Official Luthiers Forum!

I am a fan of the metric system, especially in less-than inch measurements. Another plus for the metric system is specific gravity is an easy calculation and it is a unitless measure. Whether your usual method of measuring is metric or Imperial specific gravity is outside the units of these measuring systems.
I do not know how metric weights and volumes work out so perfectly for specific gravity. I don't need to know how but it does.
To determine specific gravity you measure length, width and thickness of a piece in millimeters. Multiply all three measures together to determine volume. Then, divide the weight in grams into the volumetric figure. Once you have this figure, divide it into 1. The decimal point will have to be moved since we calculated in millimeters and kg/m3 is the equivalent of specific gravity.
The net positive is specific gravity is unitless and therefore universal across all measuring modalities.
Maybe it is too much to hope for that suppliers will list the specific gravity of individual soundboards (in particular), but at least a boy can dream.

_________________
Stay with the happy people.
--Reynolds Large

So what does this tell you? That some guitars will float better then others?

And just so anyone might want to know, the density of water at 20°C (68°F) is 62.4 lb per cubic foot.

Eh? Hombre, no comprende?

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I measure in centimetres and grams, then the calculation just works out to specific gravity, grams/cubic centimetre.
(S.G. of water at 20 deg C is one.)

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Why use specific gravity instead of density?

_________________
Jim Watts
http://jameswattsguitars.com

I am no scientist but as I understand it density is mass per unit of volume. Specific gravity is a relative quality with no units. I suppose you could use either measurement to your advantage.
I find specific gravity measure extremely useful in soundboards. I have a range of measurements that I know work well with my style of build.

_________________
Stay with the happy people.
--Reynolds Large

It works out so perfectly because grams are a metric measurement. 1 gram = weight of 1cc of water (originally at STP, then 4 degrees C, now somehow related to Planck's constant )



S.G. = mass/unit volume
Density = mass/ unit volume
S.G. = Density
The reason to use S.G. instead of imperial units (psi, pounds/cu.ft.,tons/cu. yd.) is because it is a unitless measurement it makes calculations easier to do in your head.

Specific Gravity is written without units but it is with the understanding that it refers to a specific volume of water. So in effect it carries on the units of its associated reference.

Thanks, Clay. Makes perfect sense now that you mentioned how 1 gram came to be defined and its relationship to a volume of water.

_________________
Stay with the happy people.
--Reynolds Large

Clay S -
Thanks for that, I quoted STP. but forgot about the change to 4 C and the anomalous expansion of water thing.
Not so sure about the reference to Plank's constant, or the Hubble-Carmeli time constant, and when i think about it, it's not that often.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Clay, in you formula above your calculating density not specific gravity. You can calculate density in metric units too. To get specific gravity you divide the density by the density of water which gives a ration of one mass to another.
The density calc is all you need as you point out, if you use grams and cc's you get a number very close to the Specific gravity (waters not quite 1g/cc)
It's not correct however to say the SG = Density, splitting hairs, I know.

_________________
Jim Watts
http://jameswattsguitars.com

Any way you calculate it, it's a great way to choose specific tops with particular purpose...

Agreed!

_________________
Jim Watts
http://jameswattsguitars.com

You are right , at some point you have to add back the units you are measuring in.

I’m an amateur marine aquarist if 16years.
I use a refractometer


Sent from my iPhone using Tapatalk

Which is useless here ha


Sent from my iPhone using Tapatalk