Measuring density of a sample is an important quality parameter of both raw materials and finished products. Various techniques enable the density of solid, viscous and liquid materials to be accurately determined e.g. metals, plastics, chemicals, lubricants and food.

Density for Quality Control

A variation in a raw material, indicated by a change in the density, may have a detrimental outcome on the functioning or quality of the final product. Density measurement of raw materials can be used to confirm the purity of the material. If a substance has been adulterated with a cheaper alternative, the measured density of the composite material will be different from the pure substance.

Density can also be used to ensure homogeneity. If a manufactured part is not homogenous, key performance attributes such as strength and crack resistance can be affected. For example, an internal air bubble could ultimately cause a part to fail when placed under stress. Random sampling of parts is a simple and cost-effective way to monitor ongoing quality.

Why Accurate Weighing is Crucial

Common laboratory gravimetric procedures for determining density are the buoyancy technique, the displacement principle and the pycnometer method.

The most widely used method is the buoyancy technique which utilizes Archimedes' principle: a body immersed in fluid indicates an apparent loss in weight equal to the weight of the fluid it displaces. This ancient principle, from around 200 BC, is precisely what is used nowadays to determine density gravimetrically. Accurate density measurement therefore is highly dependent upon accurate weight values.

Buoyancy Method - Archimedes' Principle in Action

Archimedes' principle states that a body immersed partially or fully in fluid experiences a buoyant force acting upwards on it. The magnitude of this force is equivalent
to the weight of the fluid displaced by the body.

If more than just the density is of interest, digital density meters measure density, specific gravity and other related attributes of liquid samples (i.e. alcohol%, BRIX°, API degrees) with high precision and short measuring time.
 

Measurement of density is an extremely important quality parameter of both raw materials and finished products. We understand there are many factors which must be considered to ensure accurate density results. If you need support with density determination or want advice on which balance or Density Kit to purchase, our team of experts is here to help.
Do not hesitate to contact us for assistance.

Bubbles

By far, the biggest source of error in density measurement is the limited wettability of the sample. When the sample is submerged in liquid, it is crucial that all bubbles adhering to the sample and equipment are removed. Any remaining bubbles will cause a buoyancy effect and distort the density calculation. (A bubble with a 1 mm diameter causes a buoyancy of 0.5 mg.) We recommend:
 

• Use a wetting agent or organic liquids (the density change experienced by distilled water by the addition of a few drops of wetting agent can be neglected).
• Degrease solvent-resistance solids
• Clean equipment regularly
• Do not touch parts to be submerged with bare hands
• Use a fine brush to remove stubborn air bubbles
   

Temperature

Solids are generally so insensitive to ambient temperature fluctuations that the corresponding density changes are of no consequence. However, as density determination is carried out with an auxiliary liquid, temperature must be taken into account. Temperature has a greater effect with liquids and causes density changes of order of magnitude 0.1 to 1‰ per °C.

This effect is already apparent in the third decimal place of the result. To obtain accurate results, we recommend that you always take the temperature of the auxiliary liquid into account in all density determinations. Values are available in dedicated books of tables. The densities of the most important reference liquids (H₂O and Ethanol) are stored in the balance.

Weighing

As mentioned above, weighing plays an important part in the accurate determination of density so it is critical that the balance used meets the needs of the density application. For small samples, the minimum net sample weight of the balance must be considered – weighing a sample below this weight cannot be trusted to be of the required level of accuracy.

Data Handling

Manual transcription of sample data, weight values and density calculations is time consuming and prone to error.

Most METTLER TOLEDO analytical and precision balances with 1 mg or higher readability can be easily used to determine density via the buoyancy technique with the use of a density kit which is mounted on the balance in a few simple steps.

To measure density of solids using a balance is an easy and convenient process that delivers highly reliable results when compared to other methods in which the volume of the part is determined independently of the weight. By converting a standard laboratory balance with the addition of a Density Kit, you avoid the necessity to purchase a dedicated piece of equipment to carry out this straightforward procedure. This makes the purchase of the Density Kit accessory a very cost-effective investment. With the addition of a glass sinker of known volume, the Density Kit can also be used for determining the density of liquid samples.

The built-in Density application provides step-by-step instructions making it easy for even untrained operators to use. Benefit further from:
 

• High flexibility to accommodate individual process needs
• Automatic density calculations for solids and liquids including temperature adjustment of the reference liquid
• Statistic evaluation of multiple samples
• All results, including User, Sample ID, Lot Number, Time and Date can either be printed or saved on a USB stick.
   

Measure Density of Bulky Samples

For bulky samples, which do not fit into the Density Kit, METTLER TOLEDO precision balances with 0.1 g and 0.01 g readability can be converted to weigh below the balance through the addition of a dedicated hook. The density measurement principle is exactly the same; the sample is weighed in air and again in a reference liquid.

Watch how simple it is to measure density on your METTLER TOLEDO laboratory balance. The Density Kit enables you to determine the density of solid, liquid, porous and viscous substances. With step-by-step instructions and automatic calculations, the whole process is easy.

The determination of density in plastic is essential for consistent plastic quality. The application note describes how the density of solid plastic parts can be easily determined using the integrated application on a METTLER TOLEDO MS-TS, ML-T or ME-T analytical balance with a Density Kit.