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ACOUSTIC SPIROMETERS IN MINERS BREATHING PROTECTION PROBLEMS

SEMYON SHKUNDIN
Moscow State Mining University, Russia.

Abstract. The new means of pulsating airgas flow rate acoustic measurement is described. The main feature - special air-metric channel, supplied with ceramics electro-acoustic transducers. The principle of its operation is based on the dependence of the velocity of the acoustic vibrations arriving at the receiver upon the air-gas velocity. The device does not disturb the aerodynamic structure of the flow, has no moving elements and is very sensitive and precise.

Key Words. Acoustic Spiro meter, respirator, flow rate, measurement, air-gas velocity.


INTRODUCTION

The urgency of new methods and means for respiratory needs invention is stipulated by the irregularity of modern pneumotachometers. The last word (in quotation marks) is widely used in medical literature indicating very low level of modern devices for breathing processes control , cause tachometers can not follow quick changes in flow velocity, loosing very important diagnostics information.

(Thought, today devices based on another physical principles are also called pneumotachometers, that is very poor compliment for them). Numerous articles , including about 40 patent disclosures have been based on the acoustics. However, almost all of them found commercial application in liquid flow measurements! There are great difficulties in gas control applications because of , first of all, huge acoustics impedance difference between the gas and the solid interface, resulting in low coupling efficiency between radiating and receiving transducers. ( This is the reason why engineers speak about hydroacoustics , but much less about airacoustics).

The general development of external medical monitoring of all vital functions of an organism means has put forward a problem of respiratory diagnostics

The express registration of exhaled air parameters gives not only so-called primary information (volume of respiration, the function of respiration and its failure), but, in combination with the information about the respiratory paths resistance and the exhaled air content, also allows to receive the information on many other important physiological performances.

Different means used for gas flow rates measurement : mechanical (tachometers, rotameters), then , means based on a pressure drop on a diaphragm or specially arranged change of the channel cross section; vortex flow meters, using the aerodynamic Carman track, find a use in pneumotachometers, spirometers and spiroanalysers. However, none of the called gears satisfies modern requests in the sense of sensitivity, inertia, measurement range and, most important, accuracy.

The problem of the new level Spiro meter creation is dictated also by appearance of the new methods for diagnostics [2], where it is necessary to control respiration at frequency 20 Hz. The problem of a precisive determination of gaseous components exists also in environmental control systems, in life supporting systems for astronauts and submarines. The analysis of the physical phenomena and effects, which would allow to create new generation of spiro information systems has initiated both in the USA and in Russia, in 70 th , beginning of 80 th years the start of the acoustics abilities researches, as the most perspective. [1,2,3,4,5,6]. Some of the named above contributors from USA informed that, despite of major amount of works, could not achieve satisfactory exactitude of measurements (according to Haden the error reached 40 %) and about not cleared up completely physical effects attending the method of measurement , used by them. The common features of their elaborations are: usage of cylindrical airduct channels, differential circuit of electro-acoustic converters reswithcing, allowing to compensate already at the analog level an error from changes of a sound speed owing to a temperature, pressure, humidity or gas content variation.

It is necessary to mark, that such compensation can not be complete because the same change of a sound speed in the channels "on"(C+V) and against the stream (C-V) results in different modulo changes (C- sound speed, V -rate of flow) of the pulses delivery time because of nonlinearity of aeroacoustics interaction.

Author of the present offer would like to stress here that we consider just cylindrical air duct as the body for simultaneous flow movement and measurement, targeting creation perfect spirometer not only from technical point of view but also from the design decision beauty point of view with minimum volume of the sensor.

2. ANOTHER FIELD OF APPLICATION

Development of measurement of the respiratory flow rate apparatus is also needed for tests (including certification one) of different mine - rescue apparatuses. Respirators are being tested in special dynamic installation with artificial lungs. During this tests actual duration of a breathing, duration of a breathing apparatus before slipping in 1% CO 2 , volume of accepted carbon acid gas, temperature characteristics of carbon acid gas, amount of reduced oxygen, aerodynamic resistance of cartridge are being measured.

During the tests it's necessary to control the temperature and moisture of gas mixture, flow rate of carbon acid gas and oxygen, aerodynamic resistance of cartridge and most important parameter-lungs ventilation, i.e. the amount of breathed out air. Now this last parameter is being measured indirectly, by estimation of result volume of gone during fixed period of time. Mine rescuers need the device for visualization and control of breathing process, i.e. changes of air flow rate at any moment during inhalation or exhalation of artificial lungs, and those are the main element of respirators tests installation.

As a rule, the actual time of protective action exceeds the guaranteed one for 20 % and more that results in partial use of a resource of protective action of a vehicle and has negative effect upon tactics of mine rescue works. At the same time, measuring the charge and volume of breathed out air past through regenerating cartridge, one can find the resource of protective time developing the acoustic means of measurement of the air-gas mixture quantities, in particular, creating the primary sensor. We took into account features of acoustic measurements of the flow rates in gases. In this connection it was necessary to decide a number of problems, namely:

1. To receive the high ratio of signal / noise ( not less than 20 Db ) at reasonably small significance of a excitation voltage of the electro-acoustic radiator.

2. To carry out experimental research to compare the device operation in current and impulse regime, and phase and time lag methods of measurements with reference to a particular primary measuring converter.

3. To investigate influence of the various destabilizing factors of a controllable gas mixture upon the sound speed, and consequently on a measuring range.

3. BASIC CHARACTERISTICS

The Moscow State Mining University has developed an ultrasonic flowmeter - spirometer for measuring of pulsing flows, e.g. for measuring of capacity of breathing in various reviving and sustaining apparatus as respirator and artificial lungs. At present , the instrument has been used

for breathing flows measurements, in dynamic automatic adjustment and tests of underground mine rescue apparatuses, including registration and indication of all general and particular characteristics. This apparatus can also be used for the performance of similar applications with medical equipment [2].

The apparatus includes a primary measuring transducer and an electronic block. The principle of its operation is based on the dependence of the velocity of the acoustic vibrations arriving at the annular receiver upon the air-gas velocity. The apparatus measures the momentary value of the volume and records the data in its memory process and integrates it during a fixed period of time; i.e. inhale and exhale, etc. The result is the determination of the air-gas mixture passed through the measuring conduit.

Merits

1.The above process of measuring does not disturb or inhibit the regime of flow being measured.

2.No moving elements and rolling parts.

3.Low air-dynamic resistance of the sensitive element, relative to the total length and cross section of the passage.

4.Wide dynamic range, up to 200.

5.The possibility of measuring low rates of flow (up to 200 ml/s).

6.Minimum inertia (less than 2 ? 10 -3 s).

Technical Data

Type of gas tested gas mixture

Range of measurements 0.2 - 10 l/s

Method of measurement acoustic

Accuracy of

measurements +/- 5% of full scale

Breathing resistance of

primary converter no more than 100 Pa

Life expectancy of

primary converter not limited

Period of switching not more than

on 1 min

4. PRINCIPLE OF MEASUREMENT

The main principle of measuring is the excitation of ultrasonic vibration in the air-gas mixture moving through the measuring conduit and the reception of these vibrations by a piezoelectric receiver. The comparison of the phases of the radiated and the received vibrations provides the momentary rate of flow. The main feature of the above method is the immediate introduction of vibration into the measured medium without reflecting/refracting surfaces and sound conductors [3].

The enclosed photo presents one implementation of the measuring principle described above.

The electronics block includes generator , which generates sinusoid electrical signal, the latter after amplification in the power amplifier agitates the acoustic radiator, which radiates the ultrasonic vibrations in power to the moving air-gas mixture. The rate of vibration propagation velocity depends on the air-gas mixture velocity and on the sound velocity in the fixed medium. The received signal is amplified, filtered, and modified into rectangular pulses, which are phase shifted relatively reference pulse proportionally only to the flow rate with the permanency of the velocity of sound and channel section.

The general deficiency of typical phase measuring power systems presently available, is the ambiguity of indication when measuring of the phase shifts lager than 2 p that may occur during the change of the sound velocity within wide limits. Broadening of the measurement range of the above spirometer is realized by means of an autophase lock mode which provides the necessary protection from electric or acoustic interferences. The essence of this method is that the outlet pulses of the phase measuring scheme containing information about measured phase differences and are formed by controlled multivibrator . The moment of the multivibrator impulse starts, it is integrally engaged with a specific phase reference signal. The duration of the outlet impulse is determined by the flow velocity only and the latter is being transmitted to the multivibrator. The outlet signal of phase comparator undergoes low frequency filtration and it compares the moment of the multivibrator's pulse termination with the signal's leading edge at the entrance of the processing channel. These three units: the multivibrator , the phase comparator, and low frequency filter maintain a reserve backup system, the aim of which is to provide inertial synchronization of the leading edge of the multivibrator signal.

The scheme of the digital signal processing system is intended for integration of momentary rates of flow and for compensation of measurement error. The special advantage of this instrument is the flowmeter channel with its special design within which the air-acoustic interaction takes place. The method of introduction of the acoustic vibrations into the channel and the isolation from any kind of disturbances are embodied in the unique system design.

5. REFERENCES

1. Puchkov L.A., Shkundin S.Z., oth. The method of measurement of airgas flow velocity. Author's certificate 16822590, 1991

2 . Schmid E R ; Knopp T J;

Rehder .K P. Intrapulmanory gas transport and propusion during high- frequency oscilation. J. Appl.Physiol. Respir.Environ.Exercise Physiol . 51: 1507-14 1981.

3. Hagen R.W, Thomas L.J ,McCartney. An Ultrasonic ventilometer.

Proceedings of the Annual Conference on engineering Medicine and Biology, 20:272,1978.

4 . Plaut D.I and Webster J.G Design and Constraction of an Ultrasonic Pneumotachometer. IEEE proceedings, 1980, January-11-19.

5. Kou A.H , Peickert W.R , Polenske E.E , Busby M.G, A Pulsed phase measurement ultrasonic flowmeter for medical gases. Annals of Biomedical Engineering. Vol 12, 1984,

6. The theory of acoustic anemometry. (In Russian). Shkundin S Z, Kremliova O.A, Rumiantceva V.A. Moscow, 2001.

 
 
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