specific gravity question...

[karen]

OK boys and girls, just doing a little "for my own edification" research. First of all- is the way this number is arrived at done as follows???- a cube is cut out of a type of material( poly, 1.60 or whatever)that is always identical in size and shape and then weighed to get this number. ( if that is NOT it- will someone please tell me how it is done) What is this number measured in- if CR39 has a Specific Gravity of 1.32-1.32 what?? How important do you feel this number is in the grand scheme of things. And if it is done in the way mentioned above then RX makes no difference-or does it? As you can see, I am very confused. Thanks in advance!

[JRS]

Density, ratio of the mass of a body to its volume, usually expressed as its specific gravity.

Specific gravity is the ratio of the density of a body to the density of water, the latter being taken as unity. In the metric system 1 cu cm of water at 4° C weighs 1 g.

Specific gravity can be determined in a number of ways. Solid bodies that have a higher specific gravity than water can be weighed separately in air and then in water. Specific gravity is obtained by dividing the weight in air by the loss of weight when the bodies are immersed in water. For measuring the specific gravity of liquids, a hydrometer is commonly used. For more accurate measurements, the weight of a known volume of liquid is determined under controlled temperature conditions. The usual method of obtaining the specific gravity of gases is similar.

Cr = 1.32, 1.6 (approx) = 1.36, 1.6 glass = 2.63, poly = 1.20, etc. etc.

Describes the weight is all, provided the rx's are equal, then Cr would weigh more than poly. I have not seen this used as a reason for pushing one material over another. Usually the focus is on thickness, safety, and that stuff.
But... after all the verbage, did this help?

[This message has been edited by JRS (edited 03-26-2001).]

[This message has been edited by JRS (edited 03-26-2001).]

[Darryl Meister]

Just to connect a dot or two on JR's very nice explanation on specific gravity and density... Specific gravity is equal to density when it is expressed in grams per cubic centimeter. For example, this why you will often see charts show the value of 1.20 as either the "specific gravity" or "density" of poly. (Though specific gravity is unitless, while density should be expressed in g/cc.)

Best regards,
Darryl

[JRS]

Thanks for cleaning up my prattle Darryl. We could have a world of fun if we "Optiboarders" ever sat down together and carried on some lenghty discussions.

You could certainly turn a patient or two 'glassy eyed' telling them about the wonders of specific gravity and their eyewear. <grin>

[Jim G]

Just a slight addition to what the professors noted about density/specific gravity as it relates to lenses.

Because a higher index lens can be thinner to provide a particular Rx, there will be less lens material than for a lower index material. In addition to spicific gravity, this should also be taken into account when comparing the weight of a particular lens as compared to another.

JRS notes: "Usually the focus is on thickness, safety, and that stuff." Virtually all consumer studies note that weight is 1st or 2nd in preference as well. Light 'n thin is the way to go.

[Darryl Meister]

Originally posted by Jim G:
Because a higher index lens can be thinner to provide a particular Rx, there will be less lens material than for a lower index material. In addition to spicific gravity, this should also be taken into account when comparing the weight of a particular lens as compared to another.

Yes, that is a very germane observation. Additionally, the recommended center thickness of the material (which may be affected by coatings) is another factor that can affect the final thickness and weight for a given lens material.

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 03-27-2001).]

[Jim G]

...and the size of the edged lens may be the biggest variable of all...!!!

For us lab rats, please try to keep those C.T.'s over 1.2; 1.5 is perfect. Those 1.0 centers are a ****** to surface...and edge!

[Prentice Pro 9000]

...and the size of the edged lens may be the biggest variable of all...!!!

For us lab rats, please try to keep those C.T.'s over 1.2; 1.5 is perfect. Those 1.0 centers are a ****** to surface...and edge!

This is all the way back from 2001 but.... I'm dying to know... what makes a 1.0 center hard to surface? I ask for it all the time and rarely get it. I just always thought the lab guys weren't paying attention.

[MakeOptics]

This is all the way back from 2001 but.... I'm dying to know... what makes a 1.0 center hard to surface? I ask for it all the time and rarely get it. I just always thought the lab guys weren't paying attention.

At that thickness, flexing, warping, distortion, and hardcoat/arc crazing become real issues. ANSI dictates a 0.2 +- tolerance so chances are that's what you'll get or at least there about. Most labs will charge a premium for 1.0 and the job may be delayed.

[RT]

Actually, section 6.1.3 of ANSI Z80.1-2010 says that if center thickness is specified, tolerance is +-0.3mm. Of course, there may be other factors that would override...US FDA drop ball requirement for example, or plus lenses where center thickness must increase to make the edge thick enough.