Expected salinity increase

[chimbo84]

I just started my first saltwater tank six days ago. It is a 14g Fluval Evo. I have a lot of experience in freshwater aquariums so I started a salt tank for a new challenge.

The problem I seem to have is my salinity variability every day. I have a monitoring system that I built for monitoring pH, temp, and EC in my freshwater tanks and it records all three parameters every 60 seconds. I am seeing a 0.5ppt increase in salinity every three hours and this is with a fully enclosed lid on the tank (see graph below). For example, this morning at 0930 it measured 31.28ppt and just now at 1230 it measured 31.79ppt (I know its a little low but I'm having trouble stabilizing it).

I do not have an ATO yet but I am wondering if this amount of increase seems normal or if there might be something wrong with my EC probe. I have calibrated it recently. The probe is currently measuring:
EC : 48730 uS/cm
TDS : 26318
Salinity: 31.82ppt
SG: 1.024

For reference I am seeing roughly 1/3gal of evaporation a day. Not much at all.

 

[Dom]

As you mentioned, you don't have ATO. And ATO will help stabilize all water chemistry and not just salinity.

You are talking about a 1/2 point swing in salinity. Is there any way to test to that level of accuracy on the hobby level?

 

[RobB'z Reef]

at those low volumes you can expect a correlation like that. I highly doubt your 14 gallon rated tank even holds anything close to 14 gallons. Minus inches from the top (it all counts), any internal structures (rock etc) you're likely holding 12 gallons? Either way .3 gallons a day equates to 2% by volume and your salinity swing isn't that far off. Besides, hobby grade measurements aren't that accurate anyhow. Get an ATO in place and match your evap rate and you'll see an improvement overall.

 

[chimbo84]

at those low volumes you can expect a correlation like that. I highly doubt your 14 gallon rated tank even holds anything close to 14 gallons. Minus inches from the top (it all counts), any internal structures (rock etc) you're likely holding 12 gallons? Either way .3 gallons a day equates to 2% by volume and your salinity swing isn't that far off. Besides, hobby grade measurements aren't that accurate anyhow. Get an ATO in place and match your evap rate and you'll see an improvement overall.

thanks, this makes sense. I am using a calibrated lab grade EC sensor from Atlas Scientific so I believe the resolution is fairly precise (it’s a K1.0 sensor rated to +-2% of EC). I just wasn’t sure if a 0.5ppt swing in three hours was typical but now that I did the math, it is within the margin of error for this sensor. I definitely plan on setting up an ATO soon but just wanted to be sure I wasn’t off base here.

 

[RobB'z Reef]

thanks, this makes sense. I am using a calibrated lab grade EC sensor from Atlas Scientific so I trust the resolution (it’s a K1.0 sensor rated to +-2% of EC). I just wasn’t sure if a 0.5ppt swing in three hours was typical. I definitely plan on setting up an ATO soon but just wanted to be sure I wasn’t off base here.

I'm a skeptic don't mind me lol. Just cuz they slap lab grade on it don't make it so. True precision like that costs serious $$. But either way you can see a correlation and that's what's most important. I'm running a much larger volume and I see a similar result when I don't keep up with evaporation. Like when I let my optical sensor get all funked up and the ato stops adding water lol