use of API gravity

[FlowBee]

I see that the definition of API gravity is just an equation that uses the specific gravity of a liquid at 60degF with respect to water at 60degF. However, looking at the usage of API gravity in handbooks and other publications, I see mention of the API gravity changing with the temperature of the fluid. How could this be, if the API gravity uses a reference temperature, not the actual temperature?

I realize measurements to determine the API gravity may be taken at temperatures other than 60degF and then corrected back to 60degF, but it seems that the fluid would have the same API gravity regardless of the process temperature.

 

[jjwamp]

At different temperatures (or pressures) the oil expands/contracts as does any fluid. As such you need a reference temperature and pressure. API gravity is a density measurement.

Absolute API gravity should not change however the density will change depending on temperature.

Oil's value alot of times depends on its API gravity as measured at 60 degrees so this number is important. However when proccessing we may be concerned with its relative API gravity at temperature. My guess is this is what they are refering too.

Kind of like saying the density of fresh DI water is 1 gr/cc but this is only at 68 F degrees at MSL. At 100 degrees the same water may have a slightly lower density to account for temp expansion.

Just remember API gravity is an oilfield term used to measure density the same way gr/cc is used for water in metric system.

 

[FlowBee]

Of course density changes with temperature. But the specific gravity of a fluid at a specified reference temperature doesn't change when that fluid is at a different temperature.

So, this usage sounds like one of those times when things are used differently than originally intended, as so often happens. When the API gravity is changed for use at a different temperature, are they using specific gravity defined as "density of fluid at temp over density of water at 60degF" or are they using "density of fluid at temp over density of water at temp"??

 

[mbeychok]

FlowBee:

It would be helpful if you gave us at least two word-for-word examples of the usage you are taking about, and include the publication names with the two examples.

Perhaps, it is just a matter of mis-interpretation of what those examples were saying.



Milton Beychok
(Visit me at www.air-dispersion.com)
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