Tank specific Gravity

[mtbiker9000]

Now I know my sg should be roughly around 1.023 for reef tank maybe even 1.024, but here comes the question.....my hydrometer is calibrated typically for 75 so the hydrometer is reading the sg as if the water is 75 right? So should I assume the sg is roughly 1.023 or do I have this all wrong and sg goes up and down with temp but the hydrometer will reflect these changes. I'm confused because I want to make sure I have my specific gravity right but I am wondering if I need to add or minus a little bit from the actual hydrometer reading to get an actual because I run my tank at 79.

Heeeeeelp me-

[roadblock]

If your hydrometer is calibrated for 75 then that is the temp. at which the reading must be taken in order to get an accurate reading. as the temp of the water rises the specific gravity will drop because it requires a greater salinity. salinity is the measure of salt concentration in the water where as specific gravity is the measure of density of the water compared to distilled water, the latter having a specific gravity of one. as the temp of the water changes so does the density. therefore, a specific gravity 1.020 requires a salinity of 29.8 gm/liter, and if the water temp rises it becomes less dense therefore the specific gravity will drop, but the salinity will remain the same. so a specific gravity of 1.023 at 79 will have a greater salinity than a specific gravity of 1.023 at 75. so a lower reading at a higher temp will give a salinity at about where you want to be. if you were to lower your tank temp to 75 then take the reading you would have the proper salinity, then raise the temp back to 79, your s.g. would drop but you would be at the proper salinity. however the most accurate measurement is made with a refractometer

[Particleman]

Ok I actually have a refractometer and am trying to explain the differences between that and my floating hydrometer. It reads a lower SG then the refractor. I know the refractor is much more accurate but if that's the case im having extreme difficulty getting my salinity to decrease. I'm at 78.5 and it won't go below 1.028. I've pulled up to 5 gallons at one time and replaced it with RO/DI and still nothing. The tank is a 55 Gallon, I loose about 1 gallon every 2 days and I replace it with RO/DI water. Whe I do my water changes every week I pull out about 5 gallons and replace it was new saltwater that's SG is registering 1.022. Again, no effect. Any ideas on what I can do? The fish and inverts seem fine but running this high makes me nervous.

[wayne]

Refractometers aren't necessarily that accurate. If adding 10% more freshwater isn't showing on the refracto, I would assume
1. It's broken
2. You're not using it right

[roadblock]

I absolutely agree with wayne! adding five gallons of fresh water should in fact show a change in specific gravity! Did you calibrate your refractometer with distilled water? Did you test at the temperature that the refractometer is calibrated for? If you want an accurate read, don't rely on the automatic temp. compensating properties of these instruments either, it's not that accurate. Lot's of lab grade refracts and hydrometers are calibrated for 60 degrees. some made specifically for this hobby are calibrated to 75, others still to 77. my recomendation is to test at the temp that you are calibrated for. Obviously you cannot lower your tank temp to 60 but you can take a sample and chill it in your fridge then take the reading. if you are calibrated for 75 then take a sample and check it when it cools to 75. you should be able to get an accurate read in this way. Remember if you read the desired s.g. at 75 and your tank is running at 79 the salinity will be correct even though the s.g. will read lower at the higher temp. For example, if I dissolve a teaspoon of salt in an 8 oz. glass of water that is 75 degrees then heat the water to 80 degrees, there is still a teaspoon of salt in the water. Yet another method with a floating hydrometer is to float it in distilled water with the same temp as your tank, and then use that point as 1.00 then go from there but this is only accurate if you have a long hydrometer and I don't recommend doing it in this way. btw refracts read the speed at which light can travel through different densities of water. a floating hydrometer read how much water it displaces in different densities of water. in order for an object to float it must displace the same amout of water that the object weighs, saltwater weighs more per gallon than freshwater, and less water needs to be displaced in order for an object to float. Therefore a floating hydrometer floats higher in saltwater than in fresh.

[Particleman]

Thanks for the help, always nice to know the science behind how things work. My refactor was calibrated at 70 degrees and the tank temp is 78 so that sounds like the cuplrit. The thing reads non-saltwater just fine and also the water from the store, so it must be the tank temp.

[Decz]

A reef tank's sg should be a little higher that 1.023.....mine are all set around 1.026