Specific Gravity Conversion

[tracklighter]

I posted this on the reef forum, but no answers so far.
I bought a glass hydrometer from this site.
The scale on the hydrometer is calibrated to 75 degrees.
I am trying to find the true specific gravity for my tank which is at 81.5 degrees at the time of measurement.
So how can I convert the reading for my temp?
IE: What is the specific gravity if my reading is 1.021 at 81.5 degrees?
There is no conversion chart included with the hydrometer.
Thanks.

 

[beth]

Look at the "What's the Problem" and "The Solution" section of this link:
http://www.epa.gov/OWOW/monitoring/v...3/school26.htm

 

[tracklighter]

Thank you, Beth.

 

[firecopemt]

Short form: Specific Gravity is temperature dependant. See the next table for a quick lookup of the recommended hydrometer readings. They are based upon our recommended S.G. of 1.025 at 60 degrees F.
Degrees F. Hydrometer reading.
50 1.0255
55 1.0252
60 1.0250
65 1.0246
70 1.0240
75 1.0233
80 1.0226
85 1.0218 (rather hot for most tanks)
90 1.0210 (very hot for most tanks)
In more detail: 1.025 recommended for reef tanks. Note that virtually all hydrometers are calibrated for measurements at a temperature of 60 F. Included below is a short table of temperature adjustments. Add the value shown to your hydrometer reading to get an accurate reading.
Degrees F. Correction
50 -0.0005
55 -0.0002
60 0.0000
65 0.0004
70 0.0010
75 0.0017
80 0.0024
85 0.0032
90 0.0040
For example: If the hydrometer reads 1.0235 at 80F, the actual Specific Gravity is 1.0235 + 0.0024 = 1.0259

 

[tracklighter]

Thank you firecoemt!! Appreciate the effort.
One question: My hydrometer says the readings on the scale are for a temperature of 75 degrees. I assume it is already adjusted from the 60 degrees. For a temp of 81 degrees I would think that the difference is pretty negligible.
I also thought that the specific gravity would go DOWN with increase in temperature. From the way you converted, it looks like the specific gravity would go up with temperature.

 

[broomer5]

read me

 

[gatorcsm]

There is a simple equation that works for MOST, not all, however hydrometers and digital hydrometers. That is for every 3 degrees off of the calibration temp (usually 80 on scientific models) is equal to either +/- .001, ergo, a 83F would be +.001 and 77 would be -.001
Check your model's instruction book, or go to the manufacturers web site. Alton Paar makes a digital one which works great. It also tells you temperature to the 1000th, and some optional other things like alkilinity/acidity, etc. Not made for the aquarium industry...
Hope that equation works out for yours

 

[tracklighter]

Broomer, that is exactly what I was looking for. Thank you.
GatorCSM, thank you also.