Re: [PSUBS-MAILIST] depth gauges

Sean

The MSP pressure transmitter I use quotes a 0.25% overall accuracy. There is no reported accuracy data at the low end vs the upper end of the range. As this unit has a range of 0-250 psig , the uncertainty of the pressure measurement is +/- 0.63 psi which translates in freshwater to +/- 1.5 ft.

As to your question on merit of having a second transmitter for better accuracy in holding periscope depth or maneuvering near the surface, the theoretical answer is yes. A 0.25% accuracy is pretty typical from my experience on pressure gages so if your range is 0-50 psig , this translate to +/- 0.13 psig or +/- 0.3 ft of fresh water. If you are using a PLC system and a pressure transmitter, it would be easy to implement this as you would just put in a few lines of ladder logic in the depth calculation that reverted to this secondary pressure transmitter if you are within it range. From a practical perspective for most cases, I think this over complicates the boat. To keep the 0- 50 psig sensor from being damaged by over pressuring , you would have to have an isolation value. To me if you really needed to know shallow depths this accurately, I would go with a 0.125% transmitter for the primary sensor and forget the secondary sensor.

As to including outboard salinity and temperature sensors to improve the density value in the depth calculation, while it is easy to make a measurement of ambient water temperature (I do this now) and outboard salinity, to use this data correctly, you would need to integrate the water temp and salinity as function of depth from the surface to the boat depth to get this second order improvement in depth accuracy. In the spirit of KISS, I don't think the small incremental improvement in depth accuracy from these added sensors (as compared with hard coding fresh water and sea water density and manually switching between in the depth calculation) is worth the effort to implement them for a psub. Can you do this, yes; should you do this, from my perspective no.

Cliff

From: Sean T. Stevenson <cast55@telus.net>
To: personal_submersibles@psubs.org
Sent: Thu, March 11, 2010 2:15:27 PM
Subject: Re: [PSUBS-MAILIST] depth gauges

On March 7, 2010 11:09:12 you wrote:
> On the R300, I have a primary and backup depth measurement. The primary is
> a 0-250 psig pressure transmitter (Digi-Key MSP6251P4-1-ND) tied into my
> PLC and touch screen display. This has worked very well as you can
> display the depth digitally and you can switch between sea water and
> freshwater in the depth calculation with a touch of the screen. As a
> backup in the event I loose both my main and auxiliary power supplies, I
> have a Weksler Altitude Gauge (BM43-YHT4-LW). The dial is for 0-450 fsw.
> The problem with a manual pressure gage like this is
> you introduce 2.5% error in the depth measurement when you move between
> sea water and freshwater because of the density difference. As with any
> pressure gage, you need to use a dead weight tester to calibrate the
> instrument.
>
> Cliff
>

Cliff - a couple of questions:

How accurate is the digital gauge in the vicinity of the zero end of the
spectrum. i.e. - Is that sensor as accurate in 0-5 psig as in 120-125 psig?
While it probably doesn't matter for the R300, if someone had a sub in which
they wanted to maintain a periscope / snorkeling depth near the surface, would
there be any merit in also incorporating a higher resolution, lower range
sensor (i.e. 0-50 psig) to be used in that range, and then switching to the
other signal when the vessel is deeper?

I notice that you move your vessel between salt and fresh operating areas. In
the interest of complicating things unnecessarily, it occurred to me that you
could include outboard salinity and temperature sensors to calculate the
density of the water on the fly, and actually create a density profile on the
way down in order to accurately calculate depth, even across a halocline. Fun
stuff.

-Sean

************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 53
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************