DE4421692A1 - Ultrasonic precision flowmeter for liquid or gas e.g. water in household water pipe - Google Patents

Abstract

The flowmeter is designed such that the influence of the instabilities of the emitter/receiver is corrected with a connected electronic unit, to the time differences ( DELTA t1) according to the following equation: DELTA t1<k> = DELTA t1 - (Trefl,R,i - Trefl,L,i). In addition by temp. or pressure measurement in the through flowing medium, the time differences ( DELTA t1<k>) on the propagation speed of the ultrasonic wave can be calibrated, based on influences by the temp., the pressure or other parameters.

Description

1. State of the art

Mechanical counters are usually used to measure liquids or gases used, which are driven directly by the medium flowing through. As An example of this is only the water meter that can be found in every household will.

The disadvantage of this method of measurement is, on the one hand, that the actual measurement equipment is directly flowed through by the medium, which is the case with aggressive or hot media leads to major problems in the manufacture of the meters.

For this reason, flow meters based on the ultrasonic measurement were developed some time ago. The principle of measurement of such ultrasonic meters consists in the physical phenomenon that the speed of propagation of the ultrasound is not constant, but is influenced by the flow rate of the medium. For this reason, the transit time of the ultrasound pulse ( Fig. 1) is measured on the one hand in the direction of flow of the medium T _in and on the other hand against the direction of flow T _against , a defined distance l having to be transmitted in the flow channel. For this purpose, an ultrasound transmitter / receiver pair S _L / E _L and S _R / E _{R are} installed on both sides of the flow channel. The transmitters S _L and S _R are activated simultaneously and a pulse with, for example, 60 wave trains is emitted. With the two receivers E _R and E _L , the arrival time of each individual wave train of the ultrasonic pulse of the opposite transmitter is recorded. The time difference Δt _i between the i-th wave train of the pulse emitted in the flow direction and the i-th wave train of the pulse emitted against the flow direction is Δt _i = T _{against, i} -T _{in, i} . With a known distance l and known propagation speed of the ultrasound V _US , the measured time difference Δt _{i is} a measure of the flow speed V of the medium. In order to increase the measuring accuracy, the average of all measured values Δt _i within an ultrasound pulse.

There is a problem with the standard flow meter described here in that this measuring method does not contain its own calibration option and thus interfering influences (e.g. temperature fluctuations) are only insufficiently compensated  can be. The measurement method thus contains systematic sources of error, which in the Practice can lead to significantly falsified measurements.

2. Description of the precision flow meter

In contrast to the flow meter described above, the precision flow meter is characterized by its additional calibration device. This consists in not only measuring T _{in, i} and T _{against, i of} each wave train, but also measuring the time differences T _{refl, L, i} and T _{refl, R, i} . T _{refl, L, i describes} the transit time after sending out the i-th wave train from the left transmitter S _L until it returns to the left receiver E _L , the sound being reflected on the opposite end face of the flow channel. Analogously, T _{refl, R, i is} the transit time after sending out the i-th wave train from the right transmitter S _R until it returns to the right receiver E _R , the sound also being reflected on the opposite end face of the flow channel. The time differences T _{refl, L, i} and T _{refl, R, i} contain no information about the flow rate of the medium, since the speed of propagation of the ultrasound _{pulse points} once in the flow direction and once against the flow direction of the medium. For this reason, T _{refl, L, i} and T _{refl, R, i can be used} immediately for the calibration of the time differences T _{in, i} and T _{against, i} . This is done as follows:

If the direction of flow ( Fig. 2) points from left to right, the following nomenclature should apply:

T _{L → R, i} : = T _{in, i} (1)

and

T _{R → L, i} : = T _{against, i} (2)

Then you get for the calibrated time differences and subsequent ones Expressions:

T _refl describes the target value of the transit time of the reflected ultrasound _pulses . This setpoint is measured under "normal conditions" or also calculated for the given arrangement from literature values under normal conditions. The calibrated time difference of a wave train, which is decisive for the flow velocity, is then calculated as follows:

Here, too, i can of course be averaged over all wave trains.

The calibration described above takes into account instabilities of the transmitter / receiver with upstream or downstream electronics. The speed of propagation of the ultrasound in the medium in question is generally not constant, but also depends on the external parameters such as pressure, temperature etc. In order to be able to calibrate this additionally, the precision flow meter is optionally equipped with a temperature and pressure sensor, for example. The time differences ΔT _i ^k are then also calibrated to the external parameters.

With a simple electronic circuit consisting of a microprocessor or A look up table (LUT) allows all calibrations carry out.

Claims (3)

1. Precision flow meter according to the principle of ultrasonic measurement, characterized in that the ultrasonic pulse reflected from the opposite end of the flow channel is used to calibrate the flow meter. 2. Precision flow meter according to claim 1, characterized in that the influence of the instabilities of the transmitter / receiver with upstream or downstream electronics on the time differences Δt _{i is} corrected using the following formula (nomenclature see description): Δt _i ^k = Δt _i - (T _{refl, R, i} -T _{refl, L, i} ) 3. Precision flow meter according to claim 1, characterized in that the time differences .DELTA.t _i ^k can be calibrated to the propagation speed of the ultrasonic wave influenced by temperature, pressure or other parameters by temperature or pressure measurement in the flowing medium.