Working Principle and Calibration of Ultrasonic Flowmeter

An ultrasonic flowmeter is a velocity-type flowmeter that uses ultrasonic pulses to measure fluid flow. It features non-contact operation, a wide measurement range, easy portability and installation, strong adaptability to pipe diameters, ease of use, and ease of digitization. It is widely used for on-site measurement of gas and fluid velocity and flow. This article introduces the design principles of the most commonly used ultrasonic flowmeters, analyzes and evaluates the measurement uncertainty of their measurement errors, and discusses methods for improving on-site measurement accuracy.

I. Measurement Principle of Ultrasonic Flowmeters

The operating principle of an ultrasonic flowmeter is shown in Figure 1. Two ultrasonic probes are installed: downstream transducer A transmits ultrasonic pulse signals, and upstream transducer B receives them. The transducers are mounted using the external clamp Z method, one on each side of a fluid pipeline at a specified distance. The pipeline's inner diameter is d, the downstream ultrasonic velocity is V, and the angle θ between the ultrasonic propagation direction and the fluid flow direction is θ.

2. Measurement uncertainty analysis

The fluid flow rate according to formula (3) consists of four parts: the inner diameter of the pipe d, the theoretical sound velocity C in the measured fluid, the tangent of the sound wave refraction angle tanθ, and the time difference Δt between the forward and reverse flow of the fluid passing through the transducer AB. Its measurement uncertainty analysis is as follows.

1. Evaluation of the uncertainty introduced by the repeatability of the measurement of the inner diameter of the pipe d

According to the ** standard, the nominal diameter of the pipe D and the thickness of the pipe s are only approximate nominal dimensions. The outer diameter of the pipe D and the thickness of the pipe s must be measured each time. Therefore, this uncertainty has two components, namely the measurement repeatability of the measured object and the measurement uncertainty of the measuring instrument used on site. According to our actual on-site measurement experience, the measurement uncertainty of the inner diameter of the pipe d is generally Urel (d) = 0.5% (k = 2); therefore, the standard uncertainty introduced by the measurement of the pipe inner diameter d is:

urel(d) = urel(d) / k = 0.5% / 2 = 0.25%

2. Evaluation of the uncertainty introduced by the measurement of the fluid sound velocity C, urel(C)

According to the technical data, this uncertainty is evaluated as Class B. The uncertainty of the sound velocity measurement in the measured fluid is:

Urel(C) = 0.6% (k = 2). This can be directly quoted:

urel (C) = Urel (C) / k = 0.6% / 2 = 0.3%

3. Uncertainty introduced by the measurement repeatability of the distance l between transducers A and B

Evaluation of the uncertainty of urel (l) The measurement uncertainty of the distance l between downstream transducer A and upstream transducer B has two components: the measurement repeatability of the measured object and the measurement uncertainty of the measuring instrument used on site. Based on our actual field measurement experience, the standard uncertainty introduced by the measurement repeatability of the distance l between transducers A and B is The uncertainty is generally

Urel(l) = 0.6% (k = 2):

Urel(l) = s/k = 0.5%/2 = 0.25%

4. Introduction to the Time Difference Δt Between Forward and Countercurrent Flow Through Transducers AB

Evaluation of the Uncertainty u(Δt) The time difference Δt between forward and countercurrent flow through transducers AB in an ultrasonic flowmeter is measured by subtracting the time t1 from the pulse transmitted from transducer A to B in the forward flow direction and t2 from the pulse transmitted from B to A in the countercurrent direction (see Figure 1). According to formula (1), its uncertainty components are mainly determined by the distance l between the downstream transducer A and the upstream transducer B, the pipe inner diameter d, and the sound velocity C in the measured fluid. The measurement accuracy of time and frequency is the highest among all measurement disciplines. The error caused by the ultrasonic flowmeter pulse timing measurement can be ignored. The distance l, the pipe inner diameter d, and the sound velocity C in the measured fluid are included in other uncertainty components. Therefore, the uncertainty u (Δt) introduced by the time difference Δt between the upstream and downstream fluids passing through transducers AB can be ignored.

III. Methods for Improving Field Measurement Accuracy of Ultrasonic Flowmeters

In field measurements, the first step is to conduct a comprehensive analysis of various factors. These factors all have a certain impact on the final measurement results, as shown below.

1. The Impact of Uncertainty in Sound Velocity C and Empirical Methods for Improving Field Measurement Accuracy

Before field measurements begin, the measured medium should be provided. If the medium is a gas, the specific gas composition, operating temperature, and operating pressure should be provided. The ultrasonic sound velocity can be obtained by consulting the relevant standards using the above information. The influence of the sound velocity C of the working medium on the ultrasonic flowmeter will have less impact on the measurement results. If the medium is a liquid, the specific liquid name, operating pressure, operating temperature, operating pressure, and the presence of suspended particles in the liquid should be provided. The sound velocity setting should take temperature effects into account. The sound velocity of aqueous solutions is greater than that of water, and for most fluids, the higher the temperature, the faster the sound velocity. When there are many particles in the fluid (but within the measurement range), there are two approaches: 1. Uniformly distributed particles. In this case, the signal is relatively stable, making it difficult to detect through measurement. The measured medium should provide the cause and type of particles. Once the particle type is known, the sound velocity of the fluid can be appropriately adjusted, and the signal quality can be compared to obtain more accurate measurement results. ② In the case of uneven particles, the signal intensity will fluctuate significantly. In this case, the best approach is to measure over a long period of time and average the readings at several points with good signal quality.

2. Distance l between transducers A and B and pipe inner diameter d

The impact of measurement repeatability and methods for improving on-site measurement accuracy: When selecting the measurement pipeline, choose a straight, stable section of the working medium, away from the pump station valve. If the medium in the pipeline is liquid, also select a pipe section that is less likely to cause sedimentation at the bottom and air accumulation at the top. Initially, measure with the probe installed vertically, then horizontally. If the difference between the two measurements is within the maximum allowable error of the ultrasonic flowmeter, with other parameters unchanged, proceed to the next measurement after further parameter settings. Otherwise, reselect the pipeline for measurement (if the difference between the two measurements exceeds the allowable error of the ultrasonic flowmeter, it indicates that the pipe section is not filled with the working medium).

When setting detailed parameters for the next measurement, the main factors affecting measurement accuracy are the distance l between transducers A and B and the pipe's inner diameter d. Distance l is generally measured with a steel ruler or vernier caliper based on the distance l. For measuring the pipe's inner diameter d, a vernier caliper can be used directly when the pipe's outer diameter is small. For larger pipes, it's best to use a precision steel ruler to measure the circumference and then calculate the diameter. When measuring pipes with severe internal deposits and fouling, the pipe wall parameter s can be increased and the wall sound velocity can be decreased. When measuring pipes with severe internal corrosion, the pipe wall parameter s can be decreased, but the wall sound velocity remains unchanged.

Based on the principle of transit-time ultrasonic flowmeters, this paper analyzes and evaluates the measurement uncertainty of ultrasonic flowmeter errors. Based on our institute's years of experience in field testing of ultrasonic flowmeters, we propose and explain several key points for improving the field measurement accuracy of ultrasonic flowmeters.