WO1987006703A1

WO1987006703A1 - Method for defining the mixture ratio in binary gas mixtures

Info

Publication number: WO1987006703A1
Application number: PCT/FI1987/000058
Authority: WO
Inventors: Pekka HIISMÄKI; Esko Kantonen; Veikko Kämäräinen; Markus Leino
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1986-05-02
Filing date: 1987-04-30
Publication date: 1987-11-05
Also published as: FI74547C; FI74547B; FI861857A0

Abstract

Method for defining the mixture ratio in a binary gas mixture, based on an accurate measurement of the speed of sound and the temperature. In the said method a wideband sound signal is made to proceed in an acoustic tube filled with the gas under measurement via two paths of differing lengths from one transmitter to one receiver, and the transit time difference (t) corresponding to the length difference (L) of the separate paths is defined as the distance of the side peaks (2, 3) of the autocorrelation function of the received signal from the origin 0. The average molecular weight M(Boolean not) of the gas mixture under measurement, as well as the partial densities of the mixture gases, are calculated from the derived formulas.

Description

METHOD FOR DEFINING THE MIXTURE RATIO IN BINARY GAS MIXTURES

The present invention relates to a method for defining the mixture ratio in binary gas mixtures, based on measuring the speed of sound and the temperature. The concept "binary mixture" here includes such multicomponent mixtures which can be divided into two parts so that the composition of each part remains unchanged and only their reciprocal mixture ratio varies.

A hygrometer based on measuring the speed of sound is described in the Finnish patent 54977, "Apparatus for measuring humidity in the exhaust steam of a drying process". The measurement with the apparatus of the specification is carried out so that the measuring sensor contains one sound transmitter and two detectors. The suggested technical embodiment comprises accurate phasewise measurement of sound transmitted, at a constant frequency. A drawback of the said method, is that it is difficult to achieve sufficient accuracy in conditions of high industrial noise. Another hygrometer based on the speed of sound is the one sold by the company Mahlo GMBH, wherein a pressure-operated injector is employed for sucking a sample flow of the atmosphere under measurement into a fluidistor oscillator, the pitch whereof is defined according to the speed of sound in the sample gas , but also accord ing to the d imens ions of the resonator which is sensitive to dirt. Sonic measuring of humidity has recently been discussed in the article by Morris & Dagle, "A Fast Response Sonic Hygrometer", Moisture and Humidity 1985, Proceedings of the 1985 International Symposium on Moisture and Humidity, Washington DC, April 15-18, 1985.

The object of the method of the present invention is to eliminate the drawbacks of the prior art arrangements. The invention is characterized in that a wideband sound signal is made to proceed in an acoustic tube filled with the gas under measurement, via two paths of differing lengths, from one transmitter to one receiver; and in that the longer path is formed of one or several parallel branches connected to the same junction of the main tube, so that the sound from the end of each side branch is reflected back into the main branch; and in that the transit time difference t corresponding to the length difference L of the separate paths is determined as the distance of the side peaks of the correlation function of the received signal from the origin; and in that the average molecular weight M of the gas mixture under measurement is calculated from the formula

or from a corresponding formula calculated for a gas more realistic than ideal gas; and that the partial densities of the mixture gases (1, 2) are calculated from the formulas

or: in that the final outcome is presented in a desired form by deriving, in a known fashion, from the above formulas or corresponding formulas applicable for a gas more realistic than ideal gas.

In the method of the present invention, the accurate measurement of the sound speed is based first of all on the use of an acoustic tube, the purpose whereof is to form an exactly defined measuring geometry and to insulate the measuring space from environmental noise. Secondly, the employed transmitted sound signal is a wideband signal, which allows the transit time to be determined on the basis of the autocorrelation function of the received signal, or on the basis of an appropriately formed crosscorrelation function. The use of the acoustic tube offers a remarkable advantage also in that the sound transmitter and receiver can be placed outside the atmosphere under measurement - which often brings about difficulties - separated by a sound-penetrating film if necessary.

With the acoustic tube, it is likewise natural to arrange two paths with differing lengths from the sound transmitter to the receiver, the transit time difference t corresponding to the respective length difference L of the separate paths being only dependent on the mechanical structure of the acoustic tube itself and on the gas contained therein, but absolutely not on the sound transmitter or receiver.

In the following the invention is explained in more detail with reference to the appended drawing, which illustrates the autocorrelation function of the sound signal recorded from the receiver.

In gases, the sound speed C is obtained from the known formula:

where

Y is the adiabatic coefficient;

R is the universal gas constant = 0,082056 [dm³. atm/mol K]; T is the temperature [K]; and is the average molecular weight.

By measuring the gas temperature and the sound speed, the average molecular weight can be calculated.

The average molecular weight in turn determines the mixture ratios in a binary gas mixture. Air, for instance, can be understood as a mixture of dry air and water vapour.

The absolute humidity of unsaturated air can be calculated from the formula

Here p is the atmospheric pressure;

M_d is the molecular weight of dry air, i.e. 28,964; M_H2O is the molecular weight of water, i.e. 18,015; and is the average molecular weight calculated from the formula (1):

The second path, needed in addition to the acoustic tube connection leading straight from the transmitter to the receiver, can be formed of a side branch placed in the atmosphere under measurement, so that the sound from the end of this side branch is reflected back to the main branch. The back-and-forth nature of the procession of the sound prevents the gas flowing speed from affecting the sound speed. The side branch may also be divided into several parallel side branches of identical lengths. The gas contained in the branch must have a sufficient connection to the atmosphere under measurement in order to allow the sensor to react quickly to the changes in the atmosphere. Thus the autocorrelation function 1 of a sound signal recorded from one single transmitter shows two side peaks 2, 3, which are located symmetrically at the distance of the transit time difference t from the origin o, as is apparent from the appended drawing.

The average molecular weight

of the gas mixture under measurement is calculated from the formula

or from a corresponding formula calculated for a gas more realistic than ideal gas. The partial densities of the mixture gases (1, 2) are calculated from the formulas

or the final outcome is presented in a desired form by deriving, in a conventional fashion, from corresponding formulas applicable for gases more realistic than ideal gas. Because the formation of the correlation function, and the calculation of the accurate transit time difference t on the basis thereof, clearly require more expensive electronics than the sensor itself, a more economical solution is often reached so that several sensors are coupled, by aid of multiplexers, in turn to the same measuring electronics.

In choosing the wideband sound signal, it must first of all be taken into account, that the simplest correlator only observes whether the signal is above or below its average (polarity correlator), but completely neglects the amplitude of the signal. Secondly it must be taken into account that while the sound proceeds in the gas-filled acoustic tube, high frequencies are clearly attenuated to a higher degree than low frequencies. The wideband signal can be either noise-type or frequencyswept. High frequencies are particularly important while aiming at an accurate definition of the transit time difference.

The method of the present invention, and particularly the mixture ratio meters constructed according to the said method, may be greatly varied within the scope of the appended patent claim.

Claims

PATENT CLAIM

A method for defining the mixture ratio of a binary gas mixture, based on an accurate measurement of the speed of sound and the temperature, c h a r a c t e r i z e d in that

- a wideband sound signal is made to proceed through an acoustic tube filled with the gas under measurement via two paths of differing lengths from one transmitter to one receiver, and

- that the longer path is formed of one or several parallel, equally long side branches connected to the same junction of the main tube, so that the sound from the end of each side branch is reflected back into the main branch, and

- that the transit time difference t corresponding to the length difference L of the separate paths is determined as the distance of the side peaks of the correlation function of the received signal from the origin, and - that the average molecular weight

of the gas mixture under measurement is calculated from the formula

or from a corresponding formula calculated for a gas more realistic than ideal gas, and

- that the partial densities of the mixture gases (1, 2) are calculated from the formulas

or that the final outcome is presented in a desired form by deriving, in conventional fashion, from corresponding formulas applicable for a gas more realistic than ideal gas.