An ultrasonic open channel flowmeter directly measures the liquid level. To measure flow in an open channel, a water trough is installed. The measuring trough transforms the flow rate into a corresponding liquid level. This measured liquid level is used to determine the flow rate based on the known relationship between water level and flow for that specific trough. The relationship between flow rate and water level in an open channel is influenced by factors such as the slope and roughness of the channel. Installing the water trough creates a throttling effect, ensuring a consistent relationship between the flow and the liquid level. This relationship mainly depends on the dimensions of the trough, with minimal impact from the channel itself.
When the flowmeter is in operation, an ultrasonic sensor is mounted above the liquid surface. The probe emits a pulsed ultrasonic wave toward the water surface. The wave travels through the air, reflects off the liquid surface, and returns to the probe as an echo. A reference rod below the probe also generates a reflection, known as a correction wave, which helps in accurate measurement. The distance from the probe to the liquid surface is calculated using the time difference between the transmitted signal, the echo, and the correction wave. The actual liquid level height H is determined by subtracting the distance L (from the probe to the zero level) from the total measured distance. As shown in the diagram.
The probe is positioned at the observation point of the measuring water trough. It measures the water level flowing through the trough. During installation, the flow-level relationship of the specific trough is programmed into the instrument’s memory via a lookup table. The microprocessor uses this table to find the corresponding flow value based on the measured water level. If the measured value falls between two entries in the table, linear interpolation is used to estimate the flow rate.
Several factors should be considered when selecting an ultrasonic open channel flowmeter:
1. Channel size and shape, flow range (including maximum and minimum flow).
2. Required measurement accuracy.
3. Installation location and environmental conditions.
4. Liquid properties, including cleanliness, solid content, and corrosiveness.
5. Allowable rise or drop in water level and channel slope.
6. Materials of the instrument parts that come into contact with the liquid.
7. For liquids with high turbidity or low conductivity, it may be necessary to investigate whether an ultrasonic or electromagnetic flowmeter is more suitable.
Estimating channel flow and determining the water level rise:
For new systems, the flow and installation location can be calculated using the process flow, allowing for proper meter selection. For existing facilities, it's essential to estimate the current flow and confirm that the upstream water level can rise within acceptable limits. The flowmeter specifications depend on peak flow and allowable water level increase. Two common methods are used: the float method and daily discharge estimation.
1. **Float Method**: This involves measuring the surface velocity of the water. Two people stand at a distance L (20–50 meters) apart along a straight section of the channel. One person releases a float (like a wooden disc), and the other starts timing when the float passes and stops when it reaches the downstream point. The surface velocity is calculated as υ = L / t. Using the cross-sectional area A, the flow rate Q can be estimated with a correction factor K (typically 0.84–0.90).
2. **Daily Discharge Estimation**: When the float method isn't feasible, the peak flow Qp can be estimated from the daily discharge Qd using equations involving the uneven coefficient Ku and the concentrated discharge hours H per day.
**Determining Water Level Rise**:
Except for full-tube electromagnetic flowmeters, the water level upstream must be raised to allow measurement. In new designs, the required rise is determined based on the flow range and environmental conditions. For existing channels, the impact of the water level rise must be considered to avoid overflow. Flowmeter specifications are then selected based on the measured water level rise and peak flow.
**Basic Requirements for Annual Calibration**:
1. Calibration should be performed under controlled environmental conditions, matching either lab standards or field conditions.
2. The calibration standard should have an error margin 1/3 to 1/10 of the device being calibrated.
3. Personnel performing the calibration must be qualified and certified. Only certified individuals can issue valid calibration certificates and reports.
This article was originally published on electromagnetic flowmeter and orifice flowmeter websites.
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