Trying to get my salinity right, this is hard!

[Scrooge2]

Randy,

First of all, excellent article. It really cleared some things up for me. Of course, in the process it also brought up more questions and generally sent my brain spinning, but that is another topic

I am getting a good general understanding, but want to check my math with my example to see if I have it right. I am trying to get 35 ppt salinity in my tank at 82 degrees. I referred to a table in Dr. Ron's article and notice that at 82 degrees and 35 ppt (actually 34.9), salinity is 1.024. My hydrometer is calibrated for 60 degrees. Using your example I divided 1.024 (I assume this is the SG of NSW at 82) by 1.0259 (sg of seawater at 60 degrees from your article) to get 0.998. So, 1.024 (my target SG for 35 salinity) times 0.998 equals 1.022. According to this, my hydrometer reading should be 1.022 for me to get where I want to be.

Is this correct? If it is, I've been way over for quite awhile and bet many others have as well!

 

[randy holmes-farley]

Scrooge:


<< . I referred to a table in Dr. Ron's article and notice that at 82 degrees and 35 ppt (actually 34.9), salinity is 1.024. >>

Just to be clear, the 1.024 is a hydrometer reading, not a salinity or even a specific gravity.


<< Using your example I divided 1.024 (I assume this is the SG of NSW at 82) by 1.0259 (sg of seawater at 60 degrees from your article) to get 0.998. So, 1.024 (my target SG for 35 salinity) times 0.998 equals 1.022. According to this, my hydrometer reading should be 1.022 for me to get where I want to be. >>

I'm confused about what you want. If you want the tank to be at S=35, then all you have to do is take the hydrometer reading from Ron's table (1.024; I believe that this table is for hydrometers calibrated at 60 deg F, but cannot guarantee its accuracy) and match that to your hydrometer (that is calibrated to 60 deg F; if it were calibrated at a different temperature, you'd have to do something different).

Am I misunderstanding something?

FWIW, sorry for the slow reply. I've been on vacation the past week.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[Scrooged]

I apoligize, scrooge. I should never be trusted with this much power.

I accidently wrote my answer over the top of your last post and wiped it out. You can either repost it here as an edit, or people can hope that I highlighted the important parts in my answer below.

[ February 01, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[randy holmes-farley]

<< Of course, this is assuming his hydrometer was calibrated for 60 degrees. I can't find where that is addressed in the article. >>

Yes, I agree that that article is confusing because it doesn't say what calibration temperature is being used. I believe it is 60 deg F from my discussions with Ron a while back.


<< So, if I have a hydrometer calibrated for 60 degrees could I simply use the above chart and know what reading to shoot for from that? >>

Yes.

<< Of course, I now have one calibrated for 75 degrees, so I'm totally lost again. >>

The correction needed for a 75 deg F hydrometer to reef tank temps is pretty small. Here's a thread at my forum where someone had a similar situation.

http://www.reefcentral.net/vbulletin/showthread.php?s=e8f73a71cf0992e5cae e28be07172e48&threadid=57134

The very easy way is to take your hydrometer in tank water at tank temp, and note the reading. Let it (the water and hydrometer) cool to 75 deg F, and get a new reading. The difference is the correction that you can always use for that actual tank temp.

If it's still unclear, I'm happy to keep at it!

<< Maybe it would be in the best interest of all concerned if the experts referred to salinity directly and we all knew how to convert that to specific gravity for our needs. >>

ABSOLUTELY. That's why I wrote the article: to get people to stop referring to 1.022 at 81 deg as if it were a salinity that everyone understood.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[Scrooge2]

LOL, you edited out my post!

At least you quoted the only parts that probably mattered.

Okay, I'm still trying to get this into the absolute simplest and most practical terms. For example, if someone asks me what there SG should be, if they have a thermometer calibrated for 60 degrees I can say shoot for 35 ppt and refer to Dr. Ron's chart to find what hydrometer reading you need.

I'm still not sure about a 75 degree calibration though. Using your method of testing at the water temp and then letting the water get to 75 and testing to find the correction stops just short because we are still only discussing SG at that point. How do I make the final and very important step to know my salinity?

I hope I'm not annoying with these questions. I'm trying to get this into the simplest and most practical terms possible.

 

[randy holmes-farley]

scrooge2:

OK, let's sort through this.


<< For example, if someone asks me what there SG should be, if they have a thermometer calibrated for 60 degrees I can say shoot for 35 ppt and refer to Dr. Ron's chart to find what hydrometer reading you need.>>

Yes. Assuming that the "thermometer" is a hydrometer.

<< I'm still not sure about a 75 degree calibration though. >>


There are a bunch of ways to do this. All will be reasonable for our needs.

I you measure S=35 water at 75 deg F with a hydrometer calibrated for 75 deg F, then the reading out to be about 1.0265.


1. If you have a table that relates S (salinity) to specific gravity (60 deg F/60 deg F), AND if you use that hydrometer at 75 deg F, you can just add 0.0004 to it, and use the table that relates salinity to "specific gravity". For example, if you read 1.0265, and added, you'd get 1.0269. Plug that into an appropriate table, and you should bet S=35.

2. For this simpler method we don't need a table. we will assume that specific gravity is linear with S. IT is pretty close over the range that reefkeepers usually use, and probably introduces less error than the hydrometer itself.

So, if S=35 has sg (75/75 degF) = 1.0265, then we can use a simple formula to relate S to sg for this hydrometer type only:

S = 35* (sg measured-1)/(1.0265-1)

So if you measure 1.027, then you calculate that S=35.7.

Remember, this last formula only works for a 75 deg F hydrometer used at 75 deg F.

If you use that hydrometer at other temperatures, here's how to correct it since you won't likely have such a correction table:

1. I

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[randy holmes-farley]

Hmmmm, part of the reply didn't show up. Anyway, use it at 75 deg F

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>