Starting a project to automate the mixing of salt water...

Hi all:
I'm beginning a project to automate the mixing of salt water. Variables to be controller are the salinity, ph, and temperature. DI water supply lines
are already available and the salt I will be using a road salt which often has large clumps in it.
At this point I'm looking for general ideas/thoughts as well as pointers to specific sensors and hardware.
I see the metering of the salt as the biggest challenge. I have been considering using an auger to pull salt out of a hopper. I'm not quire sure exactly where to start here and would love to hear any thoughts on how to best accomplish the salt metering.
The other topic I would like to discuss in this thread are sensors and ph control options. Pointers to electronic sensors for measuring specific gravity (or others to detect salinity) and ph would be great as well as systems to help raise and lower the ph.
I do not know much about either types of sensor so information on these would be helpful. I have heard that buffering chemicals must be used with electronic ph meters on a regular basis- I was hoping to find a sensor that could simply be calibrated once a year.
Thanks for any info.
David
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David wrote:

When I was involved in a batched mix-plant project many years ago (for Hydrolastic fluid production) we had to come up with means to measure the amount of a powder we were introducing. If you can keep the salt dry you might consider the use of a vibratory feed system dropping the salt over a rotating paddle (rate of rotation denotes rate of salt flow).

This is useful information about how the ph sensors work:
<http://www.sensorland.com/HowPage037.html
and
<http://accessories.picotech.com/ph_sensor.html
will give you one source of supply. A quick google on "ph sensors" will result in some suppliers of the probes and suitable equipment for your system.
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David wrote:

[%X]
Then that would work with an auger drive if you could keep the solution a bit sloppy so that it would extrude easily through the outlet nozzle. If you consider that a bag of salt (presumably the 1/2 cwt size) would last a few months the production rate of your ultimate solution is not very high. Biggish tube wet-hopper with a small auger might do the job there. A lot of that can be done with plastic parts. Look at the Igus web-site <http://www.igus.com/ for bearings that may suit.
[%X]

Are you actually trying to control a sea-water aquarium environment? If so, just say so.
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Paul E. Bennett wrote:
...

And can you tell us how you intend to control pH? Is that to be with other additives?
Jerry
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I haven't chosen a method or done much research on this yet. I have seen pH controllers and dosing systems and may end up using or canibolizing one of those.
David

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David wrote:

My question is simpler that that. If you measure a pH that you don't like, what will you do about it? Add sodium hydroxide or hydrochloric acid as needed? Sodium chloride, being a salt of a strong acid and a strong base, is pretty neutral when dissolved. That is, it disassociates pretty completely. Unwanted pH is likely caused by impurities in the water or the salt.
Jerry
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I was thinking of even mixing the salt up in the hopper to avoid the auger all together. That way only a pump would be needed to transfer the mixture from the saturated tank to the tank containing the final product.
I do see advantages with the auger concept but I am not sure where to start with coupling the motor, auger, and tank together. Perhaps I can find someone selling augers that can lend a hand. If you have advice here I'd appreciate hearing it... I'm hesisitant to ask a question about every roadblock I run into on here, perhaps because I'm not a controls engineer and I feel like I'm asking one basic question after another. I'm encouraged to see that the group is interested in other aspects of the project as well.

No, I'm trying to control salt-water that is used in environmental testing chambers for salt-fog tests. The current mixing procedure isn't very efficient and the tanks have occasionally run dry, requiring tests to be restarted (I have a BSEE and have been working as a test engineer for a few years; most of the tasks at work that I am most interested in involve DAQ or automation).
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David wrote:

Most of us here solve problems using engineering and other skills. To sensibly help you we need to know what you propose and why.
How much saline solution do you need per hour, and how long is a test? Scale matters.
How do you adjust pH? Can you do that before the salt is added?
What materials are suitable for plumbing etc? (There are heaters sheathed in stainless steel.)
Can you use a plastic tank with a magnetic stirrer?
What kind of pump is in use now? Does it suffer from corrosion? Fatigue?
Jerry
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I'm looking at a gallons per week scale; perhaps 10 gallons per week max. Tests can vary from hours to months but the salt water should always be prepared.

pH is to be adjusted after the salt is added. Buffers are currently used, I'm not familiar with the actual chemicals yet. I'd prefer a chemicalless solution but I am still looking into this. I'd like to learn a bit more about pH before trying to talk about it much.

The only restrictions that come to mind are materials that may corrode should not be used. I thought stainless steel would eventually corrode; how long might the highest grade stainless steel resist corrosion for in a saturated salt solution?

Yes, definitely. In the past a water pump has been used to circuilate the water.

I'm not sure what kind it is- not a submersible. Corrosion has been an issue in the past, I plan to look for different pumps. Fatigue has not been an issue but the pump has only been run when salt was mixed; air has been run into the resevoir following that point.
I just began discussing this project on Friday afternoon; I was looking for suggestions on metering the salt here since I have viewed that as the biggest challenge in the project. I'm happy to see you're interested in helping me with other aspects but I don't want to waste your time and should get some work done on my end first (although the magnetic stirrer suggestion will definitely be something I wouldn't haved looked into in the first place).
David

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David wrote:

...

A 200-gallon tank doesn't seem unreasonable off hand (just consider its weight when full). Hand mixing two or three times a year may be the simplest solution (pun noted).

Is seems that different batches of salt produce different acidities. Maybe recrystallized halite would be consistent enough not to need adjustment.

I don't know, but the information is available. "The highest grade" isn't the point. Some compositions better resist salt, others, acid, etc.

Have you considered a peristalsis pump using plastic tubing? The solution comes into contact only with the tube itself. There are no valves. Such pumps control intravenous drips, blood without causing it to clot, and various corrosive reagents. there are examples at http://www.eccentricpumps.com/?gclid=CJ_50qSMi5ACFSgSQQod4lJaqw and http://www.watson-marlow.com/ The tubing fatigues eventually, but it won't be an issue unless the tests are of very long duration.

A magnetic stirrer might eventually wear the tank bottom. Replacing hose now and then might be preferable.
Jerry
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Jerry Avins wrote:

...

If you plan to build a tank, look into Monel 400. Better yet, ask someone who knows. :-)
Jerry
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I presume you are aiming at something like this range of tests:-
"The neutral salt spray (fog) test (ASTM B 117): is perhaps the most commonly used salt spray test in existence for testing inorganic and organic coatings, in particular where such tests are used for material or product specifications. The duration of the test can range from 8 to over 3000 hours, depending on the product. A 5% sodium chloride solution containing not more than 200 parts per million (ppm) total solids and with a pH range of 6.5 to 7.2 is used. The temperature of the salt spray chamber is controlled to maintain 35 + 1.1 or ? 1.7 C (95 + 2 or -3 F) within the exposure zone of the closed chamber.
The acetic acid salt spray (fog) test (ASTM G 85, Annex A1): is also used for testing inorganic and organic coatings, but is particularly applicable to the study or testing of decorative chromium plating and cadmium plating on steel or zinc die castings, as well as for the evaluation of product quality. This test can be as short as 16 hours, although it normally ranges from 144 to 250 hours or more. Similar to the neutral salt spray test, a 5% sodium chloride solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid. The temperature of the salt spray chamber is controlled to the same temperature range as for neutral salt spray.
The copper accelerated acetic acid salt spray (fog), or CASS test (ASTM B 368): is primarily used for the rapid testing of chromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum. The duration of this test ranges from 6 to 720 hours. A 5% sodium chloride solution is used, with one gram of copper (II) chloride dihydrate added to each 3.8 liters of salt solution. The solution is then adjusted to a pH range of 3.1 to 3.3 by adding acetic acid. The temperature of the salt spray chamber is controlled to the same temperature range as for neutral salt spray and for acetic acid salt spray."
David wrote:

[%X]
In which case, try this for an idea. I would look at having a largish (50 to 100 gallon) strong salt water vessel for your salt solution and use a diaphragm pump to continuously circulate the solution from the bottom to the top, feeding it in so that it is a little turbulent.
A second (peristaltic) pump could then provide the take-off of this solution to feed into a smaller (2 gal) tank in which you add more water to obtain the required strength of weaker solution. Control this pump from the ph and salinity sensing of the small tank solution.
You only need then to work on your spray heads and re-circulation system.
You will probably find that many of us here have had to deal with environmental testing of some of our control equipment in the past, so there is likely to be quite a bit of experience in this sort of thing, one way or another.
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David wrote:

Help me out here. If you are dissolving salt of a known composition in reasonably pure water, then the concentration can be determined by density and temperature alone. Density can be measured intermittently by weighing a known volume or continuously by measuring reaction force, particularly at known velocity in a curved tube. Why do you need pH?
Can your brine be mixed in batches? if so, fill a vat with the right amounts of water and salt, and stir until it's dissolved. Two vats, alternately discharging and mixing, can be used if continuous flow is needed.
Jerry
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The salt water is to be used for environmental testing and both pH and salinity must be controlled.

The first procedure you mentioned describes the current manual method that I desire to automate. When mixed manually it is necessary to add more salt or water in order to bring the salinity into tolerance.
Another complication is that the temperature of the salt water must be above room temperature when mixed; I do not expect normal heater elements to resist corrosion very well... This is another issue though.

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David wrote:
...

So you expect to dissolve both salt and a pH control reagent? I can't see how else to control salinity and pH separately.

How do you measure salinity now? It seems to me that a hydrometer might do.

There are some pretty resistant conformal coatings. The immersed heater surface temperature probably won't be to high for some of them. What will you make your vat out of?
Jerry
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David wrote:

Salinity can be easily determined from conductivity. Standard conductivity cells are available. An auger should work, but since salt is hygroscopic forget trying to keep it dry. Consider going the opposite direction and use water jets to create a brine slurry.
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There are a few ways to do this.
You might consider the following before you launch off: 1. How much salt water is needed? A few gallons per minute, a millon gallons per minute? 2. How precisely do you need to keep it controlled? If it has to be to the third decimal place, I'm going to employ different techniques than if the requirement is to within 10 or 15 percent? 3. Is premixing a stock solution permitted? 4. Will this be a one step introduction or a rough/fine introduction? 5. How do you intend to mix it to make sure the salt dissolves? 6. What concentrations are you trying for? 7. What is the condition of the salt? 8. Batch or continuous operation?
A pH probe won't do any good, as salt will not alter pH. You could use conductivity or density instead.
An auger will do well in general. You can use either auger rpm, or amperage, or possibly an optical sensor that looks at the falling salt, or another metering system to weigh or infer the amout of salt coming in. Speed this up, or slow it down based upon the final concentration.
As far a chunks, you can either pulverize them, or just have a larege mixing tank with some very good mixing to even out their effects.
One odd method to consider is to dump the salt into a hopper and then just spray water on the top of the pile, letting it leach the salt away. You may find that the concentration at the bottom is fairly uniform after the water travels through a few feet of salt. Use that as a strong stock solution, and meter it in with a metering pump or some control valve. Measure its conductivity and that of final solution to determine the valve opening and leaching spray flow rate.
Michael
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