DE1573044C - Viscosity-compensated flow meter based on the differential pressure method - Google Patents

Description

The invention relates to a flow meter using the differential pressure method for flowing media a hydraulic full bridge connected to the measuring section, in the diagonals of which there is a differential pressure meter is arranged and in which the sum of the partial resistances in a bridge branch to the sum of the Partial resistances in the bridge arm parallel to it is in a constant ratio, with each in the Bridge diagonally arranged partial resistances are of the same type, in such a way that they either a substantially laminar or a substantially turbulent portion of the pressure drop exhibit.

In the case of conventional flow meters with a single throttle point, the one used for the measurement must be used Pressure signal at a cross-sectional constriction through which the medium to be measured flows (nozzle, orifice) strictly proportional to the square of the flow. Physical quantities of the flow (density, Viscosity) and device parameters may only be the proportionality factor between the pressure difference as constants and the square of the flow.

The flow measurement according to the differential pressure method is used to a large extent today. The dimensions and area of application of the throttle devices (nozzles, orifices) used for this purpose are specified in DIN 1952. The equipment required is low, the achievable measurement accuracy is high. When using nozzles and orifices, the required strict proportionality between the pressure signal and the square of the flow is only guaranteed from a certain Reynolds number R-> 10 ^{4 of} the flow in the connected pipeline. If the characteristic value Re (it characterizes the ratio of inertia to frictional forces in the flow) is smaller than the specified value, then the mentioned proportionality factor is strongly influenced by the viscosity of the flow medium to be measured. The for the flow measurement:

The required basic requirement that only the flow may cause a change in the pressure signal is then no longer met. For Re = Re _c , the lower limit of the application range of the throttle devices according to DIN 1952 has been reached. The same standard also describes special throttle devices (e.g. quarter-circle nozzles) that can be used up to a measuring range limit of Re _c = 300. In smaller flow ranges (Re <Re _c ), the use of the differential pressure method for measuring the flow of liquids and gases with the known throttle devices is only possible if the influence of the viscosity on the measurement result is taken into account, i.e. for precise flow measurements the viscosity of the measuring device is also required To capture the medium.

Measuring devices with a hydraulic bridge circuit are also known. Mention should be made e.g. French patent specification 1 427 971 in which a flow meter based on the differential pressure method for gases is described. It consists of a measuring bridge (full bridge) with four selected resistors. If one acts on one bridge diagonal with the flow stiom to be measured and put a pressure di'ference measuring device in the other diagonal, then results in the selected one Arrangement a linear relationship between pressure signal and flow. This allows for a given Measuring range of the manometer a much larger flow range can be detected than z. B. with the throttle devices, according to DIN 1952 (quadratic dependency). The influence of the viscosity of the flow medium However, there is no significant impact on the pressure signal (linear dependency). Also the company FIo-Tron, Inc., Paterson N.J., builds a flowmeter using the differential pressure method, which consists of four a full bridge interconnected but equal resistances. The flow to be measured is directed into a diagonal of the measuring bridge. The other bridge diagonal is connected to a pump constant delivery flow (this must be a multiple of the measuring flow). With this arrangement the pressure difference arising at the pump is directly proportional to the current to be measured and the flow rate of the pump, which is constant and

Δρ = pressure loss at the throttle point,
ζ = drag coefficient of the throttle element,
ρ = density of the flow medium,
V = mean flow velocity in the throttle cross-section.

The drag coefficient ζ is a function of the Reynolds number Re (Fig. 1). This can be stated as follows:

re

C _T

^6o

c _R = laminar factor.
c _T = turbulence factor.

In general, the quantities c _R and c _{r are} also dependent on Re. For certain selected line

65

must be independent of viscosity. The measurement accuracy is therefore decisive for the characteristics of the auxiliary pump dependent.

All previously mentioned measuring methods based on the differential pressure method are either in their area of application limited (nozzle, orifice) or require additional measures for the exact measurement of the Flow rate, such as measuring the viscosity of the flow medium or superimposing a strictly constant Delivery flow whose source (pump) must also be independent of viscosity.

The basic idea of the invention is to reduce the inevitable influence of the viscosity in the throttle device measuring flow medium to the pressure signal by parallel connection of suitably dimensioned resistors to compensate.

This is achieved according to the invention in that, in a device of the type mentioned at the beginning, the laminar component of the pressure drop of the resistors connected in parallel is the same and the resistors are otherwise designed so that the pressure difference signal.:] P _D in the bridge diagonal regardless of the viscosity of the flow medium is proportional to the square of the flow rate per unit of time.

This means that the device can be used with any Reynolds number Re, including the smallest. No further auxiliary measurements are required, and the device is unlimited in its area of application. As with throttle devices, the pressure signal is proportional to the square of the flow. For sizing of the line elements, their resistance behavior must be known. The invention is on Hand of the drawing explained in more detail by exemplary embodiments. It shows

F i g. 1 the resistance value of a throttle point as a function of the Reynolds number Re,

F i g. 2 and F i g. 3 the construction of a hydraulic full bridge,

F i g. 4 shows a construction example for the full bridge according to the scheme according to FIG. 2,

F i g. 5 the viscosity- _{dependent total drag coefficient ζ 0} and the viscosity-independent pressure difference _{coefficient C D} in the diagonal of a measuring bridge according to the invention as a function of the Reynolds number Re and

F i g. 6 shows an arrangement for viscosity-compensated flow control.

In general, the pressure loss at a throttle point is described as follows:

However, 50 constrictions are constant. A hydraulic full bridge is made up of such throttle elements (FIG. 2) in such a way that the resistances (R ₁ ) opposite in the bridge are identical and parallel (R ₁ , R ₂ ) are different, then the one for the flow measurement interfering viscosity influence can be compensated. The flow rate Q to be measured is applied to one diagonal of the bridge. The differential pressure measuring device is switched to the other bridge diagonal (Fig. 2). The flow rate is the same in the two bridge branches (division ratio 1: 1), since they have the same total resistance (R ₁ + R ₇ ) . For the total pressure difference I Ji ₀ gill:

C ₁ = / (R ₁ ); : ₂ = / (R ₂ ).

• The following also applies, depending on the prerequisite:
2 ■ C "

For the pressure difference in the Biückendiaoualen

I / '«= ■ (' 1- -2) · ί '/ 2 -V

- ^ι Ji ₁ - ■ -R-,

-ί + ^ .. - C ■

Rc

If one chooses the geometry of the resistors F, and R ₂ so that the following applies:

and C _T

then applies

(C ₁ -C ₂ )

= (C _{T {} -C

_T2 )

C _r .

/(Re),

that is, the pressure signal Δp _D related to the density of the flow medium is directly proportional to the square of the flow to be measured, regardless of the Reynolds number Re or the viscosity of the flow medium.

This compensation principle can generally also be used for other flow division ratios (bypass measurement) (FIG. 3). The resistances Ra, Rb, Rc and Rd only have to be selected in such a way that the ratio of the flow rates in both branches remains constant over the entire measuring range regardless of Re.

An executed construction example is shown in FIG. 4. A conical cross-sectional constriction was selected as the resistance R ₁ , and a tube with a constant inside diameter was selected _{for R 2.}

F i g. 5 shows the total resistance _{coefficient C 0 of} the measuring element and the pressure difference coefficient C ₀ in the diagonal of the measuring bridge as a function of the Reynolds number Re. The latter is constant over the entire measuring range Re <10 ⁴ , ie in this range the pressure signal is strictly proportional to the square of the flow.

If one measures the Gesamtdrückdifferehz, I p ₀ and the pressure difference in the bridge diagonal, can then be about these two values to calculate the viscosity of the through flußmediums. For the measuring bridge shown in Fig. 4, the relationship is:

or under certain simplifying conditions:

C,

\ p _D

r = kinematic viscosity of the flow medium. Ki, K ₂ , K ₃ and C ₁ are device constants.

By converting the pressure signal Ap _{0 of} the bridge into an electrical or mechanical variable, which then influences the upstream or downstream control valve, a viscosity-compensated flow controller can be built. The principle possibility of such an arrangement is shown in FIG. 6. Here (1) is the measuring bridge, the pressure signal of which is acted upon by springs

ίο Differential control cylinder (3) is routed that way forwards the path signal obtained to the control valve (2). The desired flow rate can be set at the button (4) can be set. The control valve (2) can be connected upstream or downstream of the bridge.

1 sheet of drawings

Claims (6)

Patent claims: 1. Flow meter for flowing media according to the differential pressure method with a hydraulic full bridge connected to the measuring section, in the diagonal of which a differential pressure meter is arranged and in which the sum of the partial resistances in a bridge branch is in a constant ratio to the sum of the partial resistances in the parallel bridge branch, with the Partial resistances arranged diagonally to one another in the bridge are of the same type, namely in such a way that they have either an essentially laminar or an essentially turbulent part of the pressure drop, characterized in that the laminar part of the pressure drop of the resistors connected in parallel {Ra / Rc and Rb / Rd or RJR ₂ and R ₂ IRi) is the same and the resistors are otherwise designed so that the pressure difference signal .1 P ₀ in the bridge diagonal, regardless of the viscosity of the flow medium, is proportional to the square of the flow rate per unit of time. 2. Flow meter according to claim 1, characterized in that cross-sectional widenings are used as resistors be used. 3. Duvchflußmeßgerät according to claim 1, characterized in that two identical resistors (K ₁ , R ₂ ) are arranged diagonally in the bridge. 4. Flow meter according to claim 3, characterized in that the turbulent portion of the pressure drop of one resistor (R ₁ ) is significantly greater than that of the other resistor (Ri) - 5. Flow meter according to one of claims 1 to 4, characterized in that the Full bridge further partial currents are connected in parallel to expand the measuring range. 6. Flow meter according to one of claims 1 to 5, characterized in that a second differential pressure meter is connected between the input and output of the full bridge for measuring the total pressure difference (Jp ₀ ) for the purpose of determining the viscosity of the flow medium.