Specific Gravity in the sump

[justahobby]

Here's the deal, I test my salinity with a thermo/ specific gravity floater. It reads 1.026 in the sump while my display maintains 1.022. So now I am curious if any body else has noticed this in their system.

[Mr Cob]

I know temperature plays a part in SG, but perhaps gravity too?

I would pull water from the tank and measure it or go by the readings in your main display. I would also pull the water from the tank but don't pull it from the surface under a metal halide.

We'll see what some of the pros say when they chime in...

[Mr Cob]

I have never noticed this in my system, but I test my SG during water changes. I shut all return pumps off and I pull a bucket from the main display and test it. I then test my incoming water and adjust it accordingly.

[fishypets]

Never even tested this but doesn't make sense to me.

[Gilbertg78228]

you would think they would both be the same?

[Mr Cob]

Read...

Q: Salinity & Specific Gravity – Are they the same?

A: Salinity and Specific Gravity are terms commonly used to measure the salt level in an aquarium. Although they are related, these two terms are quite different and should be understood for their own importance.
1. Salinity: is defined as the amount of dissolved salts contained in seawater.
2. Specific Gravity : is defined as the ratio of the density of a liquid (at a specific temperature) compared to the density of pure water (at a specific temperature)

a. It is imperative that temperature corrections are made to obtain a TRUE specific gravity reading.

From: (Michigan Reefers)
http://www.michiganreefers.com/forums/archive/index.php?t-34978.html

Useful information about SG. Not sure this helps but it does support the theory that temperature plays a part in SG.

[Texreefer]

If he is putting in freshly mixed saltwater into the sump when doing water changes and he has an area where there is VERY little flow.. The higher salinity will want to sink and stay put while the water in the display is getting mixed very well,,, this COULD cause the readings to be different

[Mr Cob]

If he is putting in freshly mixed saltwater into the sump when doing water changes and he has an area where there is VERY little flow.. The higher salinity will want to sink and stay put while the water in the display is getting mixed very well,,, this COULD cause the readings to be different


Good point, also if there's an auto top-off in the return area and he's testing from the intake part of the sump (higher SG) and testing the main display near the return (probably lower SG because of the top off water).

[justahobby]

When I do water changes, the new water goes to the pump return chamber and mixes through the tank. I tested the refugium section and water drain compartment in my sump. When I test the display I pull water out. I hadn't considered the temperature variable. I'll check to see if theres a diff.

[ErikH]

If you're using the same floating hydrometer it should read the same? I had trouble reading mine since the writing is so small and it's floating around in a tank. Take some sump water and display water to test at a store on a refracto. See if there is a difference there. How large is your return pump?

[rebelprettyboy]

wierd