Dilatometer
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Dilatometers are measuring instruments. The thermal expansion coefficient α of the sample is defined as the rate of change of sample volume (which presents as a change in height of sample since cross sectional area in the dilatometer is constant)with respect to the change in its temperature. Thermal expansion is a measurement of how entropy of a body reacts to changes of the volume. Dilatometers have a wide range of applications, such as in the fabrication of metallic alloys, compressed and sintered refractory compounds, glasses, ceramic products, composite materials, plastics, etc.
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[edit] Structure/types
Dilatometers consist of a heat source, usually a furnace (temperatures from -260 to 2000 °C) with a temperature distribution (rising temperature, constant temperature, changing temperature) that can be adjusted. Depending on how the expansion is measured, there are two types of dilatometer:
- Capacity dilatometers possess a parallel plate capacitor with a mobile plate (spacer sensor). Precision of measurement are possible in the range of nanometer at very low temperatures.
- Connecting rod dilatometer, the sample which can be examined is in the furnace. A connecting rod transfers the thermal expansion to a strain gauge, which measures the shift. Since the measuring system (connecting rod) is exposed to the same temperature as the sample and thereby likewise expands, it concerns not a direct measurement separates itself one obtains a relative value, which must be converted afterwards.
- Optical dilatometer is an instrument that measures dimension variations of a specimen heated at temperatures that generally range from 25 to 1400 °C. The optical dilatometer allows the monitoring of materials’ expansions and contractions by using a non-contact method: optical group connected to a digital camera captures the images of the expanding/contracting specimen as function of the temperature with a resolution of about +/-1 micrometre per pixel. As the system allows to heat up the material and measures its longitudinal/vertical movements without any contact between instrument and specimen, it is possible to analyse the most ductile materials, such as the polymers, as well as the most fragile, such as the incoherent ceramic powders for sintering process.
For simpler measurements in a temperature range from 0 to 100 °C, where water is heated up and flow or over the sample. If linear coefficients of expansion of a metal is to be measured, hot water will running through a pipe made from the metal. The pipe warms up to the temperature of the water and the relative expansion can be determined as a function of the water temperature.
For the measurement of the volumetric expansion of liquids one takes a large glass container filled with water. In an expansion tank (glass container with an accurate volume scale) with the sample liquid. If one heats the water up, the sample liquid expands and the volume changes is read. However the expansion of the sample container must be take into consideration.
The expansion and retraction coefficient of gases cannot be measured using dilatometer, since the pressure plays a role here. For such measurements a gas thermometer is more suitable.
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[edit] References
- Hans Lehmann, refuge Gatzke Dilatometrie and differential thermal analysis for the evaluation of processes ? ? , 1956
- T.Barron Generalized theory OF thermal expansion OF solids ASM? , 1998
- A.Pippard The of element OF Classical Thermodynamics Cambridge University press, Cambridge (England), 1968
- M.Paganelli The Non-contact Optical dilatometer designed for the behaviour of Ceramic Raw Materials ,
Expert Sysem Solutions S.r.l., Modena (Italy)
