How to maintain constant salinity in a mix tank with decreasing level over time?

[tonyespinoza]

I would like to setup a continuous water changer (LM3) but have run into an issue. My mix tank does increase in salinity over time due to evaporation and the amount of make-up water needed is a moving target given that water is getting pulled for daily water changes...

Is there some way to rig up a constant water level for the osmolator to read even if the mix tank is changing level over time?

(I hope there's an obvious answer that I'm missing!)

[Randy Holmes-Farley]

You already the the LM3? Other devices allow independent control of the two flow rates (like a Reef Filler).

[tonyespinoza]

Found a little info on the Reef Filler on the Premium Aquatics site... Not sure I follow how independently controlling two flow rates solves my problem.

[tonyespinoza]

The point is that the amount of evaporation will vary since the total volume is decreasing as water changes are being made on a continuous basis from this mix tank.

[sjames]

You can set litermeter III pumps to different rates (if that's what was being referred to by LM3) I use a litermeter 3 myself for automatic water changes.

For your other problems, I dont really understand what it is your doing based on your description.

[tonyespinoza]

let me try to explain the situation again:

1) Mix tank
2) LM3 w/ water changer module doing continuous water changes

My concern is that over time the mix tank is going to evaporate water and the salinity will increase. If it were a typical mix tank used once a week or once a month, I could just slap a Tunze osmolator on it and keep the salinity constant. However, the water level will be diminishing on a daily basis since the LM3 will be pulling water from it to do the continuous water changes.

How do you keep the salinity in the mix tank constant in this scenario?

[sjames]

well, you could use a conductivity meter to make adjustments, but I don't know how dependable that would be...

I assume sinse your even concerned about this your mix tank must be very large.

[tonyespinoza]

yeah, i'm thinking 100g for the mix tank.

[sjames]

with a LM3 you could do a 100gallon water change in about 6-7 days, if you have a lid on your mix tank I dont think your going to get appreciable evaporation.

[jdieck]

Originally posted by tonyespinoza
let me try to explain the situation again:

1) Mix tank
2) LM3 w/ water changer module doing continuous water changes

My concern is that over time the mix tank is going to evaporate water and the salinity will increase. If it were a typical mix tank used once a week or once a month, I could just slap a Tunze osmolator on it and keep the salinity constant. However, the water level will be diminishing on a daily basis since the LM3 will be pulling water from it to do the continuous water changes.

How do you keep the salinity in the mix tank constant in this scenario?

Usually if you keep your mix tank well covered it will not evaporate enough to be a problem, at least not in a couple of weeks so you can use a mix tank size that you refill once a week rather than using one that lasts a month.

Alternatively if your tank is large and want to use it that way, then I would adjust the salinity once a week just by manually adding some fresh water to it. That will not take more than five minutes to do.

[Randy Holmes-Farley]

How do you keep the salinity in the mix tank constant in this scenario?

I guess I'm totally lost. I make up 88 gallons at a time and put a lid on it and the salinity never changes. Then when it is gone I make up another 88 gallons.

[tonyespinoza]

Hey Randy - If you run a continuous water change with the LM3 the water level will be decreasing across ~100 small doses a day. I don't think you can seal the container since the peristaltic pump probably needs some back pressure. But if the opening is super small, i guess evap would be minimal. And to jdieck's point a weekly make-up may be sufficient. I would rather not have to chase this, so was hoping for something as automated as the Tunze Osmolator to take care of it for me, but it seems there is no such easy answer.

[DrBegalke]

As long as it is covered, doesn't have to be air tight, the salinity shouldn't change appreciably due to the small amount of evaporation that occurs. At least when I mix up 25g and let it sit for a month or more, I do not measure a change.

If you setup your system as above and you are measuring a change in the salinity... what you could do is use a container that has a one-way valve for an opening... i.e. it only lets air in (when the LM3 is pumping due to the neg pressure)... but otherwise there is no where for the air (and hence most of the humidity from evap.) to escape. I doubt that will be necessary, but its an idea if you do have the problem you describe...

[Randy Holmes-Farley]

Hey Randy - If you run a continuous water change with the LM3 the water level will be decreasing across ~100 small doses a day. I don't think you can seal the container since the peristaltic pump probably needs some back pressure.

I use trash cans with a lid. Not air tight, but there is certainly no significant evaporation.

[tonyespinoza]

gotcha. i'm probably worried about nothing!

[DrBegalke]

I use a Brute Rubbermaid container.

[virginiadiver69]

FWIW, I purposely let my display tank/sump evaporate with out make up water until I noticed a change from 1.025 to 1.026. It took a little over 7 gals (and 3 days) of water evap to get to this point in a 125 gal tank with lights and constant air flow to FORCE evap. I did this to test how long I could go without top off in case of and emergency.
My point is that I learned even with forced evaporation you have to lose a good bit of water to move your S.G. even a little bit.

[madadi]

don't mean to go off topic too much here but, does having auto water changes interfere with an ATO unit? if the pump brings in fresh saltwater and takes out exactly the same amount of old tank water it should not interfere with the ATO, right? my concern is, are the pumps so accurate that they take out and bring in the same volume of water? and if not, how long after do you notice a change if you don't correct it.

[Randy Holmes-Farley]

don't mean to go off topic too much here but, does having auto water changes interfere with an ATO unit?

Not if they are like mine, with auto in/out at the same time. The flow on the two directions is independently controllable to match them. They are also generally quite slow pumps.