DE102004022521A1 - Low pressure measurement calibration procedure uses laser Doppler anemometer to measure flow and measurement based density to calculate difference between openings in Prandtl static tube - Google Patents

Abstract

A low pressure measurement calibration (2) procedure uses a laser Doppler anemometer (4) to measure an air flow (11) rate (3) and calculate (10) the static pressure at two static tube openings (12, 13) in a Prandtl static tube (9) using air density calculated from measured temperature (7), humidity (8) and absolute pressure (6). Independent claims are included for equipment using the procedure.

Description

These The invention relates to a method and a device for displaying lower pressures. In particular, a Prandtl pitot tube in an air flow to Generation of lower pressures be used. The flow velocity becomes independent measured by the pressure generated.

by virtue of the applicable laws, e.g. Product liability law and industry standards e.g. ISO 9001: 2000 need Pressure measuring equipment industry regularly calibrated. calibration means checking one pressure gauge across from another, but more accurate pressure gauge (normal). These comparative measurements or calibrations are from simple pressure gauge up to the national standard of a country. This is called as Traceability. Supreme national authority, i. Owner of national standards in Germany the Physikalisch Technische Bundesanstalt (PTB) in Braunschweig. The PTB also has the pressure standard with the highest accuracy in Germany. Furthermore leads the PTB regular ring comparisons with the state institutes of other countries. Owns here the PTB in Braunschweig currently the highest accuracy in the presentation small pressures.

The usual today devices for displaying small pressures based on the principle of a piston gauge or a Tauchglockenmanometers. Using this method, the laboratories accredited by the DKD (German Calibration Service) can reproduce small pressures as small as 1 Pa. The measurement uncertainty (MU) is ± 0.3 Pa for pressures less than 20 Pa for submersible gauges (the curve 21 in FIG 2 ). The measurement uncertainty does not depend on the pressure for such small pressures.

In the case of piston gauges, the overpressure p _θ to be measured is determined from the force which this pressure exerts on the cross-sectional area A of the freely movable measuring piston of the piston-cylinder system and which is compensated by an external force F. The counterforce F is generated by the dead weight of the volumetric flask and by the weight of applied masses.

Furthermore, the so-called Prandtl pitot tube ( 1 in 1 ) (for example "Physik", Gerthsen et al., Springer-Verlag, Heidelberg, 16th edition, ISBN 3-540-51196-2). The Prandtl Pitot tube 1 determines directly the difference between total pressure p + 1/2 · ρv ² and static pressure p, ie the dynamic pressure 1/2 · ρv ² in flowing fluids, ie liquids or gases. Here, ρ is the density and v is the flow velocity of the fluid. The Prandtl Pitot tube thus provides a direct measure of the flow velocity.

It Object of the invention, a more accurate device and a more accurate Specify method for displaying small pressures.

Advantageous on the process developed by testo industrial services GmbH and the corresponding device is that differential pressures from 0.005 Pa can be reproducibly displayed up to 1345 Pa.

Advantageous in the method according to the invention and the corresponding device is further that with decreasing Pressure also the measurement uncertainty proportional to the generated Pressure decreases. This is not the case with any of the conventional methods.

in the The following are preferred embodiments of the invention explained in more detail with reference to the accompanying figures. Showing:

1 an inventive device for displaying small differential pressures and

2 the measurement uncertainty of a device according to the invention compared with a herkömmli chen device.

The Invention allows pressure Monitors in a range of 0.005 Pa to 1345 Pa.

1 shows an inventive device for displaying small pressures. This device comprises a Prandtl pitot tube 1 , a pressure gauge to be calibrated 2 , a wind tunnel 3 , a laser Doppler anemometer 4 , an absolute pressure gauge 6 , a temperature sensor 7 , a hygrometer 8th as well as a computer 10 , reference numeral 9 denotes streamlines. At the first opening 12 of the Prandtl pitot tube becomes the total pressure, at the second opening 13 the static pressure measured. The pressure gauge to be calibrated 2 is about connections 14 and 15 with the openings 12 respectively. 13 of the Prandtl Pitot tube that 1 connected. The connections 14 and 15 For example, they can be flexible hoses. turbine 11 pumps air through the wind tunnel 3 ,

The wind tunnel 3 has good temporal and spatial stability. To illustrate the pressure, the Prandtl pitot tube 1 used. The differential pressure pe produced by this is determined from formula (2): pe = (v / s) 2 · Ρ / 2 (2)

Out From the formula (2), it can be seen that the flow velocity v, Density ρ as well The pitot tube factor S must be known to the overpressure to be able to calculate.

The with the help of the pitot tube 1 generated overpressure pe increases with increasing flow velocity v. To represent low pressures pe, in particular gases such as air are used. In other embodiments, other fluids may be employed.

The flow velocity v is determined using the laser Doppler anemometer 4 detected. The density ρ is determined for air from the BIMP formula (3):

In formula (3) ρ denotes the air density, P _abs the static air pressure in the environment F _rel the relative humidity and T the temperature in the wind tunnel.

To determine these parameters are absolute pressure gauge 6 , Temperature sensor 7 as well as hygrometer 8th used with a corresponding accuracy. For example, computer calculates from the acquired measurements 10 the pressure gauge to be calibrated 2 applied pressure.

The exact dynamic pressure factor of Prandtl's Pitot tube 1 should be measured once. The determination of the congestion ear factor S is preceded by an accurate determination of the air density according to formula (3). Flow velocity v and overpressure pe are recorded with appropriate measuring instruments. The pitot tube factor S is determined from formula (4), which is obtained from formula (2) by solving for S: S = v / √ 2 · pe / ρ (4)

The peculiarity of the method according to the invention is that with decreasing pressure, the measurement uncertainty also decreases significantly in proportion to the pressure shown (the curve 22 in FIG 2 ). No conventional method has such a behavior.

The smallest estimated uncertainty of measurement of the method according to the invention is ± 0.001 Pa. The overall measurement uncertainty is based, in particular at low pressures shown, above all on the uncertainty of the uncertainty of the velocity measurement given by the laser Doppler anemometer, which is ± 0.5% of the measured value, but at least ± 0.01 m / s. For a flow velocity v of 0.096 m / s, a relative humidity F _rel of 40%, a temperature T of 22 ° C, a static air pressure P _abs of 930 mbar and a P _stofaction factor S of 1, an illustrated pressure pe of 0.005 Pa results , For a speed v of 0.106 m / s, all other parameters being equal, the pressure ps is 0.006 Pa. The difference of 0.001 Pa is interpreted as measurement uncertainty.

The The invention was previously based on preferred embodiments explained in more detail. For a specialist However, it is obvious that various modifications and modifications can be made without departing from the spirit of the invention. Therefore, the scope of protection by the following claims and their equivalents established.

1

Prandtlsches Pitot tube

2

to calibrating pressure gauge

3

wind Tunnel

4

Laser Doppler Anemometer

6

Absolute pressure meter

7

temperature sensor

8th

hygrometer

9

streamlines of the streaming fluid

10

computer

11

turbine

12

opening

13

opening

14

connection

15

connection

21

Best conventional method

22

inventive method

Claims (10)

A method for displaying lower pressures comprising: generating a flow in a fluid that is attached to a body ( 1 ) ( 9 ), which has a first opening ( 12 ), wherein the flow at the opening generates a back pressure; Determine ( 4 ) the flow velocity of the fluid; characterized by: determining ( 4 ) the flow velocity of the fluid independent of the back pressure at the opening ( 12 ); and calculating ( 10 ) of the dynamic pressure at the opening ( 12 ) due to the determined flow rate. Method according to claim 1, characterized in that the body has a second opening ( 13 ), wherein the dynamic pressure difference between the first opening ( 12 ) and the second opening ( 13 ) is calculated on the basis of the determined flow rate. Method according to one of the preceding claims, characterized in that it is in the body ( 1 ) is a Prandtl pitot tube. Method according to one of the preceding claims, characterized in that the fluid is air and the flow rate of the fluid by means of a laser Doppler anemometer ( 4 ) is determined. Method according to one the above claims, marked by: Determining the density of the fluid; and Consider the density of the fluid in the calculation of the dynamic pressure. Method according to claim 5, characterized in that the density of the fluid is determined on the basis of one or more of the following measured variables: temperature ( 7 ) of the fluid; Humidity ( 8th ) of the fluid; and / or absolute pressure ( 6 ) in the fluid. Apparatus for displaying low pressures comprising: a body ( 1 ), which has an opening ( 12 ) having; a pump ( 11 ) for pumping a fluid past the body, so that at the opening ( 12 ) forms a dynamic pressure; characterized by: a connection ( 14 ) for a pressure gauge to be calibrated ( 2 ), whereby the connection ( 14 ) with the opening ( 12 ) is pneumatically connected, so that the back pressure at the opening also at the port ( 14 ) is present; a speed measuring device ( 4 ) for measuring a relative velocity between the fluid and the body ( 1 ) regardless of the back pressure at the opening ( 14 ) of the body ( 1 ). Device according to claim 7, characterized in that the body ( 1 ) is a Prandtl pitot tube. Device according to claim 7 or 8, characterized in that the fluid is air and a laser Doppler anemometer ( 4 ) serves as a speed measuring device. Device according to one of claims 7 to 9, characterized in that the device comprises one or more of the following sensors: thermometers ( 7 ); Humidity sensor ( 8th ); and absolute pressure gauge ( 6 ).