in a much less visible spot for me. Food for thought...

Alec

On Tue, Sep 15, 2015 at 9:38 AM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Alec,
> I have a depth gauge inside and never look at it. I use my simple depth
> finder good for 600 feet. I am mostly interested in where the bottom is.
> Also I know if I am sinking or surfacing and it is very sensitive.
> Hank
>
> ------------------------------
> * From: * Alec Smyth via Personal_Submersibles <
> personal_submersibles at psubs.org>;
> * To: * Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>;
> * Subject: * Re: [PSUBS-MAILIST] Depth Gage
> * Sent: * Tue, Sep 15, 2015 1:14:24 PM
>
> 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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