The measurement principle of ultrasonic open channel flowmeter is based on the combination of ultrasonic propagation characteristics and fluid mechanics principles, achieving accurate flow calculation through non-contact measurement. Its core can be divided into two key steps: liquid level measurement and flow conversion:
1、 Liquid level measurement: Ultrasonic echo ranging technology
The flowmeter emits high-frequency sound pulses (usually 50-200kHz) vertically towards the liquid surface through an ultrasonic probe. The sound waves propagate at a speed of 331.4+0.6tm/s (t is the ambient temperature) in the air and reflect back to the probe after contacting the liquid surface. The device calculates the distance D from the probe to the liquid surface by measuring the time difference Δ t between transmission and reception, combined with the sound velocity v. For example, in a 25 ℃ environment with a sound velocity of 346.4m/s, if Δ t is 1ms, the liquid level height D is 0.1732m. To eliminate installation height errors, the equipment needs to preset a reference distance L from the probe to the "zero liquid level", with a final liquid level H=L-D, a resolution of up to 1mm, and an accuracy of ± 3mm.
2、 Flow Conversion: Weir Channel Water Level Flow Relationship Model
The open channel flow rate has a non-linear relationship with the liquid level, and the liquid level needs to be converted into flow rate through standard weir channels (such as Bacher channels, triangular weirs, rectangular weirs). Taking the Bacher trough as an example, its throat width b determines the flow coefficient. The equipment is equipped with an empirical formula Q=C × H ⁿ (Q is the flow rate, C is the weir trough coefficient, and n is the index), which automatically calculates the flow rate through real-time liquid level H. For example, in a Bacher groove with a throat width of 0.3m, when the liquid level is 0.5m, the flow rate Q is approximately 0.15m ³/s. For non-standard weirs, it is necessary to establish a liquid level flow curve through on-site calibration to ensure measurement accuracy.
3、 Multi channel technology: improving the accuracy of complex flow measurement
In large open channels, there may be eddies or uneven velocity distribution in the water flow, in which case multi-channel ultrasonic measurement technology is used. By installing multiple pairs of probes at different heights in the channel, synchronously measuring the flow velocity of each channel, using the weighted average algorithm to calculate the average flow velocity of the cross-section, and combining it with liquid level data to obtain the flow rate. This technology can reduce measurement errors from 5% in mono channels to within 1%, especially suitable for wide and shallow channels or non full tube states.