Flow Measurement in Plain Words: A Conversation Between a Senior and a Trainee

FLOW SENSORS

Arun is a new trainee. Meena has twenty years in a water and chemical plant. Their talk on a pump-house walk covers most of what flow sensors do.

PART 8 OF 25 | 5 MIN READ | BY SUBODH RAJ

Arun
Why are there so many types of flowmeter? Is flow not just flow?

Meena
The liquid decides. Water, oil, steam and slurry behave differently, so one meter does not fit all.

Arun
Which one do we use for dirty water?

Meena
Electromagnetic. It has nothing inside the pipe to block, so sludge does not hurt it. The only rule is that the liquid must conduct electricity, so it will not work on pure oil.

Arun
And for oil?

Meena
Positive displacement or Coriolis. Positive displacement traps a fixed volume each turn, so thick oil is no trouble. Coriolis measures mass directly and is very accurate, but it costs more.

Arun
What about that clamp-on box on the cooling line?

Meena
Ultrasonic. It sends sound through the pipe wall, so we fitted it without cutting the pipe. Good for checking a line quickly. It needs a clean, full pipe, because bubbles confuse it.

Arun
Why do we still use orifice plates? They look old.

Meena
Because they are cheap, proven and well understood. The meter measures pressure drop across a narrow point, and flow follows the square root of that drop. The cost is some lost pressure.

Arun
What is square root about it?

Meena
Double the flow and the pressure drop becomes four times. So a transmitter at the low end of its range gives poor accuracy. Pick the range carefully.

Arun
Is vortex good for steam?

Meena
Yes. A bar in the flow makes small whirls, and the number of whirls per second follows the speed. No moving parts, so steam is fine. But it needs a minimum flow to make clear whirls.

Arun
What is the one mistake to avoid?

Meena
Putting the meter in the wrong place. Most meters need straight pipe before and after them. Without it, the reading is wrong even if the meter is perfect.

Quick takeaways

Dirty water: electromagnetic.
Thick liquids: positive displacement or Coriolis.
Retrofit checks: clamp-on ultrasonic.
Gas and air: thermal mass or vortex.
Always leave straight pipe run, and always check the liquid is compatible.

The 12 sensors in this part

Each card below shows the sensor in use, what it is and how it works, and where you will find it.

SENSOR 1 OF 12

Differential-Pressure Flow Sensor

A differential-pressure flow sensor places a restriction, such as an orifice plate, a venturi or a nozzle, in the pipe. The fluid speeds up at the restriction and the pressure drops. A DP sensor measures this pressure drop, and the flow is calculated from it. It is a proven and widely used method for steam, gas and liquid.

USED IN

steamgas and liquid lines

SENSOR 2 OF 12

Electromagnetic Flow Sensor

An electromagnetic flow sensor works on Faraday's law. A magnetic field is applied across the pipe, and when a conductive liquid flows through it, a small voltage is produced that is proportional to the flow speed. It has no moving parts and no obstruction, so it is ideal for water, sewage and slurry.

USED IN

waterwastewater

SENSOR 3 OF 12

Ultrasonic Flow Sensor

An ultrasonic flow sensor measures flow using sound waves. In the transit-time method, sound pulses are sent with and against the flow, and the difference in travel time gives the speed. Clamp-on versions mount on the outside of the pipe, so the pipe need not be cut. It is useful for retrofit measurement.

USED IN

retrofit pipe measurement

SENSOR 4 OF 12

Coriolis Mass Flow Sensor

A Coriolis mass flow sensor has one or two tubes that vibrate. When fluid flows through the tubes, the vibration gets twisted, and the amount of twist is proportional to the mass flow rate. It measures mass directly, also gives density, and is very accurate, so it is used for batching and custody transfer.

USED IN

batchingcustody transfer

SENSOR 5 OF 12

Thermal Mass Flow Sensor

A thermal mass flow sensor has a heated element in the gas stream. The moving gas cools the element, and the more gas flows, the more it cools. The electronics measure how much power is needed to keep the temperature, which gives the mass flow. It is common for compressed air and other gases.

USED IN

compressed airgas

SENSOR 6 OF 12

Vortex Flow Sensor

A vortex flow sensor places a bluff body (a bar) in the flow. The fluid forms small whirls (vortices) alternately behind the bar, and the number of vortices per second is proportional to the flow speed. A sensor counts them. It has no moving parts and is used for steam, gas and liquids.

USED IN

steamgasesliquids

SENSOR 7 OF 12

Turbine Flow Sensor

A turbine flow sensor has a small rotor with blades placed in the flow. The moving fluid makes the rotor spin, and the speed of spinning is proportional to the flow rate. A pickup counts the blade passes. It is accurate for clean, low-viscosity liquids such as fuel and water.

USED IN

fuelclean liquids

SENSOR 8 OF 12

Positive-Displacement Flow Sensor

A positive-displacement flow sensor measures flow by trapping a fixed volume of liquid in chambers, such as gears or pistons, and passing it forward. The number of fills is counted, and this gives the total volume. It is very accurate even for thick liquids like oil, and it works at low flow.

USED IN

oilsviscous liquids

SENSOR 9 OF 12

Variable-Area Flowmeter (Rotameter)

A variable-area flowmeter (rotameter) has a tapered glass tube with a float inside. As the flow increases, the float rises to a bigger gap until the forces are balanced. The float position on a scale shows the flow. It needs no power, is simple and gives a direct visual reading.

USED IN

purge lineslabs

SENSOR 10 OF 12

Paddle-Wheel Flow Sensor

A paddle-wheel flow sensor has a small wheel with paddles fitted in the pipe. The flowing liquid turns the wheel, and a sensor counts the turns to give flow speed. It is low cost and easy to install, so it is used for water, cooling circuits and irrigation where high accuracy is not needed.

USED IN

cooling waterirrigation

SENSOR 11 OF 12

Averaging Pitot Tube (Annubar)

An averaging Pitot tube (Annubar) is a bar placed across the pipe with several small holes facing the flow. It measures the average of the total and static pressures, and the difference gives the flow. It causes very little pressure loss and is suited to large pipes, ducts and boilers.

USED IN

large ductsboilers

SENSOR 12 OF 12

Open-Channel Flow (Weir / Flume)

Open-channel flow is measured in channels, canals and drains using a weir or a flume. These are structures that make the water level rise in a known way as the flow changes. A level sensor measures the water height, and the flow is calculated from it. It is used for effluent, sewage and irrigation.

USED IN

effluentsewagecanals

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Written by Subodh Raj, EngineerX Academy. Part 8 of 25 in the Industrial Sensors series.