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ACOUSTIC SPIROMETERS IN MINERS BREATHING PROTECTION PROBLEMS

SEMYON SHKUNDIN

Moscow State Mining University, Leninsky ave., 6, Moscow, 117935, 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 airdynamic structure of the flow, has no moving elements and is very sensitive and precise.

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

1. INTRODUCTION

The problem of respiratory organs protection devices development in conditions, when disasters often result in unfitness of atmosphere for breathing, remains actual. The breathing apparatuses, which are in usage in Mine Rescue Troops have to meet the following requirements:

1. Small weight and dimensions.

2. Sufficient time of protective action.

3. Simplicity and reliability in operation.

4. Minimum weight per unit of protective time.

5. The resource of a breathing indicator precence.

Today mine - rescuers use devices with compressed oxygen, which provide the duration of breathing - 4 hours. The specific feature of this apparatus is the presence of oxygen reserve for gas breathing mixture feeding. Such reserve increases the weight of the device, its overall dimensions, and decrease the reliability. Modern tendency of respirator building development is elaboration of respirators with chemically bound oxygen, in which the gas mixture, intended for breathing, is regenerated in the vehicle. The regenerating respirators are a perspective means of a mine rescue service, and their development predetermines success and safety under abnormal condition - saving works and exidents liquidation. The principle, which includes oxygen generating in respirator apparatus itself, makes it possible to decrease the overall dimensions and the mass of device, to simplify the construction and, consequently, to increase the reliability, to improve microclimatic conditions of breathing.

The breathing apparatus includes a connecting box, air feed hoses, valves of a inhale and exhale, regenerating cartridge, heat exchanger, breathing bag with a redundant valve. Breathed out air, containing carbon gas and moisture, passing by a breathing out hose through a breathing out valve gets into the regenerating cartridge, in which there is an oxygen containing product. In a cartridge a regeneration of breathing out air, i.e. replacement of a carbon gas and moisture by a oxygen occurs. Cleaned and enriched by the oxygen air gets through a heat exchanger in a breathing bag. During a breathing a gas mixture from a breathing bag through a breathing valve and the air feed hose gets into person's lungs during exhale and inhale the two valves are opened or closed alternatively.

In compressed oxygen respirators the oxygen reserve in a cylinder is being determined by pressure measurement, but for the respirator with chemically bound oxygen this method is unacceptable. In this case it is necessary to elaborate a breathing flow rate gauge to measure oxygen reserve in the oxygen - including product [1].

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 time air. 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, 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 quantities of a air-gas mixture in pulsating flows, and, 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 phase and temporary 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 apparatuses as respirator and artificial lungs. At present, this instrument is used in dynamic automatic adjustment and calibration of underground mine lifesaving apparatuses, including testing and regulation system. This apparatus can also be used for the performance of similar applications on 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.Minimal inertia (less than 2 ? 10 -3 s).

Technical Data

Type of gas tested air

Range of measurements 0.2 -5 l/s

Method of test acoustic

Accuracy of

measurements +/- 5% of full scale

Inertia 2 ? 10 -3 s

Breathing resistance of

primary converter

no more than 100 Pa

Life expectancy of

primary converter not limited

Period of switching on

not more than 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 vibration 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 block-diagram provides a layout of the following components: the channel of processing of entrance signal 8, the scheme of phase autoadjusting 13, transformeter rectangular signal into sine 5, power amplifier 4, which is connected to radiator 1, frequencies support 6, source-oscillator 7, scheme of processing of digital signals 12.

Sinusoid electrical signal is sent from the power amplifier to the acoustic radiator, the latter excitat ultrasonic vibrations in 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 systems presently available, is the ambiguity of indication at measuring of phase shifts of lager than 2 radius that may occur at the change of the velocity of sound 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 contains information on measured phase differences and are formed by controlled multivibrations. The moment the multivibrator starts, it is integrally engaged with a specific phase reference signal. The wave length of the outlet impulse is determined by the velocity of control voltage only and the latter is transmitted to the multivibrator from the low frequency filter 10. The outlet signal of phase comparator 9 undergoes low frequency filtration and it compares the moment of the multivibrator's pulse termination with the signal's leading edge at the entrance to the processing channel. These three units of the multivibrator 11, the phase comparator 9 and low frequency filter 10, 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 vibration 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.: Automated acoustic means for control of gas flow rates in mine safety systems. World Mining Congress ICAMC-92. Ekaterinburg, Russia.

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

3 Shkundin S.Z. : Phase method of acoustic anemometry. 'High school transactions', Mining Journal, #5,1990


 
 
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