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Flow Indicators

Flow Indicator Definition

What is a Flow Indicator?

The term “flow indicator” is used for a few different elements in the industrial instrumentation world. It is helpful to be aware of the differences because if you don’t understand them, it can make finding the right device for your needs more difficult.

The most traditional usage of the term “flow indicator” is to refer to a device that does not measure the flow rate. Nor does it monitor the flow rate in the sense that it can provide switching capabilities. It simply provides verification that flow is occurring. In essence, it answers the question “is flow happening, yes or no?”. It generally does not provide any data on the flow, although most models enable direct viewing of the media inside. KOBOLD uses the term “flow indicator” to refer to this class of flow instrumentation.

Uses of The Term Flow indicator

Another usage of the term “flow Indicator” would more accurately termed as a “flow meter with a mechanical indicator”. This is a flow meter that does provide the measurement of flow rate and displays the rate on a mechanical indicator. Typically, these are pointer “arms” that are on a pivot point that rotate up with an increase in the flow rate. A corresponding scale is on the faceplate and the pointer arm points to the flow rate on the scale.

Another usage of the term “flow indicator” can be a flow meter that also provides a visual indication of the flow. These are typically variable area flow meters, also known as rotameters, that employ a clear glass or plastic tube and an internal float as the measuring principle. Because the tube is transparent, the flow can also be verified visually as you can see if the float has been moved up in the tube and is no longer seated at the bottom of the tube, as it would be if there was no flow present. Certain variable area flow meters can also give an indication of the flow conditions as well because the media can be seen through a sight glass.

Sometimes the term “flow indicator” is used interchangeably with the term “flow meter” because the flow meter provides an “indication” of the flow rate either through mechanical, digital, or other output means. This is not as common of a usage for the term.

What Does a Flow Indicator Do?

For purposes of our discussion and how we classify our products, we will be using the term “flow indicator” for the devices that simply let us know if flow is occurring. Flow indicators signal if flow is occurring within the piping system as most piping systems are not transparent to allow direct observation. Verifying flow requires a mechanism of some kind that is inserted into the piping system as a “window” and provides a visual cue as to what is going on inside.

How Does a Flow Indicator Work?

How a flow indicator works depends on which manner of indication that it is providing. All types of flow indicators function on the same simple principle. Flow through the devices generally moves something or sometimes nothing, in the case of a simple sight glass. Typically, the flow moves a ball or spins a paddle wheel. Flaps, chains, and other less common moving mechanisms are also used. If no mechanism is present at all, then the flow can still be verified by looking closely at the glass tube to see if there is flow happening. The advantages to a flow indicator with a moving mechanism is that they can be seen from much further away.

How a Flow Indicator Works

How Do You Read a Flow Indicator?

Unless you are using the term “flow indicator” to refer to a “flow meter”, you do not “read” a flow indicator. You can view it to see if flow is occurring, but there is nothing to “read” as an indicator does not provide a flow rate measurement to “read”. You typically view a flow indicator for indication that flow is occurring by verifying movement of the indication mechanism.

What is the Difference between Flow Indicators, Flow Meters, Flow Transmitters, Flow Switches, Flow Sensors, and Flow Restrictors?

Now that we have seen the different ways that the term “flow indicator” can be used, it is worth noting the other classes of flow instrumentation to ensure that you are searching for the right type of flow instrumentation.

To help further clarify which type of instrumentation is needed for your application, below is a general listing of most flow instrumentation, from simplest to most complex, with a brief description.

  • Flow Restrictor: A flow restrictor simply restricts the rate of flow within a system to a predetermined amount. It cannot provide flow measurement.
  • Flow Sensor: This term is more of a general catch-all terms for flow instrumentation because they all sense flow, to one degree or another. The term “flow sensor” is sometimes used interchangeably with any of the terms above, but really should be used as more of a general term for the entire class of instrumentation.
  • Flow Indicator: They show that flow is happening. They answer the question, “Is flow happening at all, yes or no?”. Most models allow a clear view of the media as well.
  • Flow Switch: They monitor the flow and trigger an electrical impulse at a certain rate of flow within the system. They do not provide continuous measurement unless the flow switch is part of a full flow meter. They answer the question, “is the flow too high, too low, or happening at a specific point?” or they trigger a specific action in the system. Some flow switches, like variable area flow switches, can also allow for direct viewing of the media as well through a plastic or glass tube.
  • Flow Transmitters: These measure the rate of flow in the system and provide the measurement as an electrical output. Some flow transmitters may be fitted also with integral displays, controllers, or totalizers. Typical flow transmitters are sometimes referred to as “blind transmitters” because looking at the device tells you nothing, unless it connected to something that can interpret the signal produced by the transmitter. They answer the question, “how much flow is happening right now”.
  • Flow Meters: These also provide a continuous measurement and are somewhat erroneously used interchangeably with “flow transmitter”. These devices also provide continuous measurement, but the rate can typically be seen directly at the device itself by a mechanical or digital means. They can also have integral switches. Some flow meter technology types allow for direct observation of the media as well. They can also have integral electronics that enable batching functions, where a precise amount of a media is dispensed. They can also have totalizers that tell you how much media has flowed in a certain period of time or for the total life of the instrument. They also answer the question, “how much flow is happening right now”, and provide many other features.

Other “Flow Indicator” Misnomers: When You Really Need a “Flow Meter” and not an “Indicator”

Featured Products
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DA - All-Metal Sight Flow Indicators - Flap, Rotor, or Drip Indication

Flap, Rotor, or Drip Indication | SS, Cast Iron, or Cast Steel | Up to 580 PSI | 1/4"...2" NPT | 1/2"...8" ANSI

Sight Flow Indicator with Self-Cleaning Mechanism
DAA - Self-Cleaning Sight Glass/Rotor Flow Indicator, Easy Maintenance

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Industrial Sight Flow Indicator
DAI - Heavy-Duty Flow Indicators - Paddle-wheel, Ball, Flap, or Chain

Rotor, Ball, Flap, Chain, No Indication | Rugged Industrial Build | Up to 580 PSI | Up to 500° F | NPT or ANSI Connections

Rotating Vane Flow Indicator
DIH - Rotating Vane Flow Indicator

Made in the USA | Compact Size | Clear Indication | Up to 230 PSI | NPT Connections | 0.05...13.2 GPM

Flow Indicator with Ball
DKB - Flow Indicators with Ball

Made in the USA | Liquid or Gas | 85 PSI | NPT Thread | Up to GPM | Up to 88 SCFM


KOBOLD USA is a subsidiary of KOBOLD Messring GmbH, a world-leading instrumentation engineering business founded in Germany in 1980 by Klaus J. Kobold. With patented technology and superior service, the company quickly established itself as one of the global leaders in sensor and control systems with high quality products. The KOBOLD brand name became synonymous with superior quality and technological advancement in instrumentation engineering.

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