Specific gravity question?

[oldsaltman]

Does anything affect the specific gravity reading of tank water other that temperature? Are there any of the chemicals that we normally use that would cause the specific gravity to change slightly if testing right before or after adding the baking soda, cal, mag supplements, etc? Are there any AM or PM changes?

 

[Randy Holmes-Farley]

Actually, temperature has only a very small effect on true specific gravity, although it does cause some ways of measuring it to be less accurate.

There are no daily changes. There are no daily supplements that will have a significant impact on specific gravity. Adding 100 ppm of calcium via calcium chloride will only boost the sg from about 1.026 to 1.0262.

These articles have more:

Temperature Corrections for Hydrometers
http://reefkeeping.com/issues/2004-07/rhf/index.htm

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

Specific Gravity Measurement
http://www.advancedaquarist.com/issues/jan2002/chemistry.htm