DIY Automated salt water mixing system

[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!

[sphelps]

Also try re-calibrating the probe, if it's faulty it probably won't calibrate at lowest tolerance. Check to make sure the cable isn't picking up interference from other electrical lines as well, ie shut everything off except the controller and see if the conductivity is effected.

[Seth81]

Originally Posted by sphelps

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

Thats just the sensor part, I would also need a transmitter to go with it. I've seen a few guys on RC have built thier own transmitter to be used with any standard sensor, but IMO off the shelf solutions would probably work better then any transmitter I built myself.

Last time I tried designing my own DC amplifier for the field winding of an AC generator it looked like crap, worked but burnt out in about 30 seconds... Hence why i'm not an electronics engineer.

something like this would work, http://www.omega.com/ppt/pptsc.asp?ref=CDTX90 I would need the transmitter for $620 plus probe $320... just a wee bit iver the budget, there must be a more economical transmitter out there.

[sphelps]

Yeah you could pick up a full aquarium controller for that, might be an option although I get the idea behind building your own controller but at the same time kind of seems like reinventing the wheel a little.

[Zoaelite]

Have you taken in account the calcium bicarbonate precipitation that will occur with highly elevated levels of both calcium and HCO3-?

I have a feeling your pump would kick out on you in the span of a day with that much abiotic precipitation.

Quoted from Randy Holmes-Farley here:

When Ω = 1, the solution is exactly saturated. When Ω exceeds one, it is supersaturated, and when Ω is less than 1, the solution is undersaturated. The higher the supersaturation, the more likely precipitation of CaCO3 will take place.

In normal seawater, Ω ~ 3 for aragonite and Ω ~ 5 for calcite, though these values have been steadily dropping as carbon dioxide has been added to the atmosphere, reducing the seawater's pH. Aragonite and calcite are just different crystalline forms of calcium carbonate. Calcite is slightly more stable, and hence slightly less soluble, than aragonite (i.e., has a lower Ksp*). Organisms can precipitate both aragonite (pteropods and corals) and calcite (foraminifera and coccoliths), but most of the precipitation in reef aquaria is aragonite (although certain organisms such as abalone form both).

Reef aquaria often have higher alkalinity and calcium levels than seawater, and hence are more supersaturated than seawater. Alkalinity is a measure of the bicarbonate and carbonate in solution. At a fixed pH, if the alkalinity is doubled, then the carbonate will also be doubled. Since many aquarists keep reef aquaria at alkalinity levels higher than natural seawater levels (2.5 meq/L; 7 dKH), the supersaturation is often higher than in the ocean.

The biggest driver of supersaturation in a reef aquarium, however, might be pH. In aquaria with a high pH (such as many aquaria using limewater) the supersaturation is much higher than in seawater. At the same alkalinity, if you raise the pH, you convert some of the bicarbonate into carbonate:

(7) HCO3 - + OH- → CO3- - + H2O

At pH 8.2 and 25°C, only 15% of the total carbonate and bicarbonate is present as carbonate. At pH 7.8, that value drops to 7%. But as the pH is raised, that portion increases to 50% at pH 8.93 and to 75% at pH 9.4. Consequently, as the pH is raised at a fixed alkalinity, the concentration of carbonate rises, thereby increasing the supersaturation of calcium carbonate. Within the pH range of most reef tanks (up to about pH 9 or so), the amount of carbonate present is approximately linear with the pH because of the relationship seen in equation (7). So if the pH rises from 7.5 to 8.5, there is approximately a ten-fold increase in the carbonate concentration. From pH 8.0 to 8.5, the increase in carbonate is about threefold. Above pH 9, the carbonate concentration continues to rise, but more slowly, and it levels off above about pH 10 as there is very little bicarbonate left at pH 10+ to convert into carbonate.

Calcium Carbonate Precipitation: Calcium, Alkalinity, and pH

Combining the various factors described above, here are some combinations of calcium, alkalinity and pH that have equal supersaturation (that is, equal propensity to cause calcium carbonate precipitation):

Ω = 1 (dissolution of aragonite takes place at all lower values of these parameters)
pH = 7.7

pH = 8.2
Calcium = 410 ppm

Calcium = 340 ppm
Alkalinity = 2.5 meq/L

Alkalinity = 1.0 meq/L

Ω = 3 (typical of normal seawater)
pH = 8.2

pH = 8.0

pH = 8.4
Calcium = 410 ppm

Calcium = 410 ppm

Calcium = 260 ppm
Alkalinity = 2.5 meq/L

Alkalinity = 4.0 meq/L

Alkalinity = 2.5 meq/L

Ω = 6 (non-biological precipitation is more likely)
pH = 8.2

pH = 8.2
Calcium = 410 ppm

Calcium = 820 ppm
Alkalinity = 5.0 meq/L

Alkalinity = 2.5 meq/L



pH = 8.0

pH = 8.7
Calcium = 410 ppm

Calcium = 410 ppm
Alkalinity = 8.0 meq/L

Alkalinity = 2.5 meq/L



pH = 8.45
Calcium = 410 ppm
Alkalinity = 4.2 meq/L

How should we think about supersaturation? The higher it is, the more likely it is that calcium carbonate will precipitate. The reason for this is straightforward: if the "pressure" to precipitate calcium carbonate becomes too high, certain inhibiting processes (described below) will be overwhelmed, and precipitation will take place.

If Ω is not too high, some precipitation will take place before the inhibiting mechanisms take control of the crystals' surface and prevent further precipitation. This is the process that happens in normal seawater. If Ω is too high, a bigger precipitation event can take place before being halted. In the worst cases, this can lead to a snowstorm of calcium carbonate particulates throughout the tank. Such snowstorms can occur, for example, when too much limewater is added to the tank. In that case, the pH rises and converts much of the bicarbonate to carbonate. Ω is then driven to unstable levels, and a massive precipitation event takes place.

[sphelps]

For what it's worth the last system I used the saturated approach I outline before had a 29 gallon tank for the salt tank, with the overflow installed it probably only held just over 20 gallons of water. I use to fill it with a full box of reefers best salt. So 20 kgs of salt to 20 gallons of water, never had a precipitation issue.

[Seth81]

Originally Posted by Zoaelite

Have you taken in account the calcium bicarbonate precipitation that will occur with highly elevated levels of both calcium and HCO3-?

I have a feeling your pump would kick out on you in the span of a day with that much abiotic precipitation.

Quoted from Randy Holmes-Farley here:

Most centrifugal pumps would not even blink if some precipitate was sucked in, however I agree that sucking in precipitate or any other non-dissolved salts should be avoided so the intake of the pump should be elevated. Alternatively I could have the pump external to the tank rather then submerged with the intake above the salt/precipitate level.


The article you cited is very interesting, I think it is relating more to how Ph effects saturation levels. I would think in my proposed setup, that once the saturated saltwater hits the RO water in the SW tank some amount of time and mixing would be required before the water should be used in the display tank. Which I have no problem with... I could program the SW tank to be filled with a mix of RO and SSW say a day or two before the scheduled water change.

I did find a yokogawa transmitter on ebay for $65 that accepts most standard probes.