[PSUBS-MAILIST] Depth Gage

[Jon Wallace via Personal_Submersibles]

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
>
>