Flow - Liquids

Flow measuring of liquids is needed in many industries and in certain applications accurate flow measurement is so important that it can make the difference between profit and loss. Flow is classified into open channel flow and closed conduit flow. Most devices measure flow indirectly and are commonly divided into those that sense or measure velocity and those that measure pressure or head.

Sigma's flow meter range includes turbine, electromagnetic, coriolis, vortex, swirl, oval gear, ultrasonic, calorimetric, variable area, and micro wave.

flow measuring systems using impeller sensors

200 series

Flow measuring system for closed pipes with “T” mount PVC impellers, that feature a six bladed impeller design with a non magnetic sensing mechanism connected to a 4-20mA transmitter, and an optional remote digital meter/controller with multipple choice of outputs (2/4 transistor relays, 0-10V/4-20mA, RS232/RS485, Profibus, DeviceNet).

Six bladed impeller PVC Tee flow sensors
Also available in brass/bronze, and stainless steel sensors together with the relevant metal Tees,as well as special versions for underground installations.

Accuracy 1% of the max range (0.3-9.0m/sec)
Linearity 1% of the max range
PVC Tee available for 1 ½, 2, 3, 4” pipes
(or threaded fittings for large diameter pipes)


Pulse converter / transmitter 4-20mA
Power supply 15-40 VDC (via panel meter).
Software for adjustment via the PC.
Maximum distance of transmitter from
sensor  500m

Digital Controller
0-20mA or 0-10 VDC  Input
1/8 DIN (48mm x 96mm)
5 Digit, 0.56" (14mm) LED
Voltage 85 to 250 VAC, 50/60 Hz, 15 VA
Field Upgradeable Option Cards (2/4 transistor relays, 0-10V/4-20mA, RS232/RS485, Profibus, DeviceNet)
Power supply :220 VAC

100 series

  • flow ratemeter
  • totalizer /
  • batch controller

for closed pipe flow measuring systems
with “T” mount PVC impellers, that feature a six bladed
impeller design with a non magnetic sensing mechanism.

Six bladed impeller PVC Tee flow sensors
Also available in brass/bronze, and stainless steel sensors
together with the relevant metal Tees,as well as special versions for underground installations.

Accuracy 1% of the max range (0.3-9.0m/sec)
Linearity 1% of the max range
PVC Tee available for 1 ½, 2, 3, 4” pipes
(or threaded fittings for large diameter pipes),


Batch Controller Model 515

Α batch controller that covers most fluid batching requirements that use one or two stage control valves. Starting , stopping or setting the preset quantity for each batch can be achieved from the front keypad or further automated via serial communications. The hardware I/O caters for signals such as End Of Batch, Flow Alarm and Pulse Output. The state of the relays can be defaulted to either "open" or "closed".

The 515 Batch Controller is a dual stage batch controller for reliable measurement of preset quantities. Used as a single or dual stage controller it is suitable for fast batch applications. It provides the operator with clear local readout and can be controlled via communications in more automated systems. There is quick access to commonly used preset values directly from the front panel if access has been authorized.
Automatic overrun compensation caters for system delays such as valve closure for precise volumes. The instrument is compatible with a wide range of flowmeter signal outputs, including analogue 4 -20mA, millivolt signals, reed switches, Namur proximity switches and pulse trains via its smart front-panel program selection. Different versions are available for pulse/frequency or analogue outputs.

• Tailored for volumetric frequency flow input
• Single or Dual stage control
• Quick access to common batch quantities
• No-flow, leakage and overflow error detection
• Allows for non-linear correction
• Storage of 100 transactions with time and date stamp
• Selection of second language and user tags
• Infra-red communications port on front panel
• Pulse width and scaling of pulse output
• 4-20mA re-transmission
• Sellectable protocols on serial ports including Modbus RTU and Printer output
• Front panel adjustment of 8-24V DC output voltage
• Backlit display
• LCD backup


Power Supply: 95...135 V AC or 190...260 V AC or 12...28 V DC
Consumption: Typically 6W
Temperature: 0°C to +60°C (conformal coating) +5°C to +40°C (no coating)
Humidity: 0 to 95% non condensing (conformal coating) 5% to 85% non condensing (no coating)
Protection: Sealed to IP65 (Nema 4X) when panel mounted
Dimensions 147mm (5.8") width 74mm (2.9") height 167mm (6.6") depth

Type: LCD with 7-digit numeric display and 11-character alphanumeric display (backlit option)
Digits: 15.5mm (0.6") high
Characters: 6mm (0.24") high
LCD Backup: Last data visible for 15min after power down
Update Rate: 0.3 second

Non-volatile Memory
Retention: > 30 years
Data Stored: Setup, Totals and Logs

Interference: CE compliance
Enclosure: ATEX, FM, CSA and SAA approved enclosures
available for hazardous areas

Real Time Clock (Optional)
Battery Type: 3 volts Lithium button cell (CR2032)
Battery Life 5: years (typical)

Frequency Input (General)
Range 0 to 10kHz
Over-voltage 30V maximum
Update Time 0.3 sec
Cutoff frequency Programmable (default at 0.25Hz)
Configuration Pulse, coil or NPS input
Non-linearity Up to 10 correction points

Signal Type CMOS, TTL, open collector, reed switch
Threshold 1.3 volts
Signal Type Turbine and sine wave
Sensitivity 15mV p-p minimum

Signal Type NPS sensor to Namur standard

Remote Logic Inputs
Signal Type Voltage free contact, open collector
Relay Output
No. of Outputs: 2 relays
Voltage: 250 volts AC, 30 volts DC maximum
Current: 3A maximum

Communication Ports
Ports RS-232 port, RS-485 port, Infra-red port (optional)
Baud Rate: 2400 to 19200 baud
Parity: Odd, even or none
Stop Bits: 1 or 2
Protocols: ASCII, Modbus RTU, Printer (RS232)

Transducer Supply

Voltage: 8 to 24 volts DC, programmable
Current: 70mA @ 24V, 120mA @ 12V maximum
Protection: Power limited output

Pulse/Digital Output
Signal Type: Open collector, non-isolated
Switching: 200mA, 30 volts DC maximum
Saturation: 0.8 volts maximum
Pulse Width: Programmable: 10, 20, 50, 100, 200 or 500ms

4-20mA Output (Optional)
Supply: 24 volts DC internal, non-isolated
Resolution: 0.05% full scale
Accuracy: 0.05% full scale (20°C) 0.1% (full temperature range, typical)


  • Tanker Loading
  • Bio-Blending
  • LPG bulk deliveries
  • Chemical IBC filling
  • Additive Injection
  • Weighbridge replacement
  • Edible oil batching
  • Ship bunkering
  • Tanker Unloading
  • Truck Registers
  • Cryogenic metering

Sigma LTF Turbine Flow Meter for full pipes


Designed for performance, this highly accurate with excellent repeatability light weight system, consists of a flow sensor and a signal converter. It is available as compact version and in separate sections version.
Sigma LTF Turbine Flow Meters are ideally suited for low viscosity without impurities industrial fluids.

Flange connection type (DN20-DN200)
Thread connection type (DN4-DN50)
Insertion type (DN150-DN1100)
Standard accuracy: 1.0% of Rate, Optional 0.5% of Rate
Output signal: Pulse output, 4-20mA
Display: available with or without LCD Display
Body Material: SS304, Optional SS316
Impeller Material: 2Cr13 or CD4Mcu Stainless Steel

Bearing Material: Tungsten Carbide
Medium teperature: --20-80? 80-150? (High Temperature version
Power Supply: +24VDC±15%
Protection rating: IP60 & IP65 (Ex versions)
Explosion rating: ExdIIBT6 (optional)

Sanitary version for Food & Pharmaceutical Liquids

The sanitary version of LTF flow meter is specifically designed for measurement of food liquids like milk, cream, juice of various fruits and pharmaceutical liquids etc. It is available with compact or remote version of transmitter, with or without display, pulse or 4-20mA output, can be installed either horizontally or vertically with a variety of optional end-fittings to meet your requirements.

Technical data
-Viscosity from 1 to 10 cst
-Pressure resistant to 10 bar
-Communication: Modbus RS485






Operating Principle:
Turbine Flow Meters use the mechanical energy of the liquid to rotate a rotor in the flow stream. Fluid entering the meter first passes through an inlet flow straightener that reduces its turbulent flow pattern. As each turbine blade passes through the magnetic field generated by the meter's magnetic pickup, an AC voltage pulse is generated. These pulses provide an output frequency that is proportional to volumetric flow. The generated signal is converted into engineering units (liters, cubic meters, gallons etc.) on the local display where applicable. Optional accessory modules can be used to export the signal to other equipment.

Ordering Options
Threaded type  (DN4-DN50)
Flanged  type (DN20-DN200)
Insertion type (DN150-DN1100)
No display; 24V DC; Pulse Output
No display; 24V DC; 4-20mA Output
Local display; Lithium Battery Power; No Output
Local display; 24V DC Power; 4-20mA Output, Optional backup power: Lithium Battery
Local display; 24V DC Power; 4-20mA Output;  Modbus RS485 Communication, Optional backup power: Lithium Battery

Low viscosity without impurities diesel; gasoline; water

Swirl design Flow Meter

Compact type with local display
Accuracy: ±1.5% of rate
Digital display; Temperature & Pressure Compensation
24V DC; 3 wires 4-20mA output; RS485;
Cast Aluminum for body material
Exd II BT6

Swirl flow meters are used for accurate measurement of the flow of gases, liquids and steam over a wide range of temperatures in almost any application. Swirl meters are ideal for use on low viscosity liquids, demineralized water, and for direct, cost-effective steam mass flow.
♦ Gas flow, oil field and urban gas distribution
♦ Petroleum, chemical, electric power, metallurgy industry
♦ Natural gas for many applications
♦ Compressed air, nitrogen gas
♦ Blast furnace gas, combustion-supporting air, mixed gas, flue gas, recycled gas etc

SIGMA ST Series Compact Ultrasonic Clamp on type flow meter without contact with the liquids.

Sigma ST is very unique in its structural design. It uses a circular magnetic steel to achieve automatic connection during installation. It takes only 2-3 minutes to complete the whole process from installation to measurement. It also adopts the current popular OLED display, which makes it easy to read in low light conditions.

Sigma ST series flow meters offer users easy installation, no process downtime, no pressure loss or leak and no contamination of the medium to be measured.

● With the world's most advanced time measurement chip (time resolution of SOps), a new
generation of stable flow meters has been developed
● The flow meter is fitted on the outside of the tube without additional pipe interventions required
● No expertise required, installation and measurement can be done according to the operating instructions
● No moving parts, no pressure loss. The flow meter can be installed without stopping production

Flow Range: 0.1 m/s~5.0 m/s
Accuracy: ±2.0%
Repeatability: 0.8%
Pipe Size: Φ6.35 ~ Φ90
Sigma ST holds statistics of daily, monthly, and annual consumption over a long period of time.
Flow Totalizer
Analogue Output: 4~20mA, Maximum load: 600Ω
Alarm Output: OCT, Upper and lower limit alarm function (optional)
Communication: RS485, support Mod bus communication protocol
Power Supply: 24 VDC
Cable Length: 2.0m
Keypad: Four light touch buttons
Screen: OLED 128*64 display screen
Units: Metric and imperial units are available, Cubic Meters(m³), Liters(L), USA Gallons(GAL)/hour, /min, Default unit setting: m³/h
Totalizer: Six bit digit
Piping Material: Stainless steel pipe, carbon steel pipe, copper pipe, plastic pipe
Housing Material: Aluminum alloy
Environment Temp. : 0℃~50℃(32°F~122°F)
Medium Temp. : 0℃~50℃(32°F~122°F)
Environment Humidity: 0-95% relative humidity, without condensation
IP rating: IP54

• Treated Water
• Water distribution flow metering
• Building Services
• Sea water
• Leak Detection
• Power/Energy Management
• Food/Hygienic flow-monitoring
• Cooling Water
• Chemical / Pharmaceutical
• Alcohols
• Chemical solvents
• Acids
• Pump inspection
• Hydraulic Oil
• Low-Conductivity Water
• Water / Glycol Solutions
• Diesel
• Survey work
Also used in the production to measure water and other liquids throughout the facility.

Technical Data
Model: SIGMA ST Series
Flow Range: 0.1 m/s~5.0 m/s
Accuracy: ±2.0%
Repeatability: 0.8%
Pipe Size: Φ6.35 ~ Φ90
Data Storage: Daily, monthly, and Annual. Flow Totalizer
Analog Output: 4~20mA, Maximum load: 600Ω
Alarm Output: OCT, Upper and lower limit alarm function (optional)
Communication: RS485, support Mod bus communication protocol
Power Supply: 24 VDC
Cable Length: 2.0m
Keypad: Four light touch buttons
Screen: OLED 128*64 display screen
Units: Metric and imperial units are available, Cubic Meters(m³), Liters(L), USA Gallons(GAL)/hour, /min, Default unit setting: m³/h
Totalizer: Six bit digit
Piping Material: Stainless steel pipe, carbon steel pipe, copper pipe, plastic pipe
Case Material: Aluminum alloy
Environmental Temp.: 0℃~50℃(32°F~122°F)
Medium Temp.: 0℃~50℃(32°F~122°F)
Environment Humidity: 0-95% relative humidity, without condensation
IP rating: IP54

Also available with optional remote display


Sigma 226 Series

Wall-mount Ultrasonic Flow meter

Flow range: 0.03m/s-10m/s
Accuracy : ±1% of the measure value
Repeatability: 0.2%
Display: 240*128 back lit LCD
Power supply: 90-245V AC
Transmitter enclosure: IP65, ABS
(Temperature: -20℃~60℃)
Output: OCT pulse output 0-10KHz, Relay
output, 4-20mA optional
Communication: RS485
Pipe size range: 25mm-1200mm
Sensor: Clamp-on type
Protect: IP68
Cable Length: 5 meters/pcs
Sensor Operating temperature: -40℃ ~ +80℃
Supports SD card


A Transit Time Ultrasonic Flow Meter measuring with Clamp-On sensors

for clean liquids in closed full pipes (< in full pipes with less than 2% solids or gas bubbles at flow velocities from ±0.07 to 39 ft/sec (±0.02 to 12 m/sec).


Model 6.1 Transit Time Flowmeter

  • Non-Contacting Clamp-on Ultrasonic Transducers
  • accuracy of ±1% of reading or 0.1 ft/sec (0.03 m/sec), whichever is greater
  • Works on Metal and Plastic Pipes
  • Large Backlit Flow Rate Display and Totalizer
  • Isolated 4-20mA Output
  • Built-in 5-Key Calibrator
  • 2 Programmable Control Relays
  • 128mb Data Logger

Accurately measure the flow rate of clean, non-aerated liquids like water, chemicals, and oils in full pipes. Ultrasonic transducers mount on the outside of a pipe without shutting down flow. Flow rate is displayed on the large, backlit display along with totalizer and signal strength. Use the built-in control relays for flow alarms or flow proportionate pulse. Connect the isolated 4-20mA output to PLC's or controllers.

Model 6.1 works on a wide range of pipe sizes and materials including carbon steel, stainless steel, ductile iron, cast iron, PVC, PVDF, fiberglass, copper, brass, aluminum and pipes with bonded liners including epoxy, rubber and Teflon.

Model 6.1 Transit Time Flowmeter operates by measuring the “transit time” or “time of flight” for ultrasonic sound pulses transmitted from one transducer to another. Depending on the mounting configuration, the signal may cross the pipe once, twice or four times. To measure flow the transit time is compared between ultrasonic signals travelling upstream and downstream.

Standard Features Include:
pair SE16B ultrasonic transducers - clamp-on for 1/2” to 48” (12 mm to 1200 mm) pipes and rated non-incendive for Class I, Division 2, Groups A, B, C & D locations
sensor cables - pair 25 ft / 7.6 m coaxial with BNC connectors
flow rate range: ± 0.07 to 39 ft/sec (± 0.02 to 12 m/sec)
installation - stainless steel transducer mounting brackets and clamps, alignment bar and silicone coupling compound kit
enclosure - watertight, NEMA4X (IP66) polyester and polycarbonate
display - white, backlit matrix
totalizer - 14 digits
calibration - built-in 5-key programmer
output - isolated 4-20mA (1000 ohm)
2 control relays - 5 amp, SPDT - programmable for flow proportional pulse output, and/or flow alarm
plug-and-play options including data logger and extra control relays
power input - 100-240VAC 50/60Hz, 4 Watts maximum (optional 9-32VDC 10W)
electrical surge protection and RFI filters - AC, sensor, 4-20mA

How it works

Transit Time transducers typically operate in the 1-2 MHz frequencies. Higher frequency designs are normally used in smaller pipes and lower frequencies for large pipes up to several meters in diameter. So operators must select transducer pairs/frequencies according to the application. Doppler transducers usually operate at 640 kHz to 1 MHz frequencies and work on a wide range of pipe diameters.
Transit Time Flowmeter with Two Ultrasonic Sensors     Transit Time flowmeters must have a pair of transducers, each containing a piezo-electric crystal. One transducer transmits sound while the other acts as a receiver.
As the name suggests, Transit Time flowmeters measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor. Upstream and downstream time measurements are compared. With no flow, the transit time would be equal in both directions. With flow, sound will travel faster in the direction of flow and slower against the flow. Because the ultrasonic signal must cross the pipe to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise the high frequency sound will be attenuated and too weak to traverse the pipe.

Α flowmeter for difficult liquids, corrosive chemicals, viscus liquids, wastewater, slurries, sludge, etc

Non Intrusive - Doppler Model 6.1

Doppler Model 6.1 is ideal for "difficult liquids" that would damage regular flow meters - wastewater, slurries, sludge, chemicals, viscous liquids and abrasives. There is no obstruction to flow and no pressure drop. The standard, strap-on ultrasonic sensor fits any pipe 1/2" (12.5 mm) Inside Diameter or larger. The electronics/display enclosure can be mounted nearby.

Use the isolated 4-20mA to transmit flow to remote displays, recorders or controllers. The 2 programmable relays can be used for flow control, pump protection or flow proportionate pulse. Calibration is easy with the built-in 5-key calibrator. Display and totalize flow in your choice of engineering units (gallons, liters etc.)

Sensor cable can be extended up to 500 ft. (152 m) with no loss of signal. Model 6.1 can be equipped with intrinsic safety barriers for sensor installation in hazardous rated locations. Other options include high temperature sensors.

Doppler Model 6.1 includes a new digital signal processor to filter background noise and interference and to measure flow rates between 0.2 to 40 ft/sec (0.06 to 12.2 m/sec) with accuracy of ±2% of full scale on liquids with entrained particles or gases of 100 microns or larger and minimum concentrations of 75 ppm. The flow meter operates with a single-head flow sensor mounted externally on any contiguous pipe material that conducts sound including: carbon steel, stainless steel, ductile iron, copper, FRP, PVC, ABS, or cement lined pipe from 1/2" to 180" (12.5 mm to 4.5 m) ID.

Main Features

  • Non-Contacting Ultrasonic Sensor
  • Large, Backlit LCD Display
  • 12-Digit Totalizer
  • power input - 100-240VAC 50/60Hz
  • Isolated 4-20mA (1000 ohm)
  • 2 control relays programmable for flow
  • reverse flow measurement
  • backflow rejection - external, voltage-free relay input
  • Automatic Sensitivity Adjustment
  • New Signal Processing System
  • Built-in 5-Key Calibrator
  • RFI Rejection Filters
  • Built-in 26-million point data logger with USB data transfer
  • sensor - strap-on, single head ultrasonic for 1/2" to 180" (12.5 mm to 4.5 m) ID pipes
  • sensor operational temperature: from -40°F to 200°F (-40°C to 93°C) High temperature optional
  • sensor cable - 20ft / 6m shielded coaxial pair
  • enclosure - watertight NEMA4X (IP66) polycarbonate

optional: Intrinsic Safety Barriers for Sensor mounting in hazardous locations,
SE3H High Temperature model rated up to 302°F (150°C),
ISE Insertion type for special applications

How it Works

Doppler Model 6.1 Ultrasonic flow meters measure flow from outside a pipe with a strap-on sensor. Doppler Model 6.1 meters continuously transmit high frequency sound (640 kHz) that travels through the pipe wall and into the flowing liquid. Sound is reflected back to the sensor from solids or bubbles in the fluid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. Model 6.1 flow meters continuously measure this frequency shift to calculate flow.

The Doppler technique only works on liquids which contain solids or gas bubbles to reflect its signal. These are "difficult" liquids that may damage regular flow meters: slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. Because the sensor mounts on the outside of the pipe, there is no pressure drop and no obstruction to flow.

For best performance Doppler sensors should be mounted away from turbulence creating devices like pipe elbows and tees, and away from velocity increasing devices like controlling valves and pumps. Typical accuracy is ±2% of full scale.

Open Channel Ultrasonic Flow Measuring Systems

Open Channel Flow Monitor - Model OC5

Continuously monitor, display, totalize and data log flow through any flume or weir. The OC 5.0 includes a non-contacting ultrasonic sensor that mounts just 8" (203.2 mm) above the flowing water. It is accurate (±0.25%) and reliable. There is no obstruction to flow and no fouling. Use the built-in Keypad/menu system to select your flume or weir from the Menu and to choose measurement units (gallons, liters etc.)

The OC 5.0 includes built-in 2-million point Data Logger and flow reporting system. Connect a flash memory stick to the flowmeter's USB output to download log files and then view your data in graph and table formats with 'Logger' software – supplied free with each OC 5.0. Daily flow reports including minimum, maximum, average and total daily flow can be viewed directly on the backlit OC 5.0 LCD display.

Use the isolated 4-20mA to transmit flow to remote chart recorders or displays, and the control relays are programmable for level/flow alarm, and flow proportionate pulse (for samplers, chlorinators or remote totalizers). An Intrinsic Safety Barrier is optional for sensor and cable installation in hazardous rated locations. The separate, watertight electronics/display enclosure is mounted at a convenient location within 500 ft (150 m) of the sensor.


  •  Non-Contacting Ultrasonic Sensor
  •  Accurate and Verifiable
  •  Built-in 2 million point Data Logger
  •  USB and RS232 Outputs
  •  4-20mA/0-5V Output
  •  2 Programmable Control Relays
  •  Simple 5-key Calibration

Area-Velocity Flow Monitor- Model-5.0

Monitor flow through open channels, partially full sewer pipes and surcharged pipes without a flume or weir. Ideal for stormwater, combined effluent, raw sewage, irrigation water and streams.
Model 5,0 uses a submerged ultrasonic sensor to continuously measure both Velocity and Level in the channel. The sensor is a completely sealed ultrasonic unit with no orifices or ports. It mounts inside a pipe or at the bottom of a rectangular, trapezoid or egg-shaped channel. Calibration is simple: enter the pipe diameter or channel width and Model-5 computes the flow volume and displays the flow rate.


  • For Open Channels and Pipes
  • Ultrasonic - Measures Velocity + Level to Calculate Flow
  • No Flume or Weir required
  • 3 Isolated 4-20mA Outputs (Flow, Level and Velocity)
  • Totalizer and 2 Programmable Control Relays
  • Simple 3-key Calibration - Password Protected
  • Optional Intrinsically Safe Sensor
  • Optional built-in 2 million point Data Logger and Software

This flow meter is recommended for pipe diameters greater than 6" (150 mm). Maximum pipe diameter or channel width should not exceed 10 ft (3 m) unless flow velocity is evenly distributed across the channel width. Avoid sensor mounting locations where sediment builds up and locations where there is an elevation drop or bend immediately upstream from the sensor. Maximum pipe or channel slope should not exceed 3%.

The flow meter  measures flow velocity from 0.1 to 20 ft/sec (0.03 to 6.2 m/sec) and reverse flow up to 5 ft/sec (1.5 m/sec)

How it Works

The ultrasonic sensor is installed at the bottom of a pipe or channel. To measure water level the sensor transmits ultrasonic pulses that travel through the water and reflect off the liquid surface. The instrument precisely measures the time it takes for echoes to return to the sensor. Based on the speed of sound in water, the level is measured with accuracy of ±0.25%.

Flow velocity is measured with an ultrasonic Doppler signal continuously injected into the water. This high frequency sound (640 KHz) is reflected back to the sensor from particles or bubbles suspended in the liquid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. With this technique the instrument measures flow velocity with accuracy of ±2%.

A separate down-looking ultrasonic sensor can be used for highly aerated or turbulent flow applications. It measures level by transmitting ultrasonic pulses through the air to the liquid surface with accuracy of ±0.25%. Along with the level sensor, a submerged Doppler velocity sensor is used to measure the water velocity.