Hydrometer "adjustment" question

[SouthBayPhoto]

Alllllrighty. I'm now thoroughly confused. (not that it's hard to do that to me, but I'm giving me the benefit of the doubt on this one. LOL)

I was reading on a forum what seems to be the best salinity rate at which to keep the aquarium. It appears that 35 ppt is best (also equal to NSW, from my understanding of the guide that I read).

It had a secondary link to read more on the subject, which I did. It was mentioning how the SG goes down as the temperature goes up. It also stated that each hydrometer should be "calibrated" at a certain temperature. Okay, cool...I'm with them, so far.

I have a Red Sea hydrometer. It states it's calibrated at 77*F. When I test my water, it's at 79*F (according to the digital thermometer on the hydrometer itself). My SG (unfortunately that's all it shows. No ppt scale) currently shows 1.021 @ 79*F.

Now, going with the understanding that the SG drops when the temp rises, that means I'm running below 1.021 SG, correct (because it's calibrated at 77*F)? If so, how do I calculate how much lower my SG really is compared to the reading on the hydrometer?

According to Red Sea's little guide (listed below thanks to Drs. F & S), I'm trying to figure out what reading I need on the hydrometer at 79*F (tank temp) to come up with 35 ppt. I'm doing a water change next weekend and would like to try and correct my SG to get close to the equivalent of 35 ppt.

Now, is everyone as confused as me? I hope someone can help me with this. I wish I could even find/design a table with these parameters so I could figure out what my ppt/SG is at different temperatures. But that's probably asking too much. :-(

Here is the link for which I got the ppt info: Link to ppt-SG Info

Here is the chart that came with the hydrometer, so I'm guessing it's "correct" for my hydrometer (because it seems as though a Deep Six hydrometer is different as it shows that it's at 35 ppt at 1.026 and on mine, the chart below, shows it at about 1.0245)

(Example care of Drs. F&S website)
Conversion of S.G. to Salinity at a constant temperature of 25°C/77°F

S.G. at 77°F - Salinity

1.018 - 26.9
1.019 - 27.8
1.020 - 28.7
1.021 - 30.6
1.022 - 31.5
1.023 - 33.4
1.024 - 34.3
1.025 - 36.2
1.026 - 37.1

Effect of temperature on S.G. for a constant Salinity of 30.6 ppt

Water Temperature (*F) - WS.G. @ Salinity of 30.6 ppt

44.6 - 1.025
55.4 - 1.024
64.4 - 1.023
71.6 - 1.022
77.0 - 1.021
82.4 - 1.020
87.8 - 1.019
93.2 - 1.018
98.6 - 1.017
(end example)

I see that at 79*F, I should be "correcting" to somewhere between 1.021 (standard reading) and 1.020 (if I was at 82.4*F). So, it almost seems like I should subtract about 0.005 from my actual reading on the hydrometer, correct? It also appears they want to use a Salinity of 30.6 ppt...isn't that WAY too low? Shouldn't it be 35 ppt (NSW) as the article above states?

Helllllpppppp!!!! :eek3:

[art_of_war]

Ya know...

That's a lot of info. to really study up on. As a matter of fact, if you're using a swing arm hydrometer, you're NEVER going to get an accurate reading; doesn't matter what temperature you're testing at. I know this b/c I've had 2 different hydrometers which I tested the same water on and both gave me conflicting readings EVERY time. I eventually broke down and went out and bought a refractometer.

A refractometer is probably one of the few things that will give you the most accurate reading. Best recommendation: get a refractometer.

[SouthBayPhoto]

I can understand that, but for now, I can't afford something like that. I'm still workin' on buying my lights and a friggin RO/DI machine! That doesn't even count the live sand and live rock I need as well.

But, I can understand what you mean. I don't exactly like the swing type either because I have to stick a darn pen in there each time and knock the bubbles off the arm or it will give a false reading.

But this is about all I can go with right now, so I figured that if I can get some help on it now, then I'll just stick with it until I can buy something better/more accurate.

[Poseidon]

Honestly I believe the variance due to temperture will be a waste of time and mental effort. You could nail down a perfect formula eventually, but still be WAY OFF due to the inaccuracy of the device.

Move the refractometer up on the list to #1, it is a wise investment. They can be found for around $40 now, which is a very minor cost for a 1 time purchase.

[TimH07]

They can be found for around $40 now, which is a very minor cost for a 1 time purchase.

thats 43 dollars not 40, please calibrate your priceometer

[Poseidon]

thats 43 dollars not 40, please calibrate your priceometer

Doh, what type of calibration fluid do I need for that? I know... You transfer me $1,000,000.00 and I'm pretty sure I can recalibrate that price-o-meter.

[TimH07]

[SouthBayPhoto]

LOL...

Thanks guys! I just happened to see one listed one time around $88 or so, so I figured that was the "norm" for pricing. I didn't really research into it. Plus, I didn't even know what the heck a refractometer was! LOL However, that one looks pretty cool. Especially since it'll handle the temperature correction and tell you in either SG or ppt. Nice.

Looks like this and an RO/DI unit have made it to the top of my "gimme, gimme, gotta have it!" list. LOL

[Ninong]

If your hydrometer is calibrated at 77 degrees Fahrenheit and your water temperature is 79 degrees Fahrenheit, that's close enough for government work. I wouldn't worry about the slight adjustment.

However, your specific gravity of 1.021 is really low. You should raise it to 1.025 - 1.026. You can do this gradually by using freshly mixed saltwater to replace evaporation until your specific gravity reaches your desired goal.

[Ninong]

Here is the link for which I got the ppt info: Link to ppt-SG Info

That's a good article. That's the one you want to listen to.

It also appears they want to use a Salinity of 30.6 ppt...isn't that WAY too low? Shouldn't it be 35 ppt (NSW) as the article above states?

Yes, it should be 35 ppt, not 30.6 ppt. You will find a lot of crap on the Internet and you have to be able to recognize it for what it is. Some people are of the opinion that fish do better at below normal salinity. There is no scientific basis for that opinion. None whatsoever. Many vendors -- both wholesale and retail -- keep their fish at lower salinity to save on the cost of salt mix. While this is not particularly harmful for fish, they would do better at natural salinity. Lowered salinities are definitely not advisable for most inverts, especially sea stars because of their water vascular system.

Surface water salinity can vary in nature depending on location, depth of the water, season of the year, etc., but that doesn't mean it's a good idea to permanently house animals at unnatural salinity levels unless you're trying to replicate the Gulf of Thailand during the monsoon season.

[SouthBayPhoto]

Thanks for the info, Ninong. I appreciate it.

I must have had a stupid bubble or two on my swing hydrometer last time because normally I keep my SG at 1.023/1.024 or so, but that was before I found out that I should be at 35 ppt/1.025 SG. And when I mixed my last batch it showed at 1.024, but I must not have taken enough care to verify that there were no bubbles on my hydrometer. Uggggh!

I'm due for a water change next weekend. I'll just have to bring up the SG of the replacement water a little bit and then keep monitoring my water and do my replacement (evaporation) water with mixed saltwater to get my SG up around 1.025/1.026.

What's even funnier (well, not really) is that my hydrometer shows the "good zone" as 1.020 through 1.023. Everything else is showing red as in it's bad for the fish. That's not too darn cool for someone just starting out. GRRRRR!

Now I know why most people are using a refractometer!!!! Looks like that's definitely on my list of must haves!

Thanks again for the info!