Ultrasonic water meter is a water meter that realizes flow measurement by generating an ultrasonic excitation signal and sensing the response of ultrasonic signal propagating in water flow. The commonly used measurement principle is based on the ultrasonic propagation time difference method, which uses the time difference between the forward and reverse propagation of the ultrasonic wave in the flowing medium. By measuring the propagation time, the average flow velocity of the fluid is calculated according to the determined mathematical model, and then multiplied by the cross-sectional area of the pipeline The instantaneous flow rate of the liquid is calculated, and the cumulative flow rate is obtained through integration.
The ultrasonic water meter mainly comprises a measuring tube, an ultrasonic transducer, a measuring circuit, and a display device. Its reality is shown in the figure below.
Structural Principle
1-Measuring tube 2-Ultrasonic transducer 3-Measuring circuit 4-Indicating device
Take the transducer structure layout in Figure 4-2 as an example, let the distance between the upstream and downstream transducers be L, the angle between the sound channel (between the transducer connection line and the axis of the measuring tube) be θ, and the water flow The average flow velocity is v, the sound velocity of ultrasonic waves is c, the propagation time in the downstream direction is t1, and the propagation time in the upstream direction is t2, then:
t1=L/c+vcosθ (4-1)
t2=L/c-vcosθ (4-2)
From formula (4-1) and formula (4-2) can get:
v=L(t2-t1)/2t1t2cosθ (4-3)
Assuming that the cross-sectional area of the pipeline is A, the flow rate q obtained according to the formula (4-3) can be obtained as:
q=A·v (4-4)
Equation (4-3) is an ideal mathematical model that has nothing to do with the speed of sound. If the arrangement of the transducer causes the ultrasonic wave to propagate in a non-measured medium or in a direction perpendicular to the flow direction of the medium, an additional delay related to the speed of sound will be generated, at this time, the model of formula (4-3) should be corrected.
In practical applications, the layout of the sound channels of ultrasonic water meters is diversified. The sound channels of small-caliber water meters are usually U-shaped, and large-caliber water meters are not only the opposite type as shown in Figure 4-2, but also V-shaped or W-shaped. Reflective.
The number of sound channels may not be limited to one pair. Large-diameter water meters usually use multiple sound channels to improve measurement accuracy and adaptability to the flow field. See the figure below for common sound channel arrangements. At this time, the average flow velocity is a function of the flow velocity measured by each channel, generally shown in the following formula.
In the formula: n is the number of channels; Wi is the weight coefficient of the i-th channel; vi is the measured flow velocity of the i-th channel.
It can be seen from formula (4-3) that the ultrasonic water meter can realize the measurement from zero flow in theory, but various factors still restrict it in practice: First, the time measurement circuit itself has zero point drift, resulting in smooth flow at extremely low flow rates. The time measurement error and dispersion of flow and reverse flow is large; second, due to cost constraints, the accuracy of time measurement cannot be achieved high enough; third, the interference of external noise; fourth, the center operating frequency of the ultrasonic transducer Drift etc.
In order to improve the measurement capability of small flow, some large-diameter ultrasonic water meters will increase the flow velocity by reducing the cross-sectional area of the measuring tube, so as to improve the accuracy of downstream and reverse flow time measurement. Reducing the cross-sectional area of the measuring tube can significantly improve the measurement capability of small flows, but it will also bring additional pressure loss.
The measurement circuit principle of the ultrasonic water meter is shown in the figure.
The measurement circuit of the ultrasonic water meter is generally designed with low power consumption and powered by a built-in battery. Some water meters are designed with remote communication function circuits, which can realize remote data transmission.
Regardless of communication power consumption, ultrasonic water meters can achieve lower power consumption under measurement conditions, and the duration of intermittent measurement can be controlled at the second level. By choosing a suitable built-in battery, a working life of 6 years or even more than 12 years can be achieved.
Ultrasonic water meters have the following advantages:
①Measurement principle can realize two-way measurement;
② A wide measurement range can be obtained, and Q3/Q has reached 400 and above;
③ There are no moving parts in the structure, and the pressure loss is very small when the diameter of the measuring tube is designed.