Converting Conductance to Specific Gravity

Anthony240

I am working on making a salt water aquarium monitoring system.· I am having a problem measuring salinity.· Using the Applied Sensors V1.3 book I have created a conductance meter using the 555 timer IC.· The meter displays a conductance reading in X E-7siemens with X being the number in siemens.· Because most salt water aquarium salinity testers measure in specific gravity, I need to convert the siemens reading to a reading in specific gravity.· Can anybody help me with this problem?·· I would appreciate any input on this subject.··
Thank You
Anthony

cyberbiota

Anthony-

There is no easy answer to your question. The conductance is proportional to the ionic concentration of the water, which depends upon what you have put into it, how it is reacting with the atmosphere, and what the fish themselves are putting into it. Specific gravity also is effected by these things. How the two are related is going to be a bit different for each mixture. What this means is that there is not likely to be an easily useable look-up table already out there for the aquarium sea water, unless a hobbyist has actually made these measurements. You may have to buy a hydrometer and make the table yourself. The following link is a good place to start understanding what is involved:

http://www.advancedaquarist.com/issues/jan2002/chemistry.htm

Good luck!

peter

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Peter C. Charles

Director, Research and Technology
CyberBiota, Incorporated
Peter.charles@cyberbiota.com
http://www.cyberbiota.com

Tracy Allen

Agreed, you should measure the specific gravity yourself, using a hydrometer, or by weighting a measured volume. Develop your own chart of calibrations. It could be a fun project!

Take care with the conductivity measurement. The measurement in Applied Sensors is kind of quick and dirty. For the best conductivity measurement, you need to establish a very good reference cell. The good ones are usually a coaxial arrangement, so the fluid is held in an accurate volume within a tube, and the electrodes are either coaxial or two rings on the inner circumferance. Sometimes there are also guard electrodes. The materials for the electrodes also matter. They have to be materials that will not corrode in the aqueous environment. Or deposits of algae and crud. It can be a harsh environment, if other metals are present, because everything looks like electrodes of a battery separated by an electrolyte. And everybody knows about the crud that forms on the walls of an aquarium. You can't let the snails and the catfish into your conductance probe to clean it off! It is one thing do these contact measurements (conductivity, pH, ion-specific) in a beaker on a lab bench, another to move them to the "real world". Often the electrodes of conductivity sensors are made from hard carbon.

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Tracy Allen
www.emesystems.com

Franklin

www.reefkeeping.com/issues/2004-06/rhf/index.php has some good info but all I've seen seems to lead to the fact there is no direct relation between conductance and specific gravity.

cyberbiota

Franklin-

I'd have to disagree with you. The page you reference has some excellent data that seems to indicate that there IS a direct relationship between salinity, specific gravity and conductance. While the material they tested was a simple sodium chloride/water solution, plotting the conductance as a function of the specific gravity yielded a linear regression with an R2 of 0.9997 (almost a perfect fit) and an equation that could be expressed as "y = 1788.8x - 1783," where y=conductance and x=specific gravity. Similarly, conductance plotted as a function of salinity (in PSU) yielded a linear regression with an R2 of 0.9999, and an equation of "y = 1.3521x + 5.7018," where y=conductance, and x=salinity. Given the caveats mentioned by Dr. Allen (rigidly defined geometry of the electrodes, carefull choice of electrode material, using an ADC rather than RCTime & etc), I think that a fairly sensitive and accurate salinity meter could be constructed that calculates whatever units you are interested in based on the measured conductivity. As you will not be using a calibrated electrode (and hence cannot express the conductivity in milliSeimens/cm2), you would likely need to make your own measurement table. But the math to calculate a conductance to specific gravity or conductance to Salinity is well within the capabilities of the Stamp. Have fun with it!!!

peter

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Peter C. Charles

Director, Research and Technology
CyberBiota, Incorporated
Peter.charles@cyberbiota.com
http://www.cyberbiota.com

Post Edited (cyberbiota) : 2/25/2006 3:39:17 AM GMT