Where the
world gathers for metal finishing
Q&As since 1989


-----

Specific gravity - powder coating

March 13, 2008

We are constantly being asked the question "How can we check the specific gravity of powders"? I have put together the following explanation that you may wish to save to file.

Specific Gravity,

As you are aware specific gravity, uses water as the universal comparison base. One-hundred cc's volume equates to one hundred grams weight. Unfortunately, powder contains voids of air and therefore we must fill those voids. This is achieved by filling them with a liquid. Such liquids should be those that do not affect the powder i.e. n-hexane or a solvent that is more readily available, kerosene or petroleum spirits.
First, check the specific gravity of the liquid you are going to use. Obtain a laboratory 100 cc glass-measuring cylinder. Weigh cylinder, add 10 grams powder, and then add liquid making sure that no air is entrapped in the powder. Continue adding liquid until the 100 cc mark is reached (not sure of its name but you view the lower part of the liquid level at 100 mark Ž the top layer of liquid forms a "U" inside the cylinder). Re-weigh.

Subtract the weight of empty cylinder and you have the weight of a known volume of liquid/powder. Subtract the weight of powder and you have the weight of liquid used.

Example
Weight of 100 cc liquid/powder = 86.225
Weight of powder = 10 gm
Weight liquid used = ((86.225) Ž 10) = 76.225gms
Specific gravity of Kerosene = 0.82
Therefore, volume of kerosene = 92.958
Volume of powder = 100 Ž 92.958 = 7.042
Specific gravity of powder is 10 divided by 7.042 =1.42 (the nearest two decimal places)

Sg powder = 1.42

If you weigh to two or only one decimal place, the answer may still be accurate for your purposes.

You could try using a pyknometer or (Weight per Gallon Cup) but this could prove expensive.

Terry Hickling
Birmingham, United Kingdom

---

Terry Hickling,

Thank you so much for your contribution.

Jose Castellanos
- Minneapolis, MN, USA
March 18, 2008