Specific Gravity Testing

Specific gravity is the ratio of the density of a material (such as a solid plastic or rubber product or material) to the density of water. Specific gravity testing is used to verify the suspected quantities of the ingredients found in a compound. While the ingredients are usually identified using analytical test methods, specific gravity testing is a quick quantitative check that can be used to help verify the compound’s formula.


Due to differences in composition, porosity, crystallinity, and thermal history, there may be slight variations in density throughout a larger sample or product. Specific gravity testing data is often used to calculate the average density of the larger product.

Specific gravity testing has many valuable applications beyond simple quantification. One example is lot to lot uniformity. Specific gravity testing may be conducted on several batches from a production lot or run of a product or compound as part of a quality check. The results of each test are compared to the original production specification or the product’s calculated specific gravity. If any changes or differences in specific gravity are detected, it may indicate that a mistake was made when weighing up raw materials—including base polymers, fillers, plasticizers, and resins, among others—or the incorrect raw material was used. Specific gravity testing enables manufacturers to catch and correct these mistakes before the product is released to ensure consistency from batch to batch.

Another application for specific gravity is understanding the end product’s weight. A lower specific gravity correlates to a lighter end product. Assuming raw materials costs remain the same, a lighter material equates to a cheaper end product. The combination of the cost per pound and the specific gravity is the pound-volume cost, which determines whether materials with different specific gravities will end up being cheaper or more expensive in the end product.

One key application of this principle is the automotive industry. The automotive industry is currently experimenting with lightweighting, or strategic reduction of overall product weight, in many parts of the vehicle, from tires to hoses to batteries and beyond. Leveraging specific gravity data to reduce weight results in a lighter overall vehicle, which translates to better fuel economy and mileage. This appeases both consumers, who want to save money on gas, and environmental policy makers, who want to reduce overall gas consumption.

Specific Gravity Test Methods

Specific gravity testing is conducted by measuring the ratio of the mass of a material to mass of an equal volume of deionized water. A solid piece of rubber—typically excised from a molded test slab or button—is weighed in air, and then weighed in water. A calculation is then done to determine the density and/or specific gravity.

If a material has a lower density than water (specific gravity less than 1), it will float, which skews the test results. There are two ways to correct for this:

  1. A sinker is used to weigh the material down. The weight of the sinker is accounted for in the final calculations.
  2. Alternative fluids, such as methanol or a narrow cut distilled from kerosene, are used instead of water.

An alternative fluid may also be used for materials that are affected by water. If the material absorbs or experiences other physical changes as a result of contact with water, apart from simply becoming wet, an alternative fluid may be more appropriate.

ASTM D792 & ASTM D297

One of the most common test specifications associated with specific gravity testing is ASTM D792—Standard for Density and Specific Gravity of Plastics. ASTM D792 describes methods for testing solid plastics and rubbers in water and alternative fluids.

An alternative specification is ASTM D297—Standard Test Methods for Rubber Products—Chemical Analysis. ASTM D297 describes a hydrostatic density test method that is used almost interchangeably with ASTM D792. ASTM 297 also includes analyses of extract, sulfur, fillers, and ash content.

Gravity and Density Expertise From ACE Laboratories

Whether you need a routine quality and consistency check, you’re investigating a potential mix-up error, or you’re in the R&D stage for a new compound, you can count on ACE Laboratories for accurate specific gravity data. Our laboratories are fully equipped to conduct both standard and custom specific gravity and density tests, and we can advise you on how to test materials that are incompatible with water as a testing fluid. Our experts understand the importance of batch consistency and the challenges of lightweighting, so we can provide the outside opinion you need to ensure quality and a better product.

To learn more, talk to a member of our team >

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