Dilatometer

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dilatometer

[‚dil·ə′täm·əd·ər]
(engineering)
An instrument for measuring thermal expansion and dilation of liquids or solids.

Dilatometer

 

a device for measuring changes in the dimensions of a body caused by the effects of temperature, pressure, electrical or magnetic fields, ionizing radiation, or any other factor. The most important property of a dilatometer is its sensitivity to absolute changes in the dimensions of a body.

Optical-mechanical, capacitance, induction, interferometric, X-ray, and radio-resonance dilatometers are widespread.

In optical-mechanical dilatometers a change in the dimensions of the test sample causes a displacement of a luminous indicator. A change in the length of the sample is determined either from preliminary calibration of the instrument or from geometric relations. Dilatometers of this type are sensitive to about 10−6-10−7cm. In capacitance dilatometers a change in the dimensions of the test sample causes a change in the capacitance of a capacitor, which serves as a sensor (sometimes the surface of the sample is one of the plates of the capacitor). Preliminary calibration is required in order to determine the change in the sample’s dimensions from the change in capacitance readings. Capacitance dilatometers are sensitive to about 10−9 cm. In induction dilatometers a change in the sample’s dimensions causes the displacement of two induction coils relative to one another, which changes the mutual inductance of the coils. Preliminary calibration is necessary to determine the change in the sample’s dimensions. Inductance dilatometers are sensitive to about 10−9 cm. In interferometric dilatometers, which are based on the Fizeau principle, an interference pattern is produced when two optical plates, between which the test sample is mounted, are illuminated by monochromatic light. Since the interference pattern is produced by superimposition of the light beams reflected only from the lower surface of the upper plate and from the upper surface of the lower plate (all other patterns are removed from the field of view because of the special arrangement of plates), a shift in the interference fringes occurs only upon a change in the length of the test sample. The change in length is calculated from the shift of the bands and from the wavelength of the light. Interferometric dilatometers are sensitive to about 10−8 cm. X-ray dilatometers, which are essentially devices for X-ray structural analysis, are used to measure changes in the parameters of the crystal lattice of the object being studied by means of radiographs taken by one of the known methods (seeX-RAY STRUCTURAL ANALYSIS). Dilatometers of the X-ray type are sensitive to about 10−5-10−6 cm (in terms of macroscopic units). In the radio-resonance type of dilatometer, a cavity resonator is used as a sensor; the resonator may be made from the material to be tested, or it may have an elastic wall fastened to the sample. In both cases, a change in the sample’s dimensions changes the volume of the resonator, thus causing a change in the resonance frequency. The change in dimensions of the sample is measured according to the shift of the resonance frequency. Dilatometers of the radio-resonance type are sensitive to as little as 10−12 cm.

As a rule, the design of dilatometers takes into account the possibility of changes in the external physical influences on the test sample (particularly changes in, and the stabilization of, the sample’s temperature). Particular attention is paid to compensation for expansion or contraction of the bodies immediately adjacent to the test samples, such as transmission linkages in the dilatometer.

For liquid or gaseous substances, only volumetric expansion is examined. Dilatometers used in the determination of volumetric expansion of liquids vary widely in design, but their principles of operation are confined mainly to the following methods: (1) a liquid fills a reservoir and a part of a carefully calibrated capillary, and the change in the liquid level in the capillary upon a change in temperature is observed; (2) a liquid filling a reservoir of known volume overflows partially upon heating; the mass of the liquid within the reservoir at the experimental temperature, and hence a measure of the density of the liquid in the reservoir as a function of temperature, is determined from the mass of the liquid that overflowed. The coefficient of thermal expansion for the reservoir must be known in both of the above cases. The method of a calibrated capillary can also be used to determine the thermal volumetric expansion of solids placed in a reservoir filled with a liquid of a known coefficient of thermal expansion. The method of communicating vessels, which was proposed by P. Dulong and A. Petit in 1818, was also used to measure the thermal expansion of liquids. Measurements of the volumetric expansion of gases are made by means of dilatometers that operate on the principle of a gas thermometer.

REFERENCES

Strelkov, P. G., G. I. Kosourov, and B. N. Samoilov. “Dilatometr dlia obraztsov malykh razmerov.” Izv. AN SSSR: Ser fizicheskaia, 1953, vol. 17, no. 3, p. 383.
Strelkov, P. G., and S. I. Novikova. “Kvartsevyi dilatometr dlia nizkikh temperatur.” Pribory i tekhnika eksperimenta, 1957, no. 5, p. 105.
Pudalov, V. M., and M. S. Khaikin. “Dilatometer With a Sensitivity of 10−4 Angstrom.” Cryogenics, 1969, vol. 9, no. 2, p. 128.
Collins, J. G., and G. K. White. “Thermal Expansion of Solids.” Progress in Low Temperature Physics, 1964, vol. 4, p. 450.
“Symposium on Thermal Expansion of Solids.” Journal of Applied Physics, 1970, vol. 41, no. 13.

IA. S. AGRANOVICH

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