Reduction of pressure induced temperature influence on the speed of sound in a gas
An apparatus for determining the proportion of gases in a gas mixture, has a measurement chamber having a chamber defining structure, a gas inlet and a gas outlet, an ultrasound source and an ultrasound detector mounted such that the ultrasound source is capable of transmitting ultrasound through the chamber to the ultrasound detector; a temperature sensor mounted such that the sensor is capable of sensing the temperature in the chamber. The chamber defining structure is adapted to amplify thermal exchange with a gas content in the chamber so as to suppress a temperature change in the chamber.
1. A method of dimensioning a measurement apparatus for determining the proportion of gases in a gas mixture by means of ultrasound detection, comprising the steps of:
determining a maximum allowable error in the determined proportion of a gas in the gas mixture;
estimating the magnitude of maximum occurring pressure variation in the gas mixture;
calculating the magnitude of maximum occurring temperature variation in the gas mixture dependent on the estimated magnitude of occurring pressure variation in the gas mixture;
calculating a maximum allowable time lag for thermal exchange between the gas mixture and the measurement apparatus dependent on the maximum allowable error in the determined proportion of a gas in the gas mixture and the magnitude of occurring temperature variations in the gas mixture;
adapting the measurement apparatus such that the time lag for thermal exchange between the gas mixture and the measurement apparatus is less than or equal to (≦) the calculated maximum allowable time lag for thermal exchange.
2. The method of claim 1 , further comprising selecting a combination of shape, size and material of a measurement chamber in the measurement apparatus in relation to properties of the gas mixture such that the time lag for thermal exchange between the gas mixture and the measurement apparatus is less than or equal to the calculated maximum allowable time lag for thermal exchange.
3. The method of claim 1 , further comprising selecting a time lag of a signal filter for application on a signal from an ultrasound detector in the measurement apparatus dependent on predetermined requirements on the response time of the measurement apparatus.
4. The method of claim 3 , wherein the maximum allowable time lag for thermal exchange between the gas mixture and the measurement apparatus is calculated also dependent on the time lag of a signal filter for application on a signal from an ultrasound detector in the measurement apparatus.
5. The method of claim 1 , wherein calculating a maximum allowable time lag for thermal exchange between the gas mixture and the measurement apparatus comprises the steps of:
calculating the magnitude of occurring error in the determined proportion of a gas in the gas mixture dependent on the magnitude of occurring temperature variation in the gas mixture;
calculating a maximum allowable detected temperature variation due to pressure variation;
determining a relation between the maximum allowable detected temperature variation and the maximum occurring temperature variation in the gas mixture due to pressure variations;
determining a relation between the time lag for thermal exchange between the gas mixture and the measurement apparatus and the time lag of a signal filter for application on a signal from an ultrasound detector in the measurement apparatus;
determining a relation between said temperature relation and said time lag relation;
calculating a maximum allowable time lag for thermal exchange between the gas mixture and the measurement apparatus dependent on said relation between said temperature relation and said time lag relation.
6. The method of claim 1 , wherein the time lag is the time constant.