Sea Water Salinity - RFI - request for ideas

J

johndk

Senior Member
I've been looking at various archived posts on measuring salinity. I haven't found much. And what I've found doesn't quite do it for my project. I'm trying to design a sea water salinity probe. Here are the characteristics I'm looking for:

Must be reasonably accurate (not looking for the n'th decimal place!)
Must be low cost; low component count; low power consumption (especially)
Probe portion needs to last only 6mo or so submerged (I know conductivity is hard on probes)
Data will be logged, analyzed after the fact

So my idea is to use the PicAxe to generate an alternating signal (as per previous posts) and using a simple gold plated 0.1 inch spacing straight header as the probe. Depending on the total surface area needed to get a good reading, I could use multiple alternating contacts (say 1,3,5,7 and 2,4,6,8 on an 8 pin header) and either the short or the long side of the header. The other side of the header would be encased in epoxy and wired to the PicAxe. If such a probe can work, it would certainly be a cheap "throw-away" that could be replaced anytime the sensor package is newly deployed. But I'm wondering if the gold plating won't stand up to the abuse quite well. We'll find out!

I've read hints at using electromagnetic techniques and wonder if they might work instead of sticking metal into sea water? If cost and power consumption can be controlled, it would be ideal. I'm not holding out much hope for that, however.

A lot of you "Axers" out there are much more astute than me when it comes to making the electronics work. I'd appreciate your thoughts and suggestions before I dive in, especially if you've played with salinity measurement. The overall project I'm working on is a low cost, multi-sensor marine module meant to be deployed for months at a time to log environmental changes. When completed, the module will be used for long term environmental studies. It will also have application in water quality measurement (and control) for recirculating aquaculture and photobioreactors (all of which I'm involved with).

Thanks in advance.
 
C

Circuit

Senior Member
johndk said:
I've been looking at various archived posts on measuring salinity. I haven't found much. And what I've found doesn't quite do it for my project. I'm trying to design a sea water salinity probe. Here are the characteristics I'm looking for:

Must be reasonably accurate (not looking for the n'th decimal place!)
Must be low cost; low component count; low power consumption (especially)
Probe portion needs to last only 6mo or so submerged (I know conductivity is hard on probes)
Data will be logged, analyzed after the fact

So my idea is to use the PicAxe to generate an alternating signal (as per previous posts) and using a simple gold plated 0.1 inch spacing straight header as the probe. Depending on the total surface area needed to get a good reading, I could use multiple alternating contacts (say 1,3,5,7 and 2,4,6,8 on an 8 pin header) and either the short or the long side of the header. The other side of the header would be encased in epoxy and wired to the PicAxe. If such a probe can work, it would certainly be a cheap "throw-away" that could be replaced anytime the sensor package is newly deployed. But I'm wondering if the gold plating won't stand up to the abuse quite well. We'll find out!

I've read hints at using electromagnetic techniques and wonder if they might work instead of sticking metal into sea water? If cost and power consumption can be controlled, it would be ideal. I'm not holding out much hope for that, however.

A lot of you "Axers" out there are much more astute than me when it comes to making the electronics work. I'd appreciate your thoughts and suggestions before I dive in, especially if you've played with salinity measurement. The overall project I'm working on is a low cost, multi-sensor marine module meant to be deployed for months at a time to log environmental changes. When completed, the module will be used for long term environmental studies. It will also have application in water quality measurement (and control) for recirculating aquaculture and photobioreactors (all of which I'm involved with).
Click to expand...
These parameters are, to put it mildly, very demanding and setting the bar really high. Yes, you can deduce salinity as a function of conductivity but, as you will realise, the conductivity of seawater varies very significantly with temperature. Therefore your system needs to log and measure the conductivity and temperature at the same time to have any meaning. You are also going to have some fun calibrating your sensor, probably using a refractometer against prepared samples. The problem of degradation of the sensor is probably minor compared with fouling of the sensor; you mention a "multi-sensor marine module...deployed for months at a time..." and "low cost" in the same sentence. Methinks there is some mismatch here. Anyway, the build-up of biofilms on the sensor will probably be a greater issue than degradation and will certainly affect the reading. Refractometers can, of course, be automated. Periodically pumping a sample of seawater through an automatic refractometer, might be the way forward. A simple bimetallic strip is used in low cost refractometers to compensate for temperature, or a fixed-index is used and then a temperature correction table is consulted. Overall an interesting and challenging project.
 
J

johndk

Senior Member
Thanks for the thoughts Circuit. Yes, I will be measuring temperature (and pH) simultaneously. And the bio-fouling problem has also been addressed. At least theoretically, this is possible. I hope to find out shortly how well my solutions survive Mother Nature in her own element.

And thanks to JulianE for suggesting wire-wrap. I had totally forgotten about those (so, yesterday!). That would give me substantially more surface area to work with if it turns out I need it.

John
 
J

johndk

Senior Member
BESQUEUT said:
This essentialy depends of how often you want to measure salinity.
Every Minute ? hour ? day ? Week ? month ?
Click to expand...
Understood. I will be taking readings somewhere between 1 and 4 times per hour. Once I have the unit finished, I'll need to calculate the battery size required to keep it energized. I'm hoping I can get away with a relatively small battery. Which is why I want to keep overall power consumption to a minimum right from the get-go.
 
C

Circuit

Senior Member
johndk said:
And the bio-fouling problem has also been addressed. At least theoretically, this is possible. John
Click to expand...
You have provoked my undivided attention; what are your thoughts on preventing the build-up of biofilms on the sensors in seawater? I am most fascinated to understand your strategy.
 
J

johndk

Senior Member
Circuit said:
You have provoked my undivided attention; what are your thoughts on preventing the build-up of biofilms on the sensors in seawater? I am most fascinated to understand your strategy.
Click to expand...
I don't know if I'm ready to share this idea publicly yet. Although a likely low cost solution, it hasn't been tested. I'd be happy do discuss it privately, however.
 
Pongo

Pongo

Senior Member
johndk said:
That would be an excellent idea. But I need a low cost sensor to measure the refractive index. Do you know of such a sensor?
Click to expand...
I was thinking of some mechanical, e.g. float based, way to measure density/specific gravity.
 
fernando_g

fernando_g

Senior Member
This density measurement may work very well, specially if temperature compensation is also applied.

But it is still susceptible to bio-fouling. Probes would have to be cleaned/replaced on a regular basis.
 
premelec

premelec

Senior Member
@johndk refractive index setup with PICAXE & servo would be quite a project... :) I hope TBT not involved in biofilm prevention... A search on BING shows a recently published IEEE paper for some new method exists for NaCl but they wanted $31 for a copy... As Pongo has suggested a hydrometer method would seem possible if you could gimbal the apparatus enough in a bobbly ocean. maybe just a heap of small vials or gel injected with the brine at certain times for later analysis - good luck...
 
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B

BESQUEUT

Senior Member
johndk said:
I will be taking readings somewhere between 1 and 4 times per hour. .
Click to expand...
So you will have to put all your circuit off after each measurement, ans find a way to wake up ...
Density measurement can be done with a pressure sensor summerged at a known depth, but it will be difficult to have an accurate depth...
It will not be sensible to biofilm.
 
G

Goeytex

Senior Member
An occasional AC current pulse to the conductivity "probes" might be useful in preventing/slowing biofilm buildup.
 
Pongo

Pongo

Senior Member
premelec said:
@johndk refractive index setup with PICAXE & servo would be quite a project... :) I hope TBT not involved in biofilm prevention... A search on BING shows a recently published IEEE paper for some new method exists for NaCl but they wanted $31 for a copy... As Pongo has suggested a hydrometer method would seem possible if you could gimbal the apparatus enough in a bobbly ocean. maybe just a heap of small vials or gel injected with the brine at certain times for later analysis - good luck...
Click to expand...
I was thinking of something totally submerged, so you would just need to mitigate currents. Maybe a balance type of measurement (like the cheap and cheerful hydrometers used by aquarium folk) with the two bodies on each side of the balance having a similar surface area and heavy relative to the possible weight of a biofilm to minimize that issue.

Refractive index with a laser point and a servo, or stepper, would be an interesting project though...
 
premelec

premelec

Senior Member
Yep... OP hasn't given data on how deep sample would be and such... Don't know the purpose of measurements - could be dilution from rain squalls :-0
 
B

BESQUEUT

Senior Member
Pongo said:
Refractive index with a laser point and a servo, or stepper, would be an interesting project though...
Click to expand...
You can make a fully static unit with a serial camera in place of servo :
camera-vga-uart-vc0706 (649 units résolution, maybe more with interpolation)
No need for 6V and lower power then servo. Easy power saving mode.
Code: 
salinité (g/kg)	variation de n(λ)	Emplacement
5	0,00097	Mers baltiques nordiques
10	0,00194	 
15	0,00290	
20	0,00386	Enfoncements du Biafra
25	0,00482	
30	0,00577	 
35	0,00673	Surface de l'océan Atlantique
40	0,00769	Mers rouges nordiques

Quantité de sel ( en g )
        Angle réfracté ( en ° )
                 Angle réfracté ( en Rad )
                          Indice de réfraction

0	41	 0.72	 1.32 
10	40	 0.70	 1.35
20	39	 0.68	 1.38
30	38	 0.66	 1.41
40	37	 0.65	 1.44
50	36	 0.63	 1.47
60	35	 0.61	 1.51
70	34	 0.59	 1.55
 
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J

johndk

Senior Member
Wow! Great ideas and suggestions. I really appreciate the interest.

Since I tend to subscribe to the KISS principle (Keep It Simple Stupid), I think I shall try deploying the conductivity method first. However, I likely will have a diode laser on board, so the suggestion by BESQUEUT for refractivity has me interested. Can you expound a bit on how you would use this camera. I understand the lookup chart that was included as a code sample, but I'm not sure how this would be set up to get a refraction angle.

One thing to keep in mind is that this unit will be completely submerged for months at a time (water-proofing will be critical). As per one question above, the expected depth will vary from sub-tidal to, for now anyway, a maximum of of 50m.

John
 
C

Circuit

Senior Member
A couple of further thoughts;

Firstly, when considering gold-plating; I have done quite a lot of gold-plating in bioengineering and there are many ways of achieving this. The main reasons that many electronic connectors are gold-plated are, obviously, to prevent oxidation, to ensure good conductivity and to ensure good solder flow. None of these aspects demand very high quality gold-plating, unless perhaps you are working to military specs. You can therefore expect the odd porosity in places and a very thin coating of gold indeed. I doubt that this would give you the durability that you are looking for. And when it comes to items made of gold itself, remember that pure gold is a rare thing; I have some gold refined to 99.9999% purity and that can only be achieved by repeated electrolytic refining - the idea of it is to have a surface that is truly oxide-free. Any metals alloyed into the gold will produce oxides on the surface and this was not what I needed for this research purpose. Many "gold-plated" items are in fact "gold-alloy plated" in reality. I guess that you understand the carat system - e.g. nine carat gold being only nine-twenty-fourths gold and the rest being other metals.

Secondly, with respect to the biofilms, I think that this needs some definition. Biofilm deposition does not usually commence with life-forms; the sea is a fascinatingly complex soup of organic and inorganic chemistry and organic chemicals may film over any substance in the seawater before any bacterial, plant or animal forms follow. Anything left in contact with seawater is likely to undergo a change in surface properties within a matter of hours (and God help you if you run across an oil-slick or anything draining out of a container ship/oil tanker/pleasure craft lavatory/military vessel etc.) - Oh, and yes, biofilms form very nicely on gold surfaces - such an inert surface is great for cohesive attachment. I have worked with biofilm formation on gold where microorganisms were allowed to colonise and then die and calcify, building up the most tenacious integuments. Somewhat paradoxically, perhaps, it is alloys of gold and copper that best resist micro-organism attachment; the copper is toxic to them, but organic films will still deposit. Platinum would be a better material for electrodes.

With regard to optical methods, there are some quite successful studies on silica nano-coatings to reduce biofilm formation and this can be applied to glass. This might just work in your application.

I am racking my brains trying to think of what sort of sensor could be used for the chemically and biologically-aggressive environment that you are envisaging for your kit; a very challenging task indeed.
 
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D

DStewert

New Member
There is a global effort to study the worlds oceans with respect to temperature and salinity
through the deployment of thousands of probes.For info goto www.argo.net
There is a plugin for google earth to view and interrogate the probes collected data.
 
J

johndk

Senior Member
Circuit said:
Though I am presuming these devices are outside of your research budget you may get some ideas from the documentation here http://www.seabird.com/sbe37smpodo-microcat-ctd-do
Click to expand...
Indeed, they are outside my budget. That is, in fact, why I'm working on my version. I suspect such probes are outside of many budgets. Eventually, I hope to be able to achieve all this probe can do (including DO) at a MUCH lower cost. I will have to sacrifice some accuracy and durability. But I think that the lower cost will more than make up for that. So far, salinity, temperature, pH, DO, pressure, and possibly micro-current are within my scope. I've also considered sound, but have yet to determine if that can translate into anything useful. If I can find the right anti-biofilm coating (as in cheap and transparent), I can also do turbidity and chlorophyll. But for now those are out of scope.

I think that the simplest approach to salinity would be to use a metal conductivity probe (perhaps stainless steel?, platinum would be a last resort due to cost) and work in some approximation of degradation. Not sure how I could measure probe efficiency on the fly, though maybe the "touch" measurement could give me some indication of conductor condition. Barring that, I would consider a time/temperature equation, based on lab studies, to approximate the compensation factor.

On the other hand, if we can get enough sensitivity out of the PicAxe, perhaps two very short lengths of very fine platinum wire would be affordable.

I'd love to form a team of interested researchers to work on this with me. I'm certainly gratified by the responses I've gotten here so far.
 
B

BESQUEUT

Senior Member
johndk said:
However, I likely will have a diode laser on board, so the suggestion by BESQUEUT for refractivity has me interested. Can you expound a bit on how you would use this camera.
One thing to keep in mind is that this unit will be completely submerged for months at a time (water-proofing will be critical). As per one question above, the expected depth will vary from sub-tidal to, for now anyway, a maximum of of 50m.
Click to expand...
What is the expected salinity range for your instrument ?
I did made a waterproof camera housing (PVC pipes...). Succefully tested to 10m. Not so easy...caissonvideo
50 m will be a chalenge, but optical Windows can be small.
 
J

johndk

Senior Member
Salinity will be within the normally expected ocean salinities, including estuary salinity which can go quite low. I haven't built the the probe yet. But an ideal scale would be 0-40 g/kg. And I agree that making a waterproof vessel from PVC pipe is a challenge. The good part is that I will not need an optical window. My thoughts on waterproofing right now are along the lines of epoxy potting as much of the unit as possible.
 
rq3

rq3

Senior Member
You might consider silver for salinity probes. Not sure how it would hold up regarding corrosion, but it's quite toxic to most micro-organisms. Might help with any bio film issues.

Rip
 
B

BESQUEUT

Senior Member
johndk said:
Salinity will be within the normally expected ocean salinities, including estuary salinity which can go quite low. I haven't built the the probe yet. But an ideal scale would be 0-40 g/kg. And I agree that making a waterproof vessel from PVC pipe is a challenge. The good part is that I will not need an optical window. My thoughts on waterproofing right now are along the lines of epoxy potting as much of the unit as possible.
Click to expand...
If you use any form of refractometry, you will need one or two optical window(s).
For densitometry, differential-pressure-sensors
with maybe an accelerometer to have vertical angle.
 
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S

SAborn

Senior Member
I have built CF meters in the past, all of which work in the microvolt range.

Firstly you need a AC voltage between the probes ( or in my case a alternating DC voltage) to prevent electrolysis of the probes.

Stainless wire is what i used at 10mm spacings and 10mm long probes, this allows for a square area for calibration.

My purpose was to monitor hydroponic solutions, which is slightly higher in soluble salts than seawater.

The next thing to consider is fluid temperature as a change in temp will affect the conductivity of the fluid.

This will need to be compensated for either in hardware or in software.

For calibration you can buy solutions made for general hand held meters to be calibrated by, and hence i suggest you do this to get calibration of your creation.
 
J

johndk

Senior Member
SAborn said:
I have built CF meters in the past, all of which work in the microvolt range.
Firstly you need a AC voltage between the probes ( or in my case a alternating DC voltage) to prevent electrolysis of the probes.
Stainless wire is what i used at 10mm spacings and 10mm long probes, this allows for a square area for calibration.
My purpose was to monitor hydroponic solutions, which is slightly higher in soluble salts than seawater.
The next thing to consider is fluid temperature as a change in temp will affect the conductivity of the fluid.
This will need to be compensated for either in hardware or in software.
For calibration you can buy solutions made for general hand held meters to be calibrated by, and hence i suggest you do this to get calibration of your creation.
Click to expand...
I believe it was some of your work I found in the archives which actually convinced me that simple conductivity probes were possible with an alternating polarity. Can you supply any specifics as to what frequency you found best and what kind of readings to expect?

As to rq3 who suggested silver. I thought of that as well in terms of biofilm prevention. But I don't believe silver holds up well in salt water. I know for a fact that silver chloride is a soluble salt and of course, there's lots of chloride in seawater.

John
 
C

Circuit

Senior Member
johndk said:
As to rq3 who suggested silver. I thought of that as well in terms of biofilm prevention. But I don't believe silver holds up well in salt water. I know for a fact that silver chloride is a soluble salt and of course, there's lots of chloride in seawater.
John
Click to expand...
You are quite right; silver is rather a non-starter in this context. If you have ever left the salt in a silver salt-cellar between dinner parties; well, you will find that you virtually have to re-drill the thing to get the salt flowing past the corrosion again - been there, seen it, done it! Platinum is really the best bet.
 
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