specific gravity

[daniel_mcgee]

hello,

i've tried searching the archive, but found nothing really that

describes specific gravity. i'm currently working on an assignment

centred around the amazing phenomena that is specific gravity,

however, apart from trolling the libraries and finding the basics

(i.e. definitions), i have been unable to locate a diagramatical

example of the ratio of stock to water - i hope this makes sense - or

anything else that expands on its purpose. so, does anyone happen to

know of any sites, or happen to be blessed with carnal knowledge on

this subject? your help will be greatly appreciated, and those who

respond, will be sent complimentary raisin treats in the post.

thanks.

[john_cook1]

Of course specific gravity has been used for years to measure the state of charge of automobile lead-acid batteries. The glass basting tube with rubber bulb can probably still be purchased at the local auto parts store.

 

But photographically, the only use I have seen for this measurement is in determining the status of various E6 chemicals in a dip-and-dunk processing line.

 

A friend used to subscribe to Kodak's Q-Lab program, which (among other things) required him to read the specific gravity of certain chemicals daily. He used a glass basting tube/rubber bulb thingie which looked exactly like the automotive version. I'm sorry I never watched him more closely to now have more information for you.

 

Perhaps on Kodak's mega-website there might somewhere be a module on Q-Lab procedures and requirements, which would tell you more.

 

So far, I may have earned only a small pile of bunny raisins...

[robert_davis2]

If I remember right most [all?] of the various companies give SG ranges for thier chemicals. It's a quick way to check if you've made gross errors in mixing chemicals. I doubt you'll find anything explaining it's purpose. Most likely you'll find something like :

 

Target SG 1.050-1.060

 

Or something like that. The people doing the checking don't need to know why it works they just need to be able to check the SG and know if it's right or wrong. If you need more then the one page section on checking your SG in the Kodak Z-131 manual I'm going to suprised if you find it. It's in section #1

[paul ron]

What exactly are you doing? Specific gravity just measures the weight of any liquid compared to water 1.000. If you disolve anything, you will change it's SG. So the question is????

[bill_napper]

Specific gravity can be a very good indicator of solution concentration in binary mixtures. (i.e. Sodium Hydroxide in water) It is used extensively in industy as a indicator of concentration. It can only be used in applications where you know what you have! It is a quick and easy way to keep a check on solution concentrations.

 

By defination it is the ratio of the density of a substance as compaired to the density of pure water at the same temperature. Density being weight per volume.

 

Easy to measure with a hydrometer.

Sorry if this response is too long. I am a chemist by trade, photographer by desire!

 

Why do you need to measure?

[bacsa]

specific what???

 

:))It's a funny name for something that has not much to do with gravity. Do they really use this expression?

 

Thanks for the good laugh.

[rowland_mowrey]

Specific gravity is simply the weight of a given volume of a solution measured in g/ml (grams / milliliter). Water, at a specified temperature (I've forgotten if it is 4 deg C or 20 deg C) has a Sg of 1.00000. Addition of liquids or solids to water changes the specific gravity to lower or higher values depending on the reaction of the water to the added material. (Specific gravity only applies to solutions, but in the case of pure water Specific Gravity = Density, but strictly speaking they are not the same thing)

 

There is an exact algebraic equation that relates to specific gravity vs temperature of most any polar material in water, such as salt (NaCl) or even silver nitrate. There are also equations to describe the specific gravity of mixed polar materials in water.

 

Specific gravity is used with flow meters to determine flow rates of liquids in ml/min or g/min and to convert flow rates back and forth between these units. In other words, knowing specific gravity, you can convert from volumetric measure to gravimetric measure and vice versa.

 

It also supplies a test method for verifying that you have mixed a solution up properly, or to test that it is at the right concentration.

 

One of photographys more useful yardsticks (sorry for the mixed units - meter sticks)

 

Ron Mowrey

[john_bean]

"specific what???

:))It's a funny name for something that has not much to do with gravity. Do they really use this expression? "

 

I'm afraid so. Used to be called "Relative Density" when I was at school, much more descriptive name IMO. But at least we don't have to worry about what units to use, it doesn't have any :-)

[chris_waller]

I knew that my time as an industrial chemist would come in useful one day! SG or as someone else noted above, Relative Density, is the weight of the liquid per unit volume, e.g. grammes per millitre. You can determine it using a float hydrometer which is calibrated directly, or you can measure out, say, 100 ml of the liquid and weigh it (i.e. weigh the measuring cylinder empty, then containing, say, 100 ml liquid, then divide the difference in weight by 100 and, hey presto, you have the SG or RD).

[john_bean]

"SG or as someone else noted above, Relative Density, is the weight of the liquid per unit volume, e.g. grammes per millitre."

 

Sorry Chris, I'm going to be a bit pedantic here. SG/RD is not what you say, it's the ratio of the density of the sample and the standardised density of water. As it's a ratio it has no units, it will always have the same value irrespecive of the units used to measure the density - density being what you described.

 

SG/RD will numerically match the true density only if you use units that give water a density of 1, like the example you gave. If you use (say) imperial lbs/gallon units then water would have a density of 10. It would still have a *relative* density of 1 though.

 

The physicist in me keeps popping up...

[kirk_keyes]

Ron wrote "Specific gravity only applies to solutions, but in the case of pure water Specific Gravity = Density, but strictly speaking they are not the same thing"

 

John's got the definition right - ratio of the mass of a body to the mass of an equal volume of water at 4C or other specified temperature.

 

But it does apply to more than just solutions - pure liquids, impure liquids, all solids, gases - they all have specific gravities. Ask a geologist or metallurgist or chemist.

 

Most things are referenced to water, but gases are commonly compared with air or hydrogen. Note that specific gravities to not have units.

 

As Ron pointed out, specific gravity can be equivalent to density, but only when density is measured in centimeter-gram-second units or equivalents. If you are measuring density in the English (or the US version of the English) system, then you are out of luck and they will not be equivalnet.

 

Anyway, I think the reason we are all discussing definitions is that the original question is kind of nebulous. What was your question??

[rowland_mowrey]

Kirk is right, of course. I always think of liquids in terms of SG or Density, but solids and gasses in terms of Density only, so I often forget that SG can apply to solids and gasses as well.

 

Density works much better, and as Kirk points out, the Metric system is ever so much better for all of this.

 

Ron Mowrey