SG vs. Density?

[Markushka]

So i'm sitting in my chem lecture and it's on density. I am wondering could measuring for density replace measuring for SG. could this be more accurate than say a swing arm hydrometer?

[Mr. Fosi]

SG is also known as "relative density". It's relative to the density of water which is 1.

 

So when you measure SG, you are measuring density.

[Markushka]

yeah that I understand. I'm wondering if I could measure density accurately, would it be better than using a hyrometer.

[cptbjorn]

If you have an accurate scale with at least .01g resolution you can measure density/sg very accurately and easily with a pycnometer. Only issue is that the scale would cost quite a bit more than a refractometer so nobody does it that way.

[Markushka]

I need to get a scale for my business with an 1 or .1g resolution so i'd rather kill two birds with one stone than blow another 50-90$ on a refractometer. So if its a valid option then thats cool.

[cptbjorn]

The most common/standard size of pycnometer is 25ml and with a .1g scale this means your SG can be off by .004, ie if you read 1.026 it could be anywhere from 1.022-1.030. Even worse with a cheap scale.

 

A solid, dependable, accurate .01g scale is going to be a couple hundred bucks minimum and you won't want to do anything with it besides carefully+gently weigh chemicals or other light objects - you can't use it to weigh packages for shipping for a business etc.

[Markushka]

haha, i'm not weighing packages . I already have a cheap postal scale for that. I'm mixing herbal powders and they have to be proportionate. I did see a scale with a resolution of .01 for about 70$ if it saves me 30$ by not having to get a refractometer for accuracy, i'll seriously consider it.

[doctaq]

probably not the best idea, some tiny amount of water off from your expected amount will throw your reading off signifigantly, unless you are measuring a huge amount of water to get a good average, but most likeley your scale should not handle a large amount of water if it is so precise, a refractometer is not the place to skimp, you can get them for 50 and under.

[Mr. Fosi]

... some tiny amount of water off from your expected amount will throw your reading off signifigantly, unless you are measuring a huge amount of water to get a good average...

 

That's what I was thinking.

 

What volume of water do you think you would measure? How accurate/precise would the volumetric container be?

[DiverDave]

Help out the challenged. Doesn't a refractometer directly measure density? The denser the solution, the greater the light refraction? Sg is dependant on temperature too yes?

[Mr. Fosi]

SG is also known as "relative density". It's relative to the density of water which is 1.

 

So when you measure SG, you are measuring density.

 

yeah that I understand. I'm wondering if I could measure density accurately, would it be better than using a hyrometer.

[cptbjorn]

For doing density/sg measurements with a scale it is best to use a pycnometer. It is a small vial with a ground glass joint and you fill it up with the fluid and then insert the top. The top has a small hole through it so the excess is forced out the hole and if you do it correctly you get the whole container full to the top of the hole in the lid with no air. The ground glass joint stops at a very repeatable place so with a bit of practice you can fill it up within .001-.002g precision between runs. Here's a pic:

 

 

You weigh it dry and empty, the fill with RO and weigh again to find the exact volume (this doesn't change so you can write it down), then fill with your test solution and weigh. Once you have done it a couple times with a particular pycnometer you'll memorize it's exact volume and it will go pretty quickly.

 

Oh yeah and point it away from your face lol.

[lakshwadeep]

A [good] glass hydrometer is just as good as a refractometer in measuring s.g./salinity. Specific gravity article:

http://www.advancedaquarist.com/issues/jan2002/chemistry.htm

 

diverdave: Refractometers work by measuring the refractive index (the extent to which light is bent when traveling from air into a sample) of a solution. In general, refractive index linearly increases with density, which allows refractometer makers to make a scale that approximates the salinities (or s.g.) of a solution.

 

cptbjorn: wouldn't one also need an accurate balance in addition to the pycnometer?

[Mr. Fosi]

cptbjorn: wouldn't one also need an accurate balance in addition to the pycnometer?

 

Ja, but the OP said he was already planning to buy one.

 

Calculating the density by volume and mass seems like more trouble than it's worth. You have the error of at least two bits of equipment whereas you only have to worry about the error of one when using a refractometer or a hydrometer.

[Markushka]

I'll play with this idea a bit, it seems doable, and the error'd probably be less than my red-sea hydrometer.

[ihatesears18]

geez quit making it all complicated. Just get a swing arm and calibrate it with your LFS's refracto. Clean it after each use and you're good.

[rollei]

The arm on the hydrometer measures the buoyant force minus the gravitational force. Buoyant force = density * volume of displacement where volume is the volume of the entire arm. The swing arm hydrometer then has scaled numbers that relate this buoyant force to a specific gravity.

 

When we see the numbers "1.026" on our hydrometer, we really are measuring the density of the water.

 

The problem is, there are so many steps and possible errors (manufacturing errors, bubbles attached to the arm that influence the volume when they're not supposed to, ect.) The huge number of errors makes the SWH so inaccurate.

[Markushka]

but I need an accurate scale anyway

[lakshwadeep]

Ja, but the OP said he was already planning to buy one.

 

doh! I should have read all the way down.

 

I don't think you can get any cheaper in total costs than a refractometer, hydrometer, or even a salinity monitor (conductivity meter).

[neanderthalman]

but I need an accurate scale anyway

 

For legal purposes?

[Walking_Target]

Assuming you can find a place to buy a NEW pycnometer that hasn't seen chemical use; you're looking at $50 and up, easily.

 

Now, assuming you also want a scale that is ACCURATE to within .01g, then you're looking to blow a couple hundered dollars. Just because a $70 scale has a measure down to .01g does not mean that it is accurate down to .01g - which is a very important thing. If your scale is off by so much as .04g, then your SG will be off accordingly.

 

Finally, you can find a tubular glass hydrometer, that is quality labware, accurate down to .001sg for around $60. Or, you can get a temperature correcting refractometer for around the same price as a pycnometer.

[Markushka]

For legal purposes?

 

well....

 

 

yes, no illegal substances here. thats bad for business. what do you think I am?

 

Assuming you can find a place to buy a NEW pycnometer that hasn't seen chemical use; you're looking at $50 and up, easily.

 

Now, assuming you also want a scale that is ACCURATE to within .01g, then you're looking to blow a couple hundered dollars. Just because a $70 scale has a measure down to .01g does not mean that it is accurate down to .01g - which is a very important thing. If your scale is off by so much as .04g, then your SG will be off accordingly.

 

Finally, you can find a tubular glass hydrometer, that is quality labware, accurate down to .001sg for around $60. Or, you can get a temperature correcting refractometer for around the same price as a pycnometer.

 

 

you make a good point. I'll keep this in mind when looking. the 70$ was just the first one I saw after a quick search. And I guess I'll have to consider the practicality of this method as opposed to the hydrometer or refractometer.

 

Thanks everyone for the informative responses!