Specific Gravity and Temp.

[NoSump]

I recently read that at 77F, specific gravity is actually .002 higher than the hydrometer reading (a reading of 1.023 would actually be 1.025).

Is it astandard practice to set hydrometer reading specific gravity at .002 lower than the desired level?

Chuck

[Ninong]

I use a cheapy ($45) ATM (automatic temperature compensating) refractometer and read the salinity side of the scale.

Specific gravity is temperature dependent. Some hobby hydrometers are calibrated at 75 degrees Fahrenheit but most better hydrometers (floating type) are calibrated at 60 degrees Fahrenheit. Anyway, if you are using a hydrometer, you need to know to which temperature it is calibrated in order to make the appropriate correction in the reading. I think most hobbyists just use the cheapy swing-arm hydrometers that are calibrated to 75 degrees Fahrenheit and don't bother to adjust the reading at all. That's OK if their water temperature is only 76 or 77 degrees Fahrenheit, but it's not close enough if their water temperature is 84 or 85 degrees Fahrenheit.

The swing-arm hydrometers are notoriously untrustworthy. Dr. Rob Toonen once purchased two dozen identical brand new swing-arm hydrometers and gave them to his students at U.C. Davis to measure the specific gravity of the same aquarium. The readings were all over the place. That was six or seven years ago. I doubt that they have made any changes in them since then.

[Reefland]

I'm one of the cheapy hobbyist that have always used the plastic swing arm hydrometers and have always targeted 1.025 on the scale. One day, one day soon I'm going to bite the bullet and find a refractometer.

[Ninong]

Always remember that we are NOT interested in specific gravity. The only thing we are interested in is salinity. Specific gravity is just an imperfect way to approximate salinity. Or you could use a refractometer and never bother with specific gravity ever again.

The average salinity in the ocean near tropical reefs is 35 ppt (parts per thousand). In the Mediterranean it runs about 37 ppt and in the Red Sea it ranges from 39 to 42 ppt. It's a good idea to keep the salinity in a reef aquarium at least 34 ppt but 35 ppt would be better. You might even want to keep it at 36 ppt if you are keeping mostly fish and invertebrates from the Red Sea. For a variety of reasons, some people like to keep the salinity in their fish-only tanks a little lower than natural seawater values. This doesn't make it any easier for the fish to osmoregulate as some people think but a lot of folks swear by lower salinities in fish tanks. Doesn't seem "natural" to me but if that's the way you want to do it, go ahead. I still think fish-only tanks would be better at salinities of at least 33 ppt and reef tanks at salinities of at least 34 ppt. And those are just minimums.

[Ninong]

I just found a recent article that Dr. Randy Holmes-Farley wrote on this subject: http://reefkeeping.com/issues/2004-07/rhf/index.htm

Randy says that the cheapy swing-arm hydrometers do not need an adjustment to their specific gravity readings. Hmmm... I guess they aren't accurate enough in the first place to bother with correction factors based on temperature.

Randy does give a chart for correction factors for Tropic Marin's hydrometer, which he says is calibrated to 77 degrees Fahrenheit. He then goes on to mention that if you are using one of the top-of-the-line laboratory hydrometers that are calibrated to 60 degrees Fahrenheit, you can find your own temperature correction table online.

Here is the link that Randy gave in his article to a chart that gives correction factors to be used with a laboratory hydrometer calibrated to 15 degrees Celsius (60 F): http://www.drydenaqua.com/Graphs_tab...ficgravity.htm Note that the temperature line at the top of the chart doesn't line up over the correct columns. You have to move it to the left so that all of the temperature numbers are above the correct column. Only 0 C is above the correct column. You have to move the 5, 10, 15, 20, 25 and 30 C temps one column to the left.

P.S. -- You can use this calculator to convert Celsius to Fahrenheit (or vice versa): http://www.fishlinkcentral.com/calcu...emperature.htm ;;

[charlie]

This may open up a can of worms, but I have heard and read that refractometers are off by about.0015 (?). Any truth to that from what you guys have heard?

[Ninong]

This may open up a can of worms, but I have heard and read that refractometers are off by about.0015 (?). Any truth to that from what you guys have heard?

I don't see how it is even possible for a refractometer to be off???

You are the one who is responsible for checking to see if it is calibrated or not. You can do that with distilled water or you can do it with a standard solution. I just use distilled water. If the color change line crosses the scale at 1.000 SG using distilled water, it's calibrated. If not, turn the adjustment screw until the color change divide lines up correctly.

You have only one real part in a refractometer and that's the prism. All you are doing is measuring the defraction of light through water. The defraction of light changes depending on the density of the water. So there are really no moving parts in a refractometer that can go bad.

P.S. -- Please be aware of the fact that most cheapy refractometers available in the hobby (such as the one I have) are NOT seawater refractometers. They are sodium chloride refractometers. True seawater refractometers are much more expensive. For a sodium chloride refractometer, a reading of 36 ppt = a true 35 ppt salinity. Maybe that's what you're talking about? Also, make sure your refractometer is an ATM (automatic temperature compensating) model. Mine is and it only cost me $45. It works like a charm.

[charlie]

I go tmine from Reeffanatic and it doesn't say anywhere it is ATM. I can't even remember how much it cost. I think it was about the same as yours. How would I find out?

[Ninong]

I go tmine from Reeffanatic and it doesn't say anywhere it is ATM. I can't even remember how much it cost. I think it was about the same as yours. How would I find out?

This is the only refractometer that I see on their website: http://www.reeffanatic.com/products/...actometers.asp

[NoSump]

Ninong:

Thanks - I'm also using one of the swing arm hydrometers. It sounds like the good news is that I do not need to adjust for minor temperature deviations from 75F, the bad news is that I am getting inacurate readings.

It sounds like there is yet another gadget in my future.

Chuck

[charlie]

Thanx Ninong, that be the one!

[Ninong]

Ninong:

Thanks - I'm also using one of the swing arm hydrometers. It sounds like the good news is that I do not need to adjust for minor temperature deviations from 75F, the bad news is that I am getting inacurate readings.

It sounds like there is yet another gadget in my future.

Chuck

Did you see this recent thread on RC: http://reefcentral.com/forums/showth...65#post4437165

[Ninong]

I'm one of the cheapy hobbyist that have always used the plastic swing arm hydrometers and have always targeted 1.025 on the scale. One day, one day soon I'm going to bite the bullet and find a refractometer.

Speaking of which, did you see this recent thread on RC? http://reefcentral.com/forums/showth...65#post4437165

There are hundreds more just like that one. :eek3:

[NoSump]

Speaking of which, did you see this recent thread on RC? http://reefcentral.com/forums/showth...65#post4437165

There are hundreds more just like that one. :eek3:

Ninong:

That's not good! And I was just beginning to think that salinity was the easiest tank variable to measure - no drops in test tubes, or colors to match.

Chuck