Ultrasonic spirometers

There are at least a half dozen companies that use an ultrasonic flowmeter in their spirometer. The first patent for an ultrasonic flowmeter was made in the 1970’s but it wasn’t until the 1990’s that the first ultrasonic spirometers came to market. The basic idea is fairly simple and that is to measure the transit time of ultrasonic pulses through flowing gas. Pulses that travel in the same direction the gas is flowing will take less time to travel a given distance, while pulses traveling against the direction of gas flow take a longer time.

This particular measurement process is called time-of-flight (as opposed to doppler shift) and has a relatively flat flow/signal curve and frequency response. An early design of this kind of flowmeter had the ultrasonic transducers sitting in the flow of gas, but this both impedes the flow of gas and is hard to clean. A transverse design was developed that put the transducers outside the path of gas flow and this configuration has been used in all ultrasonic spirometers.

Ultrasonic flowmeter

The transit time of an ultrasonic pulse depends on the distance between the two transducers, the angle of the pulses relative to the direction of gas flow, and the speed of sound. When gas is flowing as shown in the diagram, the transit time from transducer A to transducer B is:

Up_transit_formula

and from B to A is:

Down_transit_formula

Where:

D = distance between transducer A and B

S = speed of sound

V = gas velocity

cos(C) = cosign of the angle between the pulses and gas flow

One problem with these formulas is their dependence on the speed of sound which can be affected by gas temperature, gas composition and gas pressure. Fortunately these formulas can be rearranged and simplified to:

Velocity_formula

where:

Δt = the difference in A-B transit times

tavg = the average transit time

and when this is done, the measurement of velocity is insensitive to the speed of sound.

An important point is that ultrasonic flowmeters measure the average gas velocity between transducers, not the gas flow rate. In order to derive flow rate not only must the geometrical cross-sectional area of the flowmeter must accounted for, but the flow profile through the flowmeter as well.

Laminar Flow Gas Velocity

Because of shear effects near the wall of any tube that gas flows through the velocity of gas is lower near the edge and in the center. Some spirometer manufacturers maximize the central area the ultrasonic pulses travel through by having a rectangular cross-section instead of circular.

Although ultrasonic flowmeters are theoretically linear over a very broad range of flows, like other types of flowmeters they can be affected by resistance and turbulence. This limits how small (or large) they can be and at least one manufacturer sells a range of different sized ultrasonic flowmeters for small and large animal research.

For an ultrasonic flowmeter to work correctly the ultrasonic transducers need to be carefully matched in terms of their transmit/receive frequency. Ultrasonic transducers tend to be sensitive to temperature and their resonant frequencies can change when temperature changes. For this reason some ultrasonic spirometers are heated, not for BTPS correction but in order to maintain a constant transducer temperature.

One factor that limits the sampling frequency (number of measurements per second) is reflections and echoes. These do not necessarily interfere with the measurement of transit time since the primary pulse is the first pulse received and for this reason secondary pulses can be ignored, but enough time has to pass for echoes to die down before another pulse can be transmitted. Even so, sampling frequencies are usually over 100 hz which is more than sufficient to accurately measure changes in expiratory flow.

Pulse_envelope

From patent US 5,753, 824

Finally, a pulse is not a single acoustic wave but a series of them, usually over a period of 200-300 microseconds, with a characteristic rise and fall time. This means that the accurate measurement of pulse timing depends on recognition of the entire pulse envelope rather than an individual wave. Fortunately, this kind of electronic pattern recognition was developed decades ago for sonar and radar.

Several studies have shown that ultrasonic spirometers compared well with existing test systems, and that they are both accurate and stable, even when calibration was intermittent and separated by relatively large time intervals. Routine calibration is still recommended however if for no other reason than insuring that the equipment is operating correctly.

Some manufacturers line the interior of the flowmeter with plastic (that is also transparent to the ultrasonic frequency used by the device) so they can be sterilized, but do recommend the use of standard barrier filters. Others use a disposable mouthpiece insert that keeps the keeps the interior of the flowmeter clean.

Interestingly, ultrasonic flowmeters are found mostly in simple spirometry systems. After a fair amount of search I have been able to find only three test systems capable of measuring lung volumes and DLCO (which also interestingly enough use an ultrasonic molar mass gas analyzer, but that’s a completely different subject) and two plethysmographs that use an ultrasonic flowmeter but no exercise test systems. The reasons for this are unclear. There doesn’t appear to be any technical reason why ultrasonic flowmeters can’t be used in sophisticated lab test systems although it’s remotely possible that there are some acoustical interactions with the valves and tubing needed for more complex systems that is undocumented. On the other hand the patents for ultrasonic flowmeters are held by only a small number of companies that just may not be interested in that segment of the pulmonary function testing market.

Ultrasonic flowmeters appear to be well suited to inspiratory and expiratory flow measurements since they appear to be linear, accurate, stable and mostly insensitive to temperature, humidity and gas composition. For these reasons they seem to be well suited for office spirometry and this also appears to be their primary market.

References:

Buess C, Pietsch P, Guggenbuhl W, Koller EA. Design and construction of a pulsed ultrasonic air flowmeter. IEEE Trans Biomed Eng 1986; 33(8): 768-774.

Mortimer KM, Fallot A, Balmes JR, Tager IB. Evaluating the sue of a portable spirometer is a study of pediatric asthma. Chest 2003; 123: 1899-1907.

Skloot GS, Edwards NT, Enright PL. Four-year calibration stability of the EasyOne portable spirometer. Respir Care 2010; 55(7): 873-877.

Walters JAE, Wood-Baker R, Walls J, Johns DP. Stability of the EasyOne ultrasonic spirometer for use in general practice. Respirology 2006; 11: 306-310.

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