Методы измерения микротвердости для металлических материалов, методы физико-химического анализа, методы измерения плотности

The basic tool in all quantitative analyses is the analytical balance, used for the accurate weighing of samples and precipitates. For usual analytical work the balance should be able to determine differences in mass of 0.1 milligram (about 0.000004 ounce). In microanalyses the balance must be about 1,000 times more sensitive, and, for special work, balances of even higher sensitivity have been constructed.

METHODS OF DENSITY MEASUREMENT

DENSITY MEASUREMENT METHODS FOR SOLIDS

The most common method is the Archimedean immersion method, whereby the loss of weight of an object when suspended in a fluid of known density is equal to the mass of fluid displaced, from which its volume and hence density can be calculated. Unless there are good reasons for not selecting it, water is the most common immersion medium, the primary reason being its low sensitivity of density to temperature compared with many organic liquids.

Varying degrees of sophistication can be built into the technique. Ensuring that no bubbles are trapped on the immersed object, and that the suspension wire is clean and has a consistent immersion depth, are key experimental factors. For materials that have open porosity, appropriate conditioning of the material is important before liquid immersion to eliminate or stabilise the residual gas content. Techniques include evacuation and re-pressurisation under the immersion liquid, boiling while immersed in the liquid followed by equilibration, or evacuation followed by high-level pressurisation. From such tests, bulk density and apparent solid density can be determined.

The following drawing shows the legendary Archimedes weighing method of measuring the density of a solid object. (Archimedes used this to detect a jeweler's fraud of admixing silver into gold of the crown of Hiero II, the king of Syracuse). The object is suspended on a thin thread and weighed twice - measuring the force of pull on the thread when suspended in air and immersed in the liquid.  If a balance from which the thread can be suspended is not available, a modification of the method is to simply weigh the object on a top loading balance followed by weighing the container with the liquid with and without the object immersed (suspended) in the liquid.

Other less commonly used methods for density determination include:

• pyknometry (or pycnometry): the internal volume of a vessel with and without a solid material present is evacuated and filled with gas, often helium; the volume of gas used, corrected for its pressure, is used to determine the apparent solid volume
• porosimetry: a liquid, such as mercury, is forced into the material under a progressively rising pressure until a stable infiltrated volume is achieved. The amount of mercury used provides a measure of the total porosity, and the pressure/volume curve gives a measure of the open pore channel size distribution
• X-ray diffraction: the density can be estimated from the unit cell dimensions for the given crystalline phase; but this does not take defects, strain or pores into account
• gamma densitometry: the attenuation of gamma radiation transmitted through a solid body can be related to its density via some form of calibration with artefacts of known density. This technique is typically used for continuous measurement of apparent density on production lines.

DENSITY MEASUREMENT METHODS FOR LIQUIDS

Piston dilatometry has been used successfully for molten metals, provided that no leakage occurs. A given mass of metal is held between pistons in a cylinder. As the metal melts to fill the gap it pushes the pistons outwards and the change in entrained volume is determined.

Other techniques include:

• pressure method: maximum bubble pressure (for liquid oxides/glasses)
• levitation: for liquid metals, the size of a droplet levitated in an RF field

For on-line use:

• vibrating cell, vibrating wire, vibrating tube techniques: the vibration frequency is controlled by the density of the surrounding medium; works best for liquids or slurries;
• X-ray or gamma-ray densitometers based on absorption through the fluid; particularly useful for slurries and foods.

 
METHODS FOR MEASUREMENT OF DENSITY

 
DENSITY MEASUREMENT METHODS FOR GASES

Direct methods include measuring the mass of a closed vessel:

1. when evacuated,  
2. after filling with the unknown gas, and 
3. after filling with a known gas or liquid.

Key control parameters are the pressure and temperature, and the method is prone to significant errors resulting from parasitic effects, such as the small mass of the gas compared with that of the container. Devices are now available commercially for accurate measurement of the equation of state of a fluid (including density) over a wide temperature range, including the critical region.

Other simpler, but less direct methods have been employed for industrial use, particularly for on-line monitoring, including:

• flowmeter, in which the float is supported by a given mass flow rate;
• refractometry: in which the refractive index and its change with pressure or temperature is determined interferometrically, and related to density through the equation of state;
• velocity-of-sound method: this is directly linked to density through bulk modulus, which needs to be known;
• vibration method: the vibration frequency of a fork or other device changes with surrounding gas density; requires calibration
• centrifugal method: based on the pressure differential when a gas is subjected to angular velocity; requires calibration.