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How Rüchardt experiment yields the adiabatic exponent of gas

determining the temperature of the thermodynamics

in the smoked daily mercury thermometer operates on the principle of the change in volume of the metal when the temperature changes. A certain Amount of mercury, depending on the temperature reaches a certain level in the tube indicated, that points to a room temperature. But what if the area to be measured in an extreme temperature condition is that mercury would therefore useless, because as it evaporates and therefore can not serve as a measurement method? For such cases we have developed other methods whose basic skills are not least in the aviation industry and the relevant importance.

to reality: The gas thermometer
To under extreme conditions where you would encounter with a conventional mercury thermometers to the material stress limits measure (eg, 1,000 ° C), the temperature, the gas thermometer works on the principle of measuring the gas pressure of a constant amount of gas whose volume is more specific for the type of temperature change varies. This must be determined first, as the gas used behaves as air at a heating concrete. Only with this knowledge you can determine the specific extension per unit of temperature and pressure so that the potential difference of the system, which is essential for a reliable temperature measurement.

theory

compressed to a gas adiabatically, ie without flow of heat from the system, then increases more due to the temperature increase the pressure as in an isothermal compression, where the resulting Kompessionsarbeit is dissipated as heat. The exponent with which to calculate the expansion of the gas through the heat can be as isotropic or adiabatic with the Greek letter "kappa" (symbol: κ ) denotes . This value is determined by the Rüchardt experiment.

An adiabatic and revisible running state change is isentropic (entropy S remains the same, the temperature T changes), so called adiabatic exponent and the exponent or adiabatic. In the technique is generally an adiabatic change of state (eg in a steam turbine) not isentropic because produce friction, flow and impact processes entropy.

The formulas this:

p * V ^ k = constant

(P2/p1) = (V1/V2) ^ k

isentropic exponent for different gases
Temp,	gas	κ		Temp,	gas	κ		Temp,	Gas	κ
-181 ° C	H2	1.597		200 ° C	Tr. Air	1.398		20 ° C	NO	1.40
-76 ° C		1,453		400 °C		1,393		20 °C	N2O	1,31
20 °C		1,41		1000 °C		1,365		–181 °C	N2	1,47
100 °C		1,404		2000 °C		1,088		15 °C		1, 404
400 °C		1,387		0 °C	CO2	1,310		20 °C	Cl2	1,34
1000 °C		1,358		20 °C		1,30		–115 °C	CH4	1,41
2000 °C		1,318		100 °C		1,281		–74 °C		1,35
20 °C	He	1,66		400 °C		1,235		20 °C		1,32
20 °C	H2O	1,33		1000 °C		1,195		15 °C	NH3	1,310
100 °C		1,324		20 °C	CO	1,40		19 °C	Ne	1,64
200 °C		1,310		–181 °C	O2	1,45		19 °C	Xe	1,66
0 ° C	Tr. Air	1.403		100 ° C		1.399		360 ° C	Hg	1.67
20 °C		1,40		200 °C		1,397		15 °C	C2H6	1,22
100 °C		1.401		400 ° C		1.394		16 ° C	C3H8	1.13

The isentropic exponent is defined as the ratio of specific heat capacity at constant Pressure (cp) and constant volume (CV)

The value κ is 1.402 in dry air. When moist air can for expansion, coming as a result of cooling water for the failure: by the released heat of condensation exponent is lower.

process of the experiment:

for the experiment is a glass vessel (volume: 0.0025m ³), which has a tube-shaped top opening available. On the open tube is an air-tight flask dropped, compressed with its mass of 70 g air-constant air volume (Molarkonstante n) a certain volume is withdrawn. The resulting compressed air is then compared to the surrounding a compressible mission-related pressure, leading to an increase in temperature. A positive feedback occurs through this process. The pressure force pushes the falling piston back up, which then triggers the reverse process. Thus, on cushioning way in shrinking up and down movements after a certain time is reached, also due to friction influence the Stillstad. It measures the temperature and relative Pressure in the vessel.

The constant up and down by the moving piston equivalent behavior to the appropriate air pressure causes a change in temperature, as measured by laboratory instruments. If you create parallel to the measurements of a physical model of this experiment, the theoretical Scheme interaction of different sizes are investigated. By displaying the measurement results during the period of oscillation in a chart chronologically, the viewer falls on something: The modeled curve in the pt chart suggests much thicker than the measured one. This is due to the heat loss through the glass.

Theoretical Grundidee for the project is an isentropic compression of a gas (here air). As an isentropic compression (constant entropy) is impossible in practice, however, must be the Rüchardt's experiment can be modeled to the influences of the laboratory equipment (heat loss in the vessel, pressure loss, etc.) to clean up.
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* Arthur Louis was an American geophysicist who in 1900 extended the gas thermometer to a high temperature measurement method to 1150 ° C .

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Sources:

- Wikipedia: Link: http://de.wikipedia.org/wiki/R% C3% BCchardt experiment

- Haliday Physics

- System Design Prof . W. Maurer, ZHAW

Photo: Stephan Riediker, Nicole Ulrich