Investigation of the error measurement of ultrasonic sensors for measuring water flow in open canals of irrigation systems

This paper analyzes ultrasonic flow meters that provide stability of metrological characteristics during operation, low cost of meter maintenance, high reliability and sensitivity, and high performance in large diameter pipes and open chanels. Based on the analysis, the requirements of the reclamation systems to the sensors, the causes of the error are formed. The difference between the speed of sound in the opposite direction of the flow direction or in the direction of flow in the ultrasonic sensors was studied. The measurement errors of ultrasonic sensors currently widely used in open canals are investigated graphically. Based on the analysis results, it is recommended to use an ultrasonic sensor suitable for the extreme conditions of our country.


Introduction
One of the urgent problems is the introduction of large-scale automated control systems in irrigation systems to determine the true value of water flow, equipping hydraulic structures of all hydroameliorative systems with sensors for measuring water level and flow, information-measuring systems. Nowadays taking into account these problems, theoretical and practical work is being carried out in our zone on the efficient use of existing water resources, improving the reclamation of lands, increasing productivity, reducing electricity consumption at pumping stations.
We know that the main part of the irrigation network in our country is open canals. Currently, the technical condition of water flow measuring and control devices at these facilities is not satisfactory. That is, in the current age of information technology, existing water flow meters in open canals do not meet many requirements. In order to effectively use the available water resources, it is necessary to create, develop and implement a general information system and a system of hydrological monitoring, accounting, storage and planning. It is necessary to pay special attention to the development and creation of new information sensors with high sensitivity and accuracy of water flow in open canals and pipes, the development of a new information and communication system in the irrigation system [2].

Methods
Currently, various types of measuring devices are being developed to measure water flow in open canals and unpressurized pipes. Different technological processes may place different requirements on these flow meters [1]. Therefore, we formulate the general technical requirements for the development of these flow meters.
The following requirements are set for modern flow meters:  high reliability;  high accuracy in measurement;  minor error when changing the density of the liquid;  high sensitivity in measurement;  width of measurement range;  adaptive adjustment;  compactness;  low cost. In the article, we will look at modern ultrasound sensors. The principle of operation of ultrasonic (acoustic) flow meters is based on the effect that occurs when acoustic vibrations pass through liquids or gases [7,8]. Most flow meters operate in the ultrasonic range. Ultrasonic flow meters are divided into the following types: -flow meters based on the displacement of acoustic vibrations in a moving liquid medium; -Doppler effect based flow meters. The most common water flow meters are based on the principle of measuring the flow time and the transition time consumption of acoustic vibrations in the opposite direction [4,5]. In addition to the above-mentioned flow meters, long-wave acoustic flow meters have been developed that operate in the sound range of acoustic vibrations. Typically, ultrasonic flow meters measure the volumetric flow rate of liquids, but the flow rate can also be measured by placing a fluid-sensitive element in the structure [3,6]. The error of the ultrasonic flow meters available so far ranged from 0.5% to 2.5%.
Figures 1a and 1b below show an ultrasonic generator placed opposite the two sides of the unpressurized tube. As a rule, a piezoelectric crystal is used as the source of the ultrasound generator.
Each crystal is used as an ultrasound generator (exciter) or receiver. In other words, a single crystal can act as an "amplifier" or a "microphone" if necessary.
The crystals are placed at a distance D from each other and at an angle ϕ relative to the direction of flow. In addition, a small crystal can be placed inside the tube in the direction of flow (ϕ=0 0 ).
The time of sound propagation between two crystals A and B is related to the mean velocity of the current as follows [4,5]: here c -the speed of sound in the environment. In this expression, the ± sign indicates the direction of sound propagation, i.e. along or against the stream.
Vaverage is the average flow rate in the direction of ultrasonic propagation.
If the stream is laminar  cases. In order to improve the signal-to-noise relationship, the speed of propagation of the ultrasound signal is often measured in two directions [9,10]. In this case, both pesocrystals act alternately as receivers or transmitters ( Figure 2).
here t1, t2 -the times of ultrasonic propagation along and against the current. La -is the length of the active part of the acoustic channel; Lg -is the distance between the membranes of PEs; c0 -is the ultrasonic velocity in stationary water; c0 -is the velocity of water in the pipe; ϕ -is the angle between the sensor and the pipe axes.
Here: D is the inner diameter of the ultrasonic flow meter at the place of installation of piezoelectric element 1 and piezoelectric element 2.
The K-correction factor takes into account the hydrodynamic properties of the fluid and the nature of its flow.
Ultrasonic sensors type AVFM 6.1 and Beluga operate according to the velocity-area method [3] when calculating the water flow.

Q=S•v
In this case, S is the width of the channel area, m, v -is the fluid flow rate, m/s. The reasons for the error in liquid flow meters are as follows: -uncertain calculation of the influence of the flow profile; -asymmetry of electronic acoustic channels; -different speed of ultrasound in the environment; -parasitic acoustic signals; -an error introduced by the electronic circuit. The influence of the uncertainty of the flow profile is due to the fact that the average speed and average speed of acoustic vibration are not equal.

Results and Discussion
Based on the above requirements, we analyze several types of ultrasonic sensors for measuring water flow in open canals. Let's analyze the measurement errors of the above mentioned sensors.