June 26, 2009, Newsletter Issue #350: Measuring Specific Gravity

Tip of the Week

Measuring Specific Gravity, This can be done in two ways: With a hydrometer. This is a glass tube closed at both ends and weighted with lead at one end so that when floated, a scale up the side can be read at the water surface. The higher the specific gravity, the higher the viscometer floats. However, one problem with this instrument is that thick slips tend to impede free floating and give an incorrect reading. In fact, my opinion is that in most situations a hydrometer is practically useless for this reason!
With a scale. Here, you weigh a known volume and divide the weight by that volume. For example, if 10 cc's weighs 17 grams, then 17/10=1.7. A simple technique is to counterbalance a 60 cc veterinarian's syringe, then draw up a specific volume of slip, weigh it, and divide the weight by the number of cc's. Don't get sloppy on this measurement. Do it several times to make sure you get the same reading every time. If necessary, measure a larger volume, get a more accurate graduated cylinder, or a better scale. You must have a reliable way to measure specific gravity.
Some slip suppliers quote specific gravity in ounces per pint (i.e. 29 oz/pt), however, this measure is not intuitive in my opinion.

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