DIY Automated salt water mixing system

[Seth81]

So I had this idea for making salt water automatically. I already have a system to make RO/DI water, store it, top off as necessary, and also to manually mix SW water to use for water changes. But what I've been lacking is an automated way to replenish the salt water.
So my idea is this; Add a third tank for saturated saltwater (SSW), so basically dump two buckets of salt into the SSW tank and top off with RO/DI water. The salt should be able to fully dissolve to create water at a SG of about 1.185 (approx 7x that of normal SW) in the SSG tank. Alternatively I could put in 4 buckets of salt and top off with RO/DI which would result in un dissolved salt on the bottom of the tank, and saturated salt water above, and then the saturated water can be replenished one further time with RO/DI water before more salt is needed.
To fill the SW tank I could fill the SW tank in a 1:7 ratio of SSW water to RO/DI water, or perhaps use a conductivity probe to control the amount of SSW that goes into the SW tank. A second pump would be added in the SSW tank to simply circulate and mix the solution, and perhaps a heater to aid in dissolving the salts.... however water at 100 degree C can only contain 28% salt, compared to 26% salt at 0 degree C, so not too much of a gain.
I guess one question I would have to any chemist types, is would Calcium, magnesium salts dissolve as readily as Na salts. If not then I probably shouldn't use the 4 buckets of salt approach, could only stick with the 2 bucket approach.

Any thoughts or comments

[kien]

Fascinating :-)

[SeaHorse_Fanatic]

You're planning to add greater complexity and risk into the system. Imagine the disaster that would ensue if the mixing process doesn't work properly and you add saturated sw into your display. Salinity would spike and probably kill a lot of livestock. Just my thoughts.

[sphelps]

Install a conductivity probe in your tank/sump to control salinity, not your salt bin. Setup two tops offs, one from fresh water the other from saturated salt solution. Program controller to use fresh water if salinity => traget, and saltwater if < target. Set alarm if salinity drops below a certain point to remind you to refill salt. You can program a pump to remove water from your display on a timer, set top off control to allow extra top off time during this interval.

Something like this which is what I'm implementing, I've done a few similar ones before but you need a controller like profilux.

I never had issues relating to calcium, alkalinity, or other measurable elements as far as I could tell. Auto water changes were kept small and on a daily bases to prevent large swings and they system also had a reactor or dosing system due to high demand.

[Seth81]

Originally Posted by SeaHorse_Fanatic

You're planning to add greater complexity and risk into the system. Imagine the disaster that would ensue if the mixing process doesn't work proeperly and you add saturated sw into your display. Salinity would spike and probably kill a lot of livestock. Just my thoughts.

Thank you for your feedback, yes I agree there is added complexity in the system. The risk of adding saturated salt water to the sump directly is mitigated by not having a direct line to the SW mixing tank. Additionally a conductivity probe added to the SW mixing tank can be used to ensure the proper mixing has occoured. Also if I had the budget I could add another conductivity probe to the sump, although this isn't really necessary.

[Seth81]

Originally Posted by sphelps

Install a conductivity probe in your tank/sump to control salinity, not your salt bin. Setup two tops offs, one from fresh water the other from saturated salt solution. Program controller to use fresh water if salinity => traget, and saltwater if < target. Set alarm if salinity drops below a certain point to remind you to refill salt. You can program a pump to remove water from your display on a timer, set top off control to allow extra top off time during this interval.

Something like this which is what I'm implementing, I've done a few similar ones before but you need a controller like profilux.

I never had issues relating to calcium, alkalinity, or other measurable elements as far as I could tell. Auto water changes were kept small and on a daily bases to prevent large swings and they system also had a reactor or dosing system due to high demand.

I have a PLC based control system, so most of this isn't a problem, the only challenge I would have is finding economical conductivity/salinity probes that output either a 4-20mA signal or a 0 - 5 VDC signal. Any one have any suggestions? I looked on ebay and found some rosemount ones...but too much money.

Also isn't it better to do larger water changes? I personally do weekly 10% changes. But after doing some math it seems to me larger less frequent changes helps in reducing pollutants

I really like your idea of adding a high point drain to your RO/DI tank, I may do the same for some added reassurance that I don't flood anything if a relay decides to stick (even though I'm using SIL rated relays).

I personally prefer the third tank setup I'm using rather then two and mixing SSW in the sump, just for the added reasurance I guess... but realistically I am sure we all have a single point of failure in our reef setups that could cause a crash.

Also isn't larger less frequent water changes more effective in reducing pollutant levels? I personally do weekly 10% changes.

[sphelps]

Can you rig up a Neptune probe?
http://www.jlaquatics.com/product/np...ity+Probe.html

[kien]

Somewhat related to this discussion, Steve, have you ever noticed your conductivity probe randomly read an incorrect value? I have thought about doing something similar in the past but I have found my salinity probe to be quite unreliable. Maybe I just have a bad probe? My probe (for my Profilux) can read the conductivity fine, but every once in a while it will ready a very inaccurate value. When I notice this I sometimes shaking the probe which helps. Sometimes it corrects itself. I've tried putting my probe in low, medium and high flow area but it still does this from time to time. For example, my probe has been reading 1.021 for the past couple of days but I know for a fact that it is 1.026. Anyway, I really don't trust my conductivity probe at all.

[sphelps]

Originally Posted by kien

Somewhat related to this discussion, Steve, have you ever noticed your conductivity probe randomly read an incorrect value? I have thought about doing something similar in the past but I have found my salinity probe to be quite unreliable. Maybe I just have a bad probe? My probe (for my Profilux) can read the conductivity fine, but every once in a while it will ready a very inaccurate value. When I notice this I sometimes shaking the probe which helps. Sometimes it corrects itself. I've tried putting my probe in low, medium and high flow area but it still does this from time to time. For example, my probe has been reading 1.021 for the past couple of days but I know for a fact that it is 1.026. Anyway, I really don't trust my conductivity probe at all.

Mine seems pretty stable as every time I check it matches my refractometer but I measure salinity rather than specific gravity as I recall issues in the past relating to programming the controller to display readings in SG, something about a gravity offset calculation or something, can't remember. Anyway you should try recording the actual conductivity over time and see if that is changing before ruling anything out.

[kien]

Originally Posted by sphelps

Mine seems pretty stable as every time I check it matches my refractometer but I measure salinity rather than specific gravity as I recall issues in the past relating to programming the controller to display readings in SG, something about a gravity offset calculation or something, can't remember. Anyway you should try recording the actual conductivity over time and see if that is changing before ruling anything out.

hmm, good to know! I'll look into that. Thanks!