8 replies to this topic

[vibhs]

Can somebody give me the definition of specific gravity?
I am particularly confused about the temperature at which it is referred?

[ankur2061]

Vibhs,

Check this out:

http://www.cheresour...fic-gravity-sg/

http://en.wikipedia....pecific_gravity

http://en.citizendiu...wiki/Hydrometer

Regards,
Ankur.

[vibhs]

Vibhs,

Check this out:

http://www.cheresour...fic-gravity-sg/

http://en.wikipedia....pecific_gravity

http://en.citizendiu...wiki/Hydrometer

Regards,
Ankur.

Thnx Ankur Srivastava Sir for your reply.
But still the term remain confusing to me. If I go by the definition of
http://en.citizendiu...wiki/Hydrometer, then Specific gravity of water should always be 1.0 irrespective of temperature, which is not the case. Can You put some more light on this.

Vibhs

[ankur2061]

Vibhs,

The citizendium article considers a reference temperature of 4 deg C for specific gravity measuremets. At 4 deg C water has its maximum density of 1000 kg/m3 and thus its specific gravity is equal to 1 at the reference of 4 / 4 deg C. At temperatures below 4 the density off water decreases and thus the specific gravity also decreases (remember ice floats on water). As the temperature increases above 4 deg C the density of water again starts to decrease and for a sample of water at 30 degC the specific gravity would be

Specific gravity at 30 deg C = Density of water at 30 deg C / Density of water at 4 deg C

Sp. gravity @30degC = 995.65 / 1000 = 0.99565

Hope this is clear now.

Regards,
Ankur.

Edited by ankur2061, 24 September 2011 - 08:00 AM.

[katmar]

When you are dealing with liquids there must be two temperatures specified to fully define the SG. This is best shown in the Citizendium article referenced by Ankur (his third reference). The temperature in the superscript is the temperature at which the density of the fluid is measured and the subscript is the temperature at which the density of water is measured. These two temperatures are usually, but not always, the same. There is no standard that I am aware of - I have seen all sorts of temperatures used as the reference temperature.

When you are working with gases the SG is the ratio of the molecular weight of the gas to the molecular weight of dry air. This is not a universal rule, but I would say it is overwhelmingly the most used definition for gas SG. The gas SG is most usually used in valve sizing equations. The equation is then set up for air and the SG is used as a correction factor. The base equation (eg Fisher) will be set up for air at a "standard" temperature and pressure and there will also be corrections in the equation for the actual temperature and pressure. You will find lots of people argue against this definition, but as I say - it is the one that I have always seen in the literature.

[kkala]

Topic has been previously discussed, following link (concerning gases) may be useful, in addition to ones mentioned.
http://www.cheresour...h__1#entry44481.

Edited by kkala, 24 September 2011 - 04:02 PM.

[vibhs]

Its clear from the references given above that the definition of specific gravity has many versions. Though this thread was helpful to me in building understanding about specific gravity. Thank you Ankur, Katmar and Kostas Sir.....

[JMW]

It is very worthwhile ensuring you are very familiar with the terms and the instruments used to measure it. That includes the equivalents such as density, Baume Brix etc. and eahc industry often has a different way of measuring.

Though an apparently simple concept, it is surprising to see how often the meaning and the measurements are misunderstood and misapplied.

[kkala]

My understanding from the posts suggests that the European definition for specific gravity (weight per unit volume, that is density * gravity acceleration) is more clear. I try to use density as far as possible. Specific gravity is used when you want the pressure created by a fluid of h m height, for example.
English definition of specific gravity is just the relative density of the liquid to water, or the relative density of the gas to air (then the question "under what conditions" is arisen).
All these have been discussed in the mentioned threads of the past, if someone is interested.