『Gravity Flow Meter Selection: Non-Intrusive Open Channel and Partially Filled Pipe Testing Methods』Related information(clamp on meter|electromagnetic meter|venturi meterrotameter|orifice meter|ultrasonic flow meter|mass flow meter|coriolis mass flow meter|coriolis flow meter|magnetic flow meter|magmeter flow meter|magflow flow meter|mag meter flow meter|electromagnetic flow meter|vortex flow meter|turbine flow meter|thermal mass flow meter|thermal flow meter|rotameter flow meter)
Gravity Flow Meter Selection: Non-Intrusive Open Channel and Partially Filled Pipe Testing Methods
Quick Answer: For open channel gravity flow, use an ultrasonic level transmitter with a flume or weir. For partially filled pipes, select a top-mounted radar velocity area meter. Both methods are non-intrusive because no sensor part touches the liquid. They output total flow via 4-20 mA HART or Modbus.
A gravity flow meter selection starts with the pipe or channel condition. Gravity flow means the liquid moves because of slope, not pump pressure. We see this in wastewater drains, stormwater channels, irrigation canals, and partially filled sewer pipes. The pipe or channel is open to atmospheric pressure. The flow area changes with level. So you cannot use a standard full-bore electromagnetic or Coriolis meter. You need level plus velocity or a primary structure.
Non-Intrusive Method 1: Ultrasonic Level Plus Flume or Weir
An ultrasonic level sensor mounts above the channel. It sends a sound pulse to the water surface and calculates head. Pair it with a Parshall flume or V-notch weir. The flume or weir fixes the level-to-flow relationship. The transmitter then converts head into flow rate and total volume. No sensor touches the water. That matters for raw sewage, sludge, and corrosive industrial effluent.
In practice, we recommend a 300 mm Parshall flume for plant effluent from 20 to 300 m3/h. For lower flows below 10 m3/h, use a V-notch weir with a 90 degree notch. Accuracy usually lands between 1 and 3 percent of reading if the approach channel is straight and clean. Use an ultrasonic level meter with 4-20 mA HART, two relay outputs, and Modbus RS485. A customer in Vietnam on a food processing site used this setup on a 250 mm concrete channel. They measure daily discharge volume with no wetted parts and no moving parts.
Non-Intrusive Method 2: Radar Velocity Area for Open Channels
Radar flow meters send microwave pulses to the water surface. They measure both water level and surface velocity without touching the liquid. The flow computer calculates area and velocity. This method works in channels from DN150 to DN3000. It needs no flume or weir. Use it when a primary structure is impossible, for example an existing concrete drain in a chemical plant or a large stormwater outfall.
Radar is more tolerant of floating solids, grease, high temperature up to 120 °C, and sudden level changes. But it is not a magic fix. Heavy foam absorbs the signal. Wind over 5 m/s can disturb surface velocity readings. Surface velocity is not mean velocity, so you need a correction factor. Most engineers skip this part, then wonder why the totalizer drifts. We have seen this on customer sites in the Middle East with very wide channels. Result accuracy is typically 2 to 5 percent of reading.
Non-Intrusive Method 3: Top-Mounted Radar for Partially Filled Pipes
For a partially filled DN200 sewer pipe, a top-mounted radar area velocity meter is often the only non-intrusive option. The instrument installs on a nozzle or bracket at the pipe crown. It measures the distance to the water surface and the surface velocity. The internal flow computer calculates the wetted area using the pipe ID and slope. Output is flow rate and total volume. Installation takes a few hours. You do not need to empty the pipe or cut it open.
Do not select a clamp-on transit time ultrasonic meter for partially filled pipes. That me
Selection Parameters We Ask For
Send us these values and we can recommend a model in one day. First, pipe or channel size in DN or width. Second, liquid type and solid content. Third, flow range in m3/h or L/s. Fourth, temperature and hazardous area rating. Fifth, data output you need: 4-20 mA HART, Modbus RTU, pulse, or all three.
For example, a paint manufacturer in Southeast Asia needed to monitor a partially filled DN150 drain with solvent traces. We supplied a radar velocity area meter with ATEX Zone 1 rating and Modbus RS485. The meter sends flow total to their PLC. Because the sensor is non-contact, the solvent vapors do not damage it.
Silver Instruments Recommended Models
Silver Automation Instruments supplies ultrasonic level transmitters for flume and weir applications. The standard model covers 0 to 10 m range, 4-20 mA HART, Modbus RTU, IP68 sensor, and ±0.25 percent level accuracy. Pair it with a Parshall flume or V-notch weir kit made of stainless steel or GRP.
For radar area velocity, we offer a non-contact open channel radar flow meter for channels and partially filled pipes. It handles DN150 to DN3000, flow velocity 0.1 to 20 m/s, temperature up to 120 °C, and 24 V DC power. Output includes 4-20 mA HART, Modbus RTU, and pulse. Use it for stormwater, wastewater, desalination brine, and industrial drains.
On job sites, if the liquid has very low conductivity like demineralized water, radar is the better choice because electromagnetic meters need conductivity above 5 µS/cm. But for full pipes, a Coriolis mass flow meter or electromagnetic flow meter gives higher accuracy. Gravity flow is different. You trade some accuracy for a non-intrusive installation and total volume tracking.
FAQ
Can I use a clamp-on ultrasonic flow meter on a partially filled pipe?
No. Clamp-on transit time meters need a full pipe. Use a top-mounted radar area velocity meter instead.
What is the smallest pipe size for non-intrusive radar flow measurement?
DN150 for most radar area velocity meters. Below DN150, use an ultrasonic level meter with a small V-notch weir or a flume.
Do I need a flume or weir for open channel flow measurement?
Not always. A radar velocity area meter works without a primary structure. But a flume or weir improves accuracy to ±1 to 3 percent.
What accuracy can I expect from non-intrusive gravity flow meters?
Ultrasonic level plus flume or weir: ±1 to 3 percent. Radar velocity area: ±2 to 5 percent depending on surface turbulence and installation.
How do I send flow data to a PLC or SCADA?
Most Silver Instruments meters include 4-20 mA HART and Modbus RTU. Pulse output is available for totalizer panels.
Contact Silver Automation Instruments for a non-intrusive gravity flow meter selection. Tel +86-25-68650347. WhatsApp +86-25-52155837. WeChat +86 15365082610. Send us your liquid type, pipe size in DN, flow range in m3/h, temperature in °C, and hazardous area requirement. We will reply with a model and price within 24 hours.
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