Device and method for determining mixing ratios of flowing media

Abstract

The present invention relates to a device and a method for determining mixing ratios of flowing media, in particular for determining the mixing ratios of two gases by using two flow resistances with different characteristic curves, each flow resistance containing a differential pressure sensor and being connected in series, where one flow resistance is formed by a sintered metal filter and another flow resistance is formed by an orifice.

Claims

1. A device for determining the fluid concentration of a flowing fluid mixture, the fluid mixture substantially containing two fluid components, wherein a flow conduit which contains at least two flow resistances each containing a differential pressure sensor and being connected in series, the at least two flow resistances having different characteristic curves.

2. The device according to claim 1, wherein one of the flow resistances has a linear characteristic curve and another flow resistance has a square characteristic curve.

3. The device according to claim 1, wherein one flow resistance is formed by a sintered metal filter and another flow resistance is formed by an orifice.

4. The device according to claim 1, wherein the fluid mixture is gaseous.

5. The device according to claim 4, wherein the fluid mixture contains CO2 and air.

6. A method for determining the fluid concentration of a flowing fluid mixture, the fluid mixture substantially containing two fluid components, comprising the steps of: a) conducting the fluid mixture through a flow conduit which contains at least two flow resistances, wherein at least one flow resistance contains a differential pressure sensor and the other flow resistance contains either a differential pressure sensor or a mass flow sensor and which are connected in series, wherein the at least two flow resistances have different characteristic curves, and b) determining the fluid concentration by determining the respective pressure drop over the flow resistances by means of a calibration curve.

7. The method according to claim 6, wherein one of the flow resistances has a linear characteristic curve and another flow resistance has a square characteristic curve.

8. The method according to claim 6 or 7 wherein one flow resistance is formed by a sintered metal filter and another flow resistance is formed by an orifice.

9. The method according to claim 6, wherein one of the flow resistances includes a thermal mass flow sensor.

10. The method according to claim 6, wherein the fluid mixture is gaseous.

11. The method according to claim 10, wherein the fluid mixture contains CO2 and air.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an illustration of pneumatic circuit measurement set-up.

(2) FIG. 2 shows an illustration of calibration set-up.

(3) FIG. 3 shows calibration curves for air and CO.sub.2.

(4) FIG. 4 shows a simplification for determining the dp1/(dp2) values of air and CO.sub.2.

(5) FIG. 5 shows reasons for simplification by First Intercept Theorem of elementary geometry.

(6) FIG. 6 shows the dimensions of an orifice.

(7) FIGS. 7A and 7B show a sintered metal filter hollow cylinder made from CrNi-steel. FIG. 7A shows a sinter metal filter as a hollow cylinder (view from the outside) and FIG. 7B shows a sinter metal filter with a pore size of 35 m.

DETAILED DESCRIPTION OF THE INVENTION

(8) Theoretical Approach

(9) Due to the dependence of the flow resistances on the flow properties of the medium, the composition of gas mixtures can be determined. The condition is that the flow properties of the gases do not significantly differ. In principle, this correlation also applies for liquids.

(10) Measurement and Calibration Set-Up

(11) For determining the concentration ratios, the medium is conducted through a series connection of two flow resistances. The respective differential pressure drop over the flow resistance is recorded and used as a basis for determining the concentration ratios. The condition for the differentiation of different media portions is the use of two flow resistances with different characteristic curves, e.g., linear and square. In the measurement set-up shown here, this is achieved by a sintered metal filter (linear) and an orifice (square). Furthermore, by means of a pressure sensor, the absolute pressure (p_abs) is obtained, by means of which errors caused by pressure variations are compensated (see below). Another aspect of the set-up is a temperature measurement of the gas for correcting any potential temperature variations (see below).

(12) For using the measurement set-up, a prior calibration with the corresponding media is required. This is necessary for both mixture components to be measured. An exemplary set-up for calibration is shown in FIG. 2. By plotting the differential pressure dp1 versus the differential pressure dp2, a characteristic curve is generated for the medium (FIG. 3), which is used for the later calculation of the concentration percentages (see below).

(13) For generating a volume flow, a pneumatic pump is employed, with the control of the volume flow being effected by a throttle valve 1. The throttle valve 2 serves for generating different underpressure levels over the complete measurement path, for determining the parameters of the absolute pressure correction (see below).

(14) In the example of the gas mixture of gas 1 and gas 2 to be measured here, a calibration is made with pure gas 1 or pure gas 2. The characteristic curves determined from such a mixture are shown in FIG. 3 as an example of a mixture of air (gas 1) and CO.sub.2 (gas 2).

(15) Herein, as an example, a calibration and subsequent measurement of an air/CO.sub.2 mixture was made. The gas conduits had a diameter of 8 to 10 mm. Gas flows up to 30 l/min were achieved. Before the measurement path (see FIG. 1), pressures up to 150 mbar relative to the atmosphere were measured, by means of a sensor from the HDI series of the company Sensotronics, which operates in the range from 1 bar (14.5 psi) to +1 bar (+14.5 psi). The differential pressure sensors over the flow resistances had a measurement range from 0 to 10 mbar (0.145 psi). For this purpose, sensors of the HDI series of the company Sensotronics were employed (HDIM010DUF8P5). As flow resistances, an orifice (FIG. 6) and a sintered metal filter (FIG. 7) with a pore size of 35 m were used. As shown in FIG. 7, the sintered metal filter was configured in the form of a hollow cylinder made from CrNi-steel.

(16) It was found that an accuracy of the calibration being sufficient for many purposes can already be achieved by calibration with one gas only, with the characteristic curve of the second gas being calculated. This enables, for instance, a regular calibration for an air/CO.sub.2 measurement by determination of the characteristic curve for air with calculation of the characteristic curve for CO.sub.2. In this way, the determination of the characteristic curve of CO.sub.2 being otherwise necessary can be dropped.

(17) Basic Equations

(18) The following equations or conditions apply:

(19) $\begin{array}{cc}R=\frac{\mathrm{dP}}{Q}\begin{array}{c}\mathrm{applies}\mathrm{for}\mathrm{small}\mathrm{pressure}\mathrm{variations}\\ \mathrm{or}\mathrm{for}\mathrm{linear}\mathrm{flow}\mathrm{resistances}R\end{array}& i.)\\ \frac{c\left[\mathrm{Gas}1\right]}{c\left[{\mathrm{CO}}_2\right]}=\frac{R_{\mathrm{Gas}1}}{R_{\mathrm{Gas}2}};c=\mathrm{concentration},R=\mathrm{flow}\mathrm{resistance}& \mathrm{ii}.)\\ \frac{\mathrm{dp}{2}_{\mathrm{Gas}2}}{R{2}_{\mathrm{Gas}2}}=\frac{\mathrm{dp}{1}_{\mathrm{Gas}2}}{R{1}_{\mathrm{Gas}2}}\frac{\mathrm{dp}{2}_{\mathrm{Gas}1}}{R{2}_{\mathrm{Gas}1}}=\frac{\mathrm{dp}{1}_{\mathrm{Gas}1}}{R{1}_{\mathrm{Gas}1}}& \mathrm{iii}.)\end{array}$

(20) In words: The characteristic curves of the flow resistances significantly differ depending on the media concentration.

(21) Calculation of the Mixing Ratio for an Example Gas 1 and Gas 2

(22) In the following calculations, the gas 2 portion is the target value to be determined. If there is a mixture of gas 1 and gas 2, the flow resistances are made up approximatively as an addition of the flow resistances R.sub.Gas2 and R.sub.Gas1 together:
R1.sub.misch=n.Math.R1.sub.Gas1+(1n).Math.R1.sub.Gas2; R2.sub.misch=n.Math.R2.sub.Gas1+(1n).Math.R2.sub.Gas2
n=portion of Gas1 or Gas2, 0n1

(23) For the measurement of a gas mixture follows from (i.):

(24) $\frac{\mathrm{dp}2}{R{2}_{\mathrm{misch}}}=\frac{\mathrm{dp}1}{R{1}_{\mathrm{misch}}}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{R{2}_{\mathrm{misch}}}{R{1}_{\mathrm{misch}}}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{n*R{2}_{\mathrm{Gas}1}+\left(1-n\right)*R{2}_{\mathrm{Gas}2}}{n*R{1}_{\mathrm{Gas}1}+\left(1-n\right)*R{1}_{\mathrm{Gas}2}}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{n*R{2}_{\mathrm{Gas}1}+R{2}_{\mathrm{Gas}2}-n*R{2}_{\mathrm{Gas}2}}{n*R{1}_{\mathrm{Gas}1}+R{1}_{\mathrm{Gas}2}-n*R{1}_{\mathrm{Gas}2}}$$\mathrm{With}$$R=\frac{\mathrm{dp}}{Q}$$\mathrm{follows}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{\left(n*\mathrm{dp}{2}_{\mathrm{Gas}1}+\mathrm{dp}{2}_{\mathrm{Gas}2}-n*\mathrm{dp}{2}_{\mathrm{Gas}2}\right)*Q}{\left(n*\mathrm{dp}{1}_{\mathrm{Gas}1}+\mathrm{dp}{1}_{\mathrm{Gas}2}-n*\mathrm{dp}{1}_{\mathrm{Gas}2}\right)*Q}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{n*\mathrm{dp}{2}_{\mathrm{Gas}1}+\mathrm{dp}{2}_{\mathrm{Gas}2}-n*\mathrm{dp}{2}_{\mathrm{Gas}2}}{n*\mathrm{dp}{1}_{\mathrm{Gas}1}+\mathrm{dp}{1}_{\mathrm{Gas}2}-n*\mathrm{dp}{1}_{\mathrm{Gas}2}}$

(25) Assume a Vertical Concentration Behavior:

(26) For the calculation of the gas concentration, the following assumption is made:
dp2=dP2.sub.Gas2=dP2.sub.Gas1

(27) This corresponds to a simplification, since the real concentration behavior from one to another characteristic curve, that is, e.g., from 100% gas 2 to 100% gas 1 does not proceed vertically, but along an inclined line. This experimentally found effect is shown in FIG. 4:

(28) This simplification for determining the gas 2 concentration is admissible, since the characteristic curves of gas 1 and gas 2 can be assumed as extending in parallel to each other in a sufficiently small interval. Thereby, then the First Intercept Theorem of elementary geometry can be applied:

(29) If you have two lines intersecting at a common point, and the two lines are intersected by two parallels, which do not pass through the intersection point of the lines, then applies:

(30) The ratio of the two segments of a line formed by the intersection point of the lines and the parallels equals the ratio of the corresponding segments on the second line.

(31) Applied to the case of interest here, the following relationship shown in FIG. 5 will result (shown here, too, in an exemplary manner with reference to the characteristic curves of air and CO.sub.2):
VZ:VV=RZ:RR

(32) Thus follows:

(33) $\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{n*\mathrm{dp}2+\mathrm{dp}2-n*\mathrm{dp}2}{n*\mathrm{dp}{1}_{\mathrm{Gas}1}+\mathrm{dp}{1}_{\mathrm{Gas}2}-n*\mathrm{dp}{1}_{\mathrm{Gas}2}}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{\mathrm{dp}2}{n*\mathrm{dp}{1}_{\mathrm{Gas}1}+\mathrm{dp}{1}_{\mathrm{Gas}2}-n*\mathrm{dp}{1}_{\mathrm{Gas}2}}$$\frac{\mathrm{dp}2}{\mathrm{dp}1}=\frac{\mathrm{dp}2}{n*\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)+\mathrm{dp}{1}_{\mathrm{Gas}2}}$$\mathrm{dp}1=n*\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)+\mathrm{dp}{1}_{\mathrm{Gas}2}$$n=\frac{\mathrm{dp}1-\mathrm{dp}{1}_{\mathrm{Gas}2}}{\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)}$$n=\frac{\mathrm{dp}1-\mathrm{dp}{1}_{\mathrm{Gas}2}}{P{1}_{\mathrm{ges}}}$

(34) Limiting Behavior of the Equations:
dp1=dp1.sub.Gas2n=0Gas2content 100%
dp1=dp1.sub.Gas1n=1Gas2content 0%

(35) Normalization:

(36) In order to directly indicate the gas 2 concentration, the following conversion is made:

(37) $\begin{array}{c}\mathrm{Gas}2\mathrm{concentration}=\left(1-n\right).Math.100\%\left[\mathrm{Gas}2\right]\\ =1-\frac{\mathrm{dp}1-\mathrm{dp}{1}_{\mathrm{Gas}2}}{\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)}*100\%\left[\mathrm{Gas}2\right]\\ =\frac{p{1}_{\mathrm{ges}}}{p{1}_{\mathrm{ges}}}-\frac{\mathrm{dp}1-\mathrm{dp}{1}_{\mathrm{Gas}2}}{p{1}_{\mathrm{ges}}}*100\%\left[\mathrm{Gas}2\right]\\ =\frac{\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)}{p{1}_{\mathrm{ges}}}-\frac{\left(\mathrm{dp}1-\mathrm{dp}{1}_{\mathrm{Gas}1}\right)}{p{1}_{\mathrm{ges}}}*\\ 100\%\left[\mathrm{Gas}2\right]\\ =\frac{\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}{1}_{\mathrm{Gas}2}\right)}{p{1}_{\mathrm{ges}}}+\frac{-\mathrm{dp}1+\mathrm{dp}{1}_{\mathrm{Gas}{2}_2}}{p{1}_{\mathrm{ges}}}*\\ 100\%\left[\mathrm{Gas}2\right]\\ =\frac{\left(\mathrm{dp}{1}_{\mathrm{Gas}1}-\mathrm{dp}1\right)}{p{1}_{\mathrm{ges}}}*100\%\left[\mathrm{Gas}2\right]\end{array}$ dp1.sub.Gas1 and dp1.sub.Gas2 have to be determined with the respective measurement value dp2 from the calibration characteristic curves! dp1 is a measurement value.
Absolute Pressure Correction

(38) Absolute pressure variations will lead to a change of state of the gas mixture. This is accompanied by a change of the flow properties, whereby a measurement error is caused. This error has to be corrected correspondingly. For this purpose, a correction formula is experimentally determined using the shown calibration set-up.

(39) Temperature Correction

(40) In an analogous manner to absolute pressure variations, a temperature variation will affect the measurement result. The correction is made by using the temperature in an experimentally determined correction formula.

(41) Method

(42) The method of measuring according to the invention is based therefore on the measurement of the pressure drop of a fluid flow over two flow resistances compared with a calibration curve. For carrying-out the method, the set-up device has therefore first to be calibrated by means of two fluids, for instance of two liquids or of two gases. The mentioned fluids can in turn represent mixtures, provided that the mixing ratio remains constant, such as for instance when using air. Preferred fluids are gases.

(43) The method can be applied in different fields of engineering. For example, the composition of process gases in the chemical industry can be determined according to the invention. Those skilled in the art will of course understand that with very high flows, as it is the case in many fields of chemical engineering, an arbitrary partial flow can be separated, within which the measurement is made. A preferred application of the method according to the invention is the determination of the CO.sub.2 percentage of air in medical devices, e.g., respirators or insufflators.