Apparatus and Method for Measuring a Gas Flow

Technical Field

The present invention relates to an apparatus for measuring a gas flow as defined in the preamble of claim 1. It also relates to a method of measuring a gas flow as defined in the preamble of claim 14.

Background and Prior Art

It is well known to measure a gas or liquid flow using ultrasound technology. The measurement results in a value of the volumetric flow through the flow sensor. Such measurements have the advantage that the result is independent of the composition and other properties of the fluid. Some other methods of measuring the flow of a fluid are dependent on the mechanical properties, i.e. density and viscosity, of the fluid or the thermal properties such as thermal conduction and thermal capacity. This measurement method may be used, for example, to determine the volumetric flow of respiration gas in a ventilator. Such gas consists mainly of N2, O2, CO2 and H2O.

In some cases when measuring a gas, the volumetric flow is not relevant. Instead there may be a desire to measure the mass flow. If temperature, pressure and composition of the gas are all constant, the volumetric flow could be used to calculate the mass flow. This is also possible if the variations are slow, so that they can be measured by available pressure, temperature and gas concentration sensors. If the variations are faster sensors having sufficiently low time constants may not be available. In particular it is difficult to find temperature sensors that have a sufficiently low time constant, that is, that will react fast enough to changes in the temperature. The same is valid for the composition of the gas. Gas concentration sensors that will react fast enough are generally not available.

US 6 199 423 discloses a method of measuring the mass flow of a gas using ultrasound sensors. Time-of-flight signals are obtained from the ultrasound sensors

in the way common in the art and are used together with the pressure and the temperature of the gas flow to calculate the mass flow. The method is adapted to non-ideal gas mixtures of complex components. Some of the adaptations made according to US 6 199 423 make this method unsuitable for measuring a flow of breathing gas in a ventilator.

Object of the Invention

It is an object of the invention to enable the reliable measurement of the mass flow of a gas, such as a mixture of oxygen and nitrogen, using ultrasound transducers.

Summary of the Invention

This object is achieved according to the present invention by an apparatus for measuring a gas flow φ through gas flow channel, comprising at least one ultrasound transducer arranged to emit and receive at least a first ultrasound signal through the gas flow in the channel to obtain at least a first time-of-flight signal, a pressure sensor P, and means for determining a volumetric gas flow through the gas flow channel, said apparatus being characterized in that it comprises calculating means arranged to calculate a mass flow of gas by the following steps: obtaining the at least one time of flight signal, calculating the sound velocity for the gas, and calculating a relationship between the actual volumetric flow, the actual sound velocity, the actual pressure and a constant γ ₀ .

The object is also achieved by a method of calculating a mass flow of gas using ul- trasound sensors, said method comprising the following steps: obtaining at least one time of flight signal for the upstream and downstream, measuring the gas pressure in the flow, determining the instantaneous volumetric flow and sound velocity for the gas and calculating a relationship between the actual volumetric flow, the actual sound ve- locity, the actual pressure and a constant γo.

The apparatus and method of the invention are particularly useful in ventilation of patients. In this case the gas is substantially comprised of oxygen and nitrogen in varying concentrations. In the expiration gas levels of up to 5% CO2 and H2O may be present, but the variations in these levels are limited and the thermal properties of such a gas mixture are close to those of pure two-atomic gas mixtures, and vary mainly due to variations in the O2-N2-composition . In gas flow measurements during ventilation it is a problem that both pressure and temperature may vary quickly during the respiratory cycle. Therefore, measuring a momentary volumetric gas flow will not be meaningful. In this case, a mass flow will be more relevant.

It is also possible to define a reference state, for which the mass flow can be used to calculate a volumetric flow, if the composition of the gas is known.

Preferably, the calculating means is arranged to calculate the instantaneous volumetric flow φactuai according to

φ tofup - tofdo actual = *1 where tof _up and tof _dO are the time-of- flight upstream and downstream and k _t is a first calibration constant.

Further, the calculating means is preferably arranged to calculate the sound velocity Cactuai for the gas according to

tof _up + tof _do

^C actual — ^2 tof _up tof _do where tof _up and tof _do are the time-of-flight upstream and downstream and k ₂ is a second calibration constant.

In case the volumetric flow is obtained from a separate sensor, an ultrasonic sensor may be placed orthogonal to the gas flow to obtain one time-of-flight signal. Alter-

natively, two ultrasonic sensors placed opposite from each other may be used to obtain one time-of-flight signal. If only one time-of-flight signal, tof, is obtained, orthogonal to the gas flow, the sound velocity may be calculated according to c _actual = 2k ₂ /tof.

The calculating means may be arranged to calculate the mass flow W(t) according to

where p _actua i(t) is the pressure measured in the gas flow and γ ₀ is a constant.

Yo may be selected such that γo is a good approximation of γ(t).

The calculating means is further arranged to calculate a volumetric flow of gas. This is particularly useful in applications where both the volumetric flow and the mass flow are needed, since it enables the calculation of both the volumetric and the mass flow with only one flow sensor.

The apparatus according to the invention is particularly well suited for measuring a gas flow in a gas tube of a ventilator, said gas tube being arranged to transport breathing gas to a patient or expiratory air from the patient.

Brief Description of the Drawings

The invention will be described in more detail in the following, by way of example and with reference to the appended drawings in which: Figure 1 shows, schematically, an arrangement for measuring the gas flow in a channel, known per se.

Figure 2 is an overall flow chart of the method according to the invention.

Figure 3 illustrates a measurement arrangement for measuring the washout of O2 in which the inventive arrangement shown in Figure 1 may be used. Figure 4 illustrates a second embodiment of the invention.

Detailed Description of Embodiments

In Figure 1, a first preferred embodiment of the invention, a gas flow φ passes through a gas flow channel. A first and a second ultrasound transducer, Tl, resp. T2, are arranged to measure the gas flow in the channel, according to the prior art. A pressure sensor P is also included. As is common in the art, both transducers Tl, T2 act as both transmitter and receiver. A pulse train is transmitted from the first transducer Tl and received by the second transducer T2, that is, in essentially the same direction as the gas flow. The time of flight downstream tof _do is measured. Then a pulse train is transmitted in the opposite direction, from T2 to Tl, and the time of flight upstream tof _up is measured. The time of flight upstream and the time of flight downstream will differ and can be used to indicate the gas flow volume. The skilled person is familiar with such methods. The relative positions of the transducers Tl, T2, and the pressure sensor P, can be varied, as is well known in the art.

According to the invention, the time of flight values tof _up and tof _do can be used, together with the pressure value, to determine the mass flow of gas, according to the following:

The time of flight signals obtained from the ultrasound transducers are used to calculate two values:

1) φ _actuab which is proportional to the flow velocity, and thus to the volumetric flow at a particular state of the gas:

_ tof _up - tof _do

^actual ~ ^λ l , _r , _r V ¹ / tofup tofdo

2) c _actual , which is proportional to the velocity of sound in the gas:

tof _up + tof _a do

^C actual — ^2 (2) tof _up tof _do

In equations (1) and (2), Ic ₁ and k ₂ are calibration constants dependant on the distance between the transducers. In addition, Ic ₁ is dependent on the cross-sectional area of the channel and the distribution of the flow across it. The ultrasound measurements are performed fast, typically in a tenth of a millisecond, and therefore gives essentially instantaneous values for φ _actua i and c _actUa i- The subscript "actual" indicates that the flow and the sound velocity values obtained represent the flow and sound velocity at the present state of the gas, that is, the present pressure and temperature.

In a second embodiment, which will be discussed in connection with Figure 4 below, a separate sensor is used to determine the volumetric flow. The time-of-flight signal is obtained from an ultrasonic sensor placed orthogonal to the gas flow.

In this embodiment, equation 1 is not needed, and equation 2 above can be modified to

C _actual = 2 Jc ₂ /tof (T)

The velocity of sound in the gas depends on the molecular weight (M) and the absolute temperature (T) according to equation (3)

R c = γ M

(3)

M

where γ is the quotient of the specific heat capacities at constant pressure and constant volume, respectively (C _p /C _v ), R _M is the universal gas constant (R _M = 8.3143 J/mol K), M is the mean molecular weight of the gas mixture (expressed as kg/mol), and

T is the absolute temperature (expressed as Kelvin).

The quotient γ= 1.40 for two-atomic gases at temperatures up to approximately 400 K.

The volumetric flow can be used to calculate a reference state according to the following: The mass flow through the ultrasound sensor can be expressed at a given state of the gas {p,T}, or at a reference state, as the volumetric flow multiplied by the density of the gas. That is:

massflow = φ _actual p _actual = φ _ref 9 _ref (4)

φ _ref is the volumetric flow at a (theoretical) reference state [m ³ /s] P _ref is the density of the gas at the reference state [kg/m ³ ] φ _actua i is the volumetric flow at the current state [m ³ /s]

P _actua i is the density of the gas at the current state [kg/m ³ ]

Thus, this equation can be seen as a definition of the volumetric flow at the theoretical reference state.

The density, p, is determined from the ideal gas law:

P _ ^R M _τ

(5) p M

Substituted in equation (4) this yields (after eliminating the gas constant R _M and the molecular weight M):

ref rp actual j, (6)

-* ref * ^■ actual

Equation (6) may be used to calculate the flow in a reference state defined by the selected pair {p _ref ,T _ref }, based on the volumetric flow for the current state.

According to the invention an instantaneous value for the following expression is calculated:

W(t) =φ _actual (t)^— _Pactual (t) (7)

^C actual \ * )

Thus, the current values of the volume flow and the sound velocity, φ _actua i and c _actua i, are obtained from the ultrasound sensor, while the current pressure p _actua i is obtained from a separate pressure sensor. γ ₀ is a constant factor.

Substituting (3) in (7) yields:

Yo M(t) Pactualit)

Wφϊ →actuati) (8)

Y(O K τ _actual (t)

By comparison with equation (6) equation (8) can be rewritten as

using the ideal gas law (5), we find that equation (9) may be rewritten as

This means that if the constant γo has been selected in such a way that it is a good approximation of γ(t), the expression W(t) can be interpreted as the instantaneous mass flow through the ultrasound sensor (see equation 4). If γ ₀ is not absolutely correct the expression will still yield a value that is proportional to the mass flow as long as γ(t) is substantially constant.

In summary, by instantaneously calculating the following expression based on measurement data from the ultrasound sensor and the pressure sensor, {φ _actua i ₅ c _ac- tuai} and {pactuai}, respectively:

the instantaneous mass flow through the sensor can be obtained, provided that the factor γ ₀ is selected correctly. This is valid as long as γ(t) is substantially constant, which is known to be the case for mixtures of two-atomic gases at moderate temperatures. As can be seen from Eq. (10), if γo/γ(t)=l, W(t) is equal to the mass flow. Thus, if γo/γ(t)=l, the mass flow can be expressed as in Eq. (11).

Thus, the method according to the present invention can be summarized as in figure 2

Step Sl: Obtain the time of flight signals for the upstream and downstream Step S2: Calculate the instantaneous volumetric flow and sound velocity for the gas (equations (1) and (2)) Step S3: measure the pressure in the gas flow.

Step S4: calculate a relationship between the actual volumetric flow, the actual sound velocity, the actual pressure and a constant γo according to equation (11), which can be interpreted as the instantaneous mass flow through the ultrasonic sensor.

As will be understood, step S3 can be performed at any point in the procedure before step S4. As discussed briefly above, only one time-of-flight signal is needed if a separate flow sensor is used, which may be obtained by one or two ultrasonic sensors placed orthogonal to the gas flow in step Sl.

Figure 3 illustrates the use of the inventive apparatus in the ventilation of a patient, represented by a pair of lungs 51. The patient is ventilated using a ventilator 53 which is connected to the patient through a Y piece 55 which interconnects a first tube 57 for breathing gas provided by the ventilator and a second tube 59 for remov- ing expired air from the patient, with the patient's lungs in the way common in the art. As is well known, the breathing gas will normally comprise an appropriate mixture of air and O2. The expired air is output to the surroundings from the ventilator at a gas outlet 61, for example through a valve 63. The gas flow may be measured at the gas outlet 61 or the valve 63. As shown in Figure 3 according to the first em- bodiment of the invention, an ultrasonic flow sensor 65 as shown in Figure 1 is provided near the valve 63, upstream or downstream of the valve 63. The ventilator itself, as well as the flow sensor 65 are controlled by one or more control units, which are represented in Figure 3 by one control unit 69. The sensor 65 as well as the control unit 69 may be located externally or may be integral parts of the ventila- tor 53. In Figure 3, the control unit 69 is also used for performing the calculations discussed in connection with Figure 2. These calculations may of course be performed in a separate calculation unit of the ventilator, or in an external unit. By using an inventive flow sensor as the flow sensor 65 both the volumetric flow and the mass flow of gas can be determined at the same time. This is advantageous in appli- cations where both these values are needed. One such application is when monitor-

ing the amount of a particular gas that is washed out. To do this, the composition of the gas, and the volume of gas are needed, as discussed in more detail in the co- pending application filed on the same date as the present application.

Figure 4 illustrates an alternative embodiment, which is similar to that shown in Figure 3, except that a separate flow sensor 67 is used to obtain the volumetric flow. The sensor may be placed upstream or downstream of the valve 63 and may, for example, be one of the following:

• a differential pressure flow sensor • a thermal flow sensor

• a vortex shedding sensor

In this case, the time-of flight signals upstream and downstream are not needed. The velocity of sound is preferably measured orthogonal to the flow. In this case only one ultrasonic transducer Tl is needed, instead of two as shown in Fig. 1, since the signal will be reflected by an object such as a wall placed opposite of the transducer Tl. Alternatively, two ultrasonic transducers may be used, placed on opposite sides orthogonal to the flow. The relative positions of the transducer/s, and the pressure sensor P, can be varied, as is well known in the art.

In this embodiment, equation 1 is not needed. Equation 2 above can be modified to C ac _t t _u ua _l l = 2 k 2 ₂ /tof J

The embodiment shown in Figure 4 is useful in applications where the volumetric flow is already measured according to another method.