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Weir Flow Meter: Open Channel V-Notch Wastewater Volume Calculations
Quick Answer
A V-notch weir flow meter converts upstream water level H into open channel wastewater volume. The 90 degree V-notch is the most common shape for wastewater because it covers roughly 0.5 L/s to 50 L/s. Pair the weir plate with an ultrasonic level transmitter and use the built in totalizer to record daily volume in m3.
What Is a V-Notch Weir Flow Meter
This meter uses a thin plate with a sharp V shaped opening. Wastewater passes through the opening. The upstream water level H rises as flow increases. An ultrasonic sensor mounted above the channel measures H without touching the water. The flow meter calculates flow Q from that H value. Silver Instruments supplies open channel flow meters for this task.
The method works well in open channels, final effluent drains, and industrial wastewater sumps. No moving parts contact the water. That matters in wastewater with grit, rags, and solids.
V-Notch Flow Equation and Common Angles
The standard flow formula is Q = Cd x 8/15 x square root of 2g x tan(angle/2) x H raised to 5/2. H must be in meters. Q comes out in m3/s. For a 90 degree V-notch, tan(45) is 1. A practical Cd value is about 0.58. The simplified working formula is Q = 1.37 x H^2.5. At H = 0.150 m, Q is about 0.012 m3/s. That equals 12 L/s or about 1,037 m3 per day.
Other angles exist. A 60 degree notch gives more resolution at very low flow under 2 L/s. A 30 degree notch suits flows under 1 L/s. A 90 degree notch is the workhorse for municipal and food plant wastewater.
Do not use a V-notch if the channel carries heavy floating debris or if H can exceed the plate height. In those cases, a rectangular weir or a flume may be better. But for most wastewater discharge monitoring, the V-notch is the simplest reliable choice.
Sensor Selection for Open Channel Wastewater
Most installations use a non contact ultrasonic level sensor. Silver Instruments recommends a range of 0 to 5 m. Accuracy of 2 mm or better is enough for H values from 30 mm to 600 mm. Output should be 4-20 mA HART plus RS485 Modbus. The transmitter should have a totalizer for daily flow m3.
Temperature compensation is useful in the Middle East and Southeast Asia because open channels can heat up quickly. A PT100 temperature sensor in the probe helps keep the speed of sound correction accurate.
Here is the thing: ultrasonic works well for most wastewater, but foam or steam can weaken the echo. In that case, radar is more stable. For ordinary wastewater, ultrasonic gives the best balance of price and accuracy.
Installation Details That Affect Volume Accuracy
Install the weir plate vertical and centered in the channel. The crest must be sharp and clean. The upstream channel should be straight for at least 10 times the channel width. For a 0.5 m wide channel, allow 5 m of straight run before the plate. Mount the ultrasonic sensor upstream at 3 to 4 times Hmax. For Hmax of 0.5 m, mount the sensor 1.5 to 2.0 m from the plate.
Set the zero correctly. The zero is the exact crest elevation. Most eng
Wastewater Volume Calculation Example
A customer in Vietnam installed a 90 degree V-notch in a 300 mm wide channel. H was 0.180 m during peak discharge. The flow meter displayed 18.6 L/s. Daily volume at steady peak was 1,607 m3. The plant used this value to report discharge compliance to the local environmental authority.
For variable flow, the totalizer integrates Q over time. The formula is volume equals the sum of flow times sample interval. In a PLC, this is simple. In a Silver Instruments open channel meter, the totalizer does it internally and stores the daily total in RAM.
Common Mistakes We See on Customer Sites
One mistake is placing the sensor in a dead zone. Ultrasonic sensors have a blanking distance, often 0.3 m. If the sensor is too close to the plate or the channel wall, the echo may be lost. Another mistake is ignoring downstream submergence. If the tailwater rises above the crest, the flow reading becomes too high. Keep the downstream level below the bottom of the V.
Algae and sludge also change the crest shape. Clean the plate weekly in warm climates. A stainless steel 304 or 316 plate lasts longer in wastewater. Do not paint the crest. Paint changes the edge and ruins accuracy.
Recommended Configuration
For a standard 90 degree V-notch wastewater flume, specify a Silver Instruments open channel flow meter with a 0 to 5 m ultrasonic sensor, 4-20 mA HART output, RS485 Modbus, two relay outputs, and a weatherproof IP66 transmitter. The transducer should be IP68 for flooded conditions. The display shows H in mm, flow in L/s, and total volume in m3.
Send us your weir type, maximum head Hmax, channel width, power supply, and target country. We will return a quote with the right range and mounting bracket. Contact Silver Automation Instruments. Tel +86-25-68650347. WhatsApp +86-25-52155837. WeChat +86 15365082610.
FAQ
What is the best V-notch angle for low wastewater flow?
A 90 degree V-notch covers most low to medium flows from 0.5 L/s to 50 L/s. For flows under 1 L/s, use a 30 or 60 degree notch.
How do I calculate wastewater volume from a V-notch weir flow meter?
Measure H in meters. Use the formula Q = 1.37 x H^2.5 for a 90 degree notch. Multiply Q in m3/s by seconds to get volume. Or use a totalizer output from the flow meter.
Can a V-notch weir flow meter handle sludge and solids?
Yes if the channel carries low to medium solids. The sensor does not contact water. Clean the plate regularly to keep the crest sharp.
What sensor works for open channel wastewater?
Ultrasonic level is the most common. It is affordable and accurate. Radar works better in foam or steam but costs more.
Does Silver Instruments supply the full weir flow meter package?
Yes. We supply the ultrasonic transmitter, sensor, mounting bracket, and commissioning support. Send us your Hmax and channel size for a quote.