Calibrating for temp

[kevin95695]

Hi!

At one point I had a source that showed how to calibrate salinity/specific gravity for any given water temp., but I lost the link somehow. Anybody got that? TIA.

-Kevin

 

[funman1]

Calibrate what?
A good refractometer does this for you.

 

[kevin95695]

I just remember reading a table that showed that while the salt in solution remained the same, you would get a very different reading if the temp was 70 vs. if the temp was 80...

 

[dots]

Your correct Kevin, specifc gravity does change with temperature.

Specific gravity (sg) is a ratio of two densities, one being the density of pure water at 3.98 degrees C, (some say 4).

The definition of density is mass/volume

Because the volume changes due to thermal expansion and contraction that in of itself is caused by changes in temperature, specific gravity is temperature dependent.

(Did you know the last piece of the Golden Gate Bridge could only be fitting in place once the morning sun had warmed the metal, expanding it ever so?)

The key is knowing what standard your refractometer is calibrated at. The real challenge lies in the resolution of the refractometer.....i don't know about you guys, but I have problems telling 1.025 from 1.026 sometimes and guess on 1.0255. I won't even begin to think myself, or the instrument is that precise for anything further, and isn't.

So the real question is: how close is "close" enough for you?

From the below article link:

"The density of pure water at 20 °C is 0.998206 g/cm3, and at 60 °F it is 0.9990247 g/cm3. While these seem close to 1, and are often simply claimed to be 1.00 in many contexts, the difference can be substantial. For example, the specific gravity of natural seawater (S =35) is 1.0278 using the 3.98 °C standard, 1.0269 using the 60 °F standard, 1.0266 using the 20 °C standard, and 1.0264 using the 77 °F standard."-Randy Holmes-Farley

http://www.advancedaquarist.com/issues/jan2002/chemistry.htm

Here is your chart:

http://www.reefkeeping.com/issues/2004-07/rhf/index.php

And one on refractometers:

http://reefkeeping.com/issues/2006-12/rhf/index.php

With that said, I think the difference is far more precise than the average user with the average equipment can accuratly achieve to see the difference from 70-80 degrees.

 

[Reefugee]

I agree with Doug. Here's a quote from a previous post:

<a href=showthread.php?s=&postid=9205106#post9205106 target=_blank>Originally posted</a> by Reefugee
ATC is more of a marketing ploy than anything else. If you look at these refractometers, they work in a relatively narrow band of temperature (50F/10C - 86F/30C). I did a google search and found that the density of water at various temperatures:

Temp. Density (kg/m^3) Specific Volume (m^3/kg) (calculated)
10C 50F 999.8 0.0010002
15C 59F 999.2 0.0010008
20C 68F 998.3 0.0010017
25C 77F 997.1 0.0010029
30C 86F 995.7 0.0010043

If my calculation is correct, kg/m3 is the same as g/l. Anyway - salinity of ocean is 35 psu - which by definition means exactly 35 grams of salt per liter of water. So let's see what happens when we go from from 10C to 30C in solution. Lets assume we have 35 PSU at 10C and heat it up to 30C.

Assuming we have exactly 1 L of water and exactly 35g of salt, we would have a reading of exactly 35PSU. (I think PSU is similar to PPT) Increase the water to 30C, the volume of the water would increase by 0.23% [ (0.0010043-0.0010002)/ 0.0010002 x 100% = 0.23% ] or 1.0023L. Since the amount of salt has not changed, the new salinity is 35g/1.0023L = 34.92PSU . The water temperature change caused the salinity to change 0.23% [ (35-34.92)/35 * 100% = 0.23%).

Just looking at some of the refractometer out there, they have an accuracy of +/- 1ppt or accuracy of 0.2%. Remember - this is when the refractometer is calibrated at 20C, and I calculate the error over the whole range. So even with the changes in salinity caused by the temperature change within the refractometer temperature range - the refractometer is still reading within it's accuracy.

Conclusion: ATC is a bunch of marketing hype. Within the operating range, the refractometer is still within an acceptable error range. Regardless - a refractometer still works great!

Minh

PS - I used PSU instead of PPT because PSU is easier to calculate. I just want to show how the salinity changes very little over that temperature range.
PSS - for you science buff - I ignored significant figure in the calculation.

 

[GreshamH]

Huh.

 

[kevin95695]

Thanks. I sorta get it. Steve's right, my refracto is ATC, whether that's hype or real, still makes me feel good. Turns out the range I was concerned about was within tolerances.