A flow indicator is a simple flow instrument that verifies flow is occurring in a piping system without measuring the flow rate or providing switching capabilities. It answers the question “is flow happening, yes or no?” and most models enable direct viewing of the media inside, usually through a glass tube or a glass window.
Key Takeaways
- Flow indicators work on one simple principle: flow through the device usually moves a ball, paddle wheel, flap, or chain that you can see.
- You do not read a flow indicator: you view it to verify movement of the indication mechanism, since it provides no flow rate measurement.
- A flow indicator with a controller, totalizer, or transmitter is really a flow meter, since indicators do not measure and cannot totalize.
- High pressure flow indicators use higher pressure capability metal, glass, and seals, and KOBOLD’s DA models handle pressures up to 580 PSIG.
- Flow indicators are generally low in price compared to other industrial flow instrumentation, with construction materials as the biggest pricing variable.
What is a Flow Indicator?
A flow indicator, in its most traditional usage, is a device that does not measure the flow rate and does not monitor it in the sense of providing switching capabilities. It simply provides verification that flow is occurring.
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.
| How the term “flow indicator” is used |
Typical devices |
What the device actually does |
| A true flow indicator (the traditional usage, and the one KOBOLD uses) |
Sight glass, ball, paddle wheel, chain, and similar mechanisms |
Shows and affirms that flow is occurring. It does not measure the flow rate and does not switch. |
| A flow meter with a mechanical indicator |
Flow meters with a pointer arm on a pivot and a scale on the faceplate |
Measures the flow rate and displays it with the indicator arm pointing to the scale. |
| A flow meter that also gives a visual indication of flow |
Variable area flow meters (rotameters) with a clear glass or plastic tube and an internal float, or a sight glass |
Measures the flow rate, and the transparent tube also lets you verify flow by seeing the float lifted off the bottom of the tube. |
| “Flow indicator” used as another word for “flow meter” |
Flow meters with mechanical, digital, or other output means |
Provides an “indication” of the flow rate through its output. This is not as common a usage of the term. |
Another usage of the term “flow indicator” would be 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.
What Does a Flow Indicator Do?
A flow indicator signals whether flow is occurring within a piping system. Most piping systems are not transparent, so there is no way to observe the flow directly. 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?
All types of flow indicators function on the same simple principle: flow through the device moves something, typically a ball or a paddle wheel. Flaps, chains, and other less common moving mechanisms are also used. In the case of a simple sight glass, nothing moves, and flow is verified by looking closely at the glass tube.
How a flow indicator works depends on which manner of indication it is providing. The advantage of a flow indicator with a moving mechanism is that it can be seen from much further away.
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?
A flow restrictor only limits the rate of flow and cannot measure it. A flow indicator shows that flow is happening. A flow switch triggers an electrical impulse at a certain rate of flow. Flow transmitters and flow meters measure the rate of flow continuously, and “flow sensor” is a general catch-all term for the whole class of flow instrumentation.
To help further clarify which type of instrumentation is needed for your application, the table below lists most flow instrumentation, from simplest to most complex, with a brief description.
| Instrument |
What it does |
Question it answers |
| Flow Restrictor |
Simply restricts the rate of flow within a system to a predetermined amount. It cannot provide flow measurement. |
Not applicable |
| Flow Sensor |
More of a general catch-all for flow instrumentation, because all of these devices sense flow to one degree or another. The term “flow sensor” is sometimes used interchangeably with any of the other terms in this table, but really should be used as a general term for the entire class of instrumentation. |
Not applicable |
| Flow Indicator |
Shows that flow is happening. Most models allow a clear view of the media as well. |
“Is flow happening at all, yes or no?” |
| Flow Switch |
Monitors the flow and triggers an electrical impulse at a certain rate of flow within the system. It does not provide continuous measurement unless the flow switch is part of a full flow meter. Some flow switches, like variable area flow switches, also allow direct viewing of the media through a plastic or glass tube. |
“Is the flow too high, too low, or happening at a specific point?” It can also trigger a specific action in the system. |
| Flow Transmitter |
Measures the rate of flow in the system and provides the measurement as an electrical output. Some flow transmitters are also fitted 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 is connected to something that can interpret the signal produced by the transmitter. |
“How much flow is happening right now?” |
| Flow Meter |
Provides continuous measurement, and the rate can typically be seen directly at the device itself by a mechanical or digital means. The term is somewhat erroneously used interchangeably with “flow transmitter“. Flow meters can have integral switches, integral electronics that enable batching functions (where a precise amount of a media is dispensed), and totalizers that tell you how much media has flowed in a certain period of time or for the total life of the instrument. Some flow meter technology types allow for direct observation of the media as well. |
“How much flow is happening right now?” plus many other features. |
Other “Flow Indicator” Misnomers: When You Really Need a “Flow Meter” and not an “Indicator”
If you are searching for a flow indicator with a controller, a totalizer, or a transmitter, or for a magnetic or volume flow indicator, the device you actually need is a flow meter. Flow indicators do not measure, so they cannot totalize or provide a flow rate, either volumetric or mass.
Flow Indicator with Controller
If you are looking for a “flow indicator” with controller, you will have more success looking for a “flow meter with controller“. “Flow indicator“, in this sense, is a bit of a misnomer. You can view our comprehensive line of flow meters here.
Flow Indicator with Totalizer
If you are looking for a “flow indicator with a totalizer“, again, you are really looking for a “flow meter with totalizer“. Flow indicators do not measure, so they cannot totalize. You can view our comprehensive line of flow meters here.
Flow Indicator Transmitter
If you are looking for a “flow indicator with a transmitter“, the better terminology would be “flow transmitter” or “flow meter transmitter“. You can view our comprehensive line of flow meters here.
Magnetic Flow Indicator
The usage of the term “magnetic” is suggesting again that the actual device that is needed is a flow meter and not a flow indicator. Simple flow indicators are not made in magnetic flow technology as it would be expensive overkill if all that needs to be known is if flow is happening or not. Magnetic flow meters are a type of flow meter that use no moving parts or mechanical means to provide flow measurement. You can view our comprehensive line of magnetic flow meters here.
Volume Flow Indicators
Because flow indicators do not provide a flow rate, either volumetric or mass, it is likely that the type of device needed is a flow meter and not an indicator. Flow meter technology types that operate by a volumetric principle of measurement are all the flow meter technologies with the exception of thermal dispersion and Coriolis flow meters.
What are Sight Flow Indicators?
Sometimes referred to as “sight glass flow indicators“, sight flow indicators do exactly what their name states. They enable sight of the flow, usually through a glass tube or a glass window. Sometimes “sight flow indicators” refer to indicators that have an integral mechanism to clearly display flow is occurring, and sometimes they refer to clear glass tubes or windows that simply allow the media to be seen. This is really a general term for the entire class of “flow indicators” as all indicators indicate flow is occurring by something being seen by the user. You can view our sight flow indicators here.
Our most popular sight flow indicator, the DAA Sight Flow Indicator, which has an integral cleaning mechanism, comes with either a high visibility red paddle wheel for indication or without any mechanism at all. Our unique design features a mechanism that can remove periodic build up of media contaminants by simply twisting the device. It requires no system shut down or line removal to maintain and is made in the USA.
KOBOLD Flow Indicator Models at a Glance
The table below summarizes the current KOBOLD flow indicator models.
| Model |
Indication mechanism |
Body material |
Max. pressure |
Max. temperature |
Connections |
Media |
| DA |
Flap, rotor, or drip tube |
Cast iron, cast steel, or stainless steel; soda-lime or borosilicate glass windows |
230 PSIG (optional 360 or 580 PSIG) |
250 or 300 °F depending on model (optional 530 °F / 260 °C) |
¼"...2" NPT, ½"...8" ANSI flange |
Liquid |
| DAA |
Red paddle wheel (rotor) or no mechanism; twist to clean |
Brass or stainless steel, borosilicate glass |
232 PSIG |
212 °F (100 °C) |
¼"...1½" NPT |
Liquid |
| DAF |
Paddle wheel (low flow) |
Brass, stainless steel, or polysulfone |
235 PSIG (model dependent) |
230 °F (110 °C) (model dependent) |
⅛"...1½" NPT |
Liquid |
| DIG |
Paddle wheel |
Brass, stainless steel, or polypropylene |
230 PSI |
176 °F (80 °C) |
⅛"...1" NPT |
Liquid |
| DIH |
Rotating vane |
Brass, stainless steel, or POM |
230 PSI |
176 °F (80 °C) |
⅜" and 1" NPT |
Liquid |
| DKB |
Ball |
Brass |
87 PSIG |
248 °F (120 °C) |
⅛"...1" NPT |
Liquid or gas |
| DKF |
Paddle wheel |
Brass |
87 PSIG |
248 °F (120 °C) |
⅛"...1" NPT |
Liquid |
Can Flow Indicators Handle High Pressures?
Yes, flow indicators can handle high pressures when they are built for it. High pressure applications require a more rugged build: higher-pressure capability metal, glass, and seals, and a more durable flow indication mechanism than the plastic indication elements of lower pressure flow indicators. KOBOLD’s DA High-Pressure Sight Flow Indicators are built for pressures up to 580 PSIG.
Most standard pressure capability flow indicators are a combination of a metal and/or glass housing and typically employ a plastic means of indication within the line. The metal used is typically brass or stainless steel.
The DA is of a heavy-duty industrial grade build and is available with a variety of indication mechanisms. To see if any of our flow indicators may fit your needs, visit our flow indicator line.
What are Paddle Wheel Flow Indicators?
Probably the most common type of flow indicator, a paddle wheel flow indicator, employs a paddle wheel to indicate flow. The same simple technology is also used in paddle wheel flow meters, except in paddle wheel flow meters, flow rate is provided by measuring the speed of the paddle rotation. KOBOLD's paddle wheel and vane style flow indicators include the DA, DAA, DAF, DIG, DIH, and DKF models.
Although no flow measurement is provided by a paddle wheel flow indicator, the speed of the turning wheel can very, very roughly indicate whether the flow is low or high by observing the speed at which the paddle is revolving around its axis. Visit our product page to learn more about the paddle-wheel flow indicators that we offer.
What is a Rotameter Flow Indicator?
Rotameter technology is also referred to as “variable area” technology. Simple flow indicators are not usually created in variable area technology. Most variable area flow devices are flow meters and not indicators, although variable area flow meters provide visual flow indication if they are built with a translucent tube that houses the float. Because the only extra step needed to create a flow meter out of what would be a “rotameter flow indicator” is a scale printed on the glass tube by which the top of the float can be read against for a flow rate, most variable area devices are flow meters.
KOBOLD carries one of the industry’s largest lines of rotameter variable area flow meters and many of our meters also provide visual indication of flow via the float movement in the glass tube. We also provide low-flow flow meters in variable area technology. Visit our article to learn more about rotameters or view our rotameters.
What are Water Flow Indicators?
Most liquid flow indicators are compatible with water and can be used as water flow indicators. Besides the general application parameters such as pressure and temperature, the other consideration to be aware of when choosing a water flow indicator is accounting for the characteristics of the water in the system.
Is it clean, ultra-pure, dirty, or mixed with other media that causes it to be aggressive or viscous? These factors can determine the suitability of certain models of flow indicators for a specific water application. To see which of our water flow indicators are compatible with different kinds of water, view our flow product comparison page.
What are Air Flow Indicators?
Most gas flow indicators are suitable for use as air flow indicators. To determine whether an air flow indicator is right for your application, make sure that it meets the pressure and temperature requirements of your system and pay attention to whether or not the air has any characteristics that could damage the visual indication mechanisms, such as large particulate matter that may be carried in the air flow stream.
Our suggested air flow indicator is our DKB Air Flow Indicator that offers indication via a ball mechanism. The DKB is rated for liquid or gas, with air ranges from 0.11...14 SCFM up to 0.32...88 SCFM; our other flow indicators are rated for liquid service.
What are Water Cooling Flow Indicators?
A water cooling flow indicator verifies flow in various parts of a water cooling process, where a loss of proper water flow can compromise the system and cause a lot of costly damage. For a simple water cooling flow indicator, KOBOLD recommends the DA Industrial Flow Indicator.
Water cooling systems protect expensive equipment by removing heat produced by the process by transferring it to water and moving the water into a cooling process where the temperature is reduced and then the cooled water is sent back to reduce the heat once again.
Flow meters or flow transmitters go one step further than a flow indicator and provide a continuous rate of flow. They are an even better safeguard for water cooling systems as they can offer more information and they can control functions that a simple flow indicator cannot.
For example, a flow meter would be able to detect leaks by sensing a significant unexpected drop in the flow rate. Flow meters also offer more automation to the process because they can transmit the data to control centers for continuous monitoring. A simple flow indicator usually requires on-site visual confirmation of flow.
Flow meters can also provide switching options that, in tandem with the pumps in the system, can keep the flow rate steady to enable consistent cooling to the process.
Can Flow Indicators Be Used with Liquid Fertilizer?
Because flow indicators typically only use a mechanical means to indicate flow and no electrical components are part of the device, most any liquid flow indicator can be used as a liquid fertilizer flow indicator as long as the liquid characteristics are taken into account. Certain indicators will not tolerate particulate matter or higher viscosity liquids very well. The DAA, for example, is rated for liquids up to 150 cSt. Because of the corrosive nature of liquid fertilizer, flow indicators made of stainless steel are usually recommended, although the chemical composition of liquid fertilizer can vary, and a flow indicator should be specified for the exact composition of the fertilizer.
Because liquid fertilizer can also accumulate build up on the sight glass, our unique model DAA Sight Glass Flow Indicator with an integral cleaning mechanism, which does not require system shut down or device removal for cleaning, delivers an extra value for liquid fertilizer.
How Do You Install a Flow Indicator?
Installing a flow indicator is relatively easy as they are mechanical in nature and do not require any menu programming. With little to no exception, all simple flow indicators, that are providing visual indication of flow only with no flow rate measurement, are inline devices that are inserted into the piping of the system.
Each end of the device will have an inlet for the flow and an outlet for the flow that are on opposite sides of the device. The inlets and outlets will have a connection type, here in the US it is usually an NPT thread or an ANSI flange that will connect with the pipe on each side of the device, effectively inserting itself directly into the piping system and the flow path of the media. KOBOLD flow indicators cover ⅛" through 2" NPT threads depending on the model, and the DA is also available with ½" through 8" ANSI flanges.
Here are some other considerations for installation and commissioning a flow indicator:
- Ideally it is best if the pipe remains full so that it properly engages the indication mechanism.
- Suddenly starting the inflow may cause pressure peaks exceeding the working pressure of the instrument. This could cause damage to the indicator.
- Remove any coarse foreign matter before installing the instrument in the pipe.
How Much Do Flow Indicators Cost?
Flow indicators are generally low in price compared to most other industrial flow instrumentation. The most prominent pricing variables will be the materials used in the construction. For example, high pressure models require higher grade materials to ensure longevity and will cost more as a result.
It is important to verify that the flow indicator that you are considering is an industrial grade flow indicator and not a flow indicator meant for more residential use. Cheap, all-plastic flow indicators are not going to stand up to the rigors of industrial applications.
Flow Indicator Manufacturers
There are many types of flow indicators available that are made by a wide variety of manufacturers. KOBOLD manufactures, sells, and supports our flow indicators and we are well versed in helping you select the best option, whether it be for a single flow indicator or for bulk quantity OEM flow indicators.
Most of our flow indicators are made in the USA, in our manufacturing facilities in Pittsburgh, PA. Our flow indicators are built and tested by hand for quality assurance. BABA (Build America, Buy America) compliant models of the DAA, DAF, DIG, DIH, DKB, and DKF are available.
Contact our expert engineers now for free assistance in selecting the optimum flow indicator for your needs and your budget, schedule a free engineering consultation, or learn more about why you should partner with KOBOLD for your application.
References
- American Society of Mechanical Engineers (ASME). ASME B1.20.1-2013 (R2018), Pipe Threads, General Purpose, Inch. 2013. https://www.asme.org/codes-standards/find-codes-standards/b1201-pipe-threads-general-purpose-inch
- American Society of Mechanical Engineers (ASME). ASME B16.5-2020, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard. 2020. https://asme.org/getmedia/5cc627a0-d379-4186-89b4-4f552381a98f/b16-5_pipe-flanges-and-flanged-fittings-2020.pdf
Frequently Asked Questions
The common types are grouped by what moves inside them: ball flow indicators, paddle wheel flow indicators (likely the most common type), flap indicators, chain indicators, and plain sight glass indicators with no mechanism at all. Variable area devices with a translucent tube also show flow visually, but most of them are flow meters rather than indicators.
No. A simple flow indicator uses only a mechanical means of indication, and the device contains no electrical components. That is also why installation is relatively easy: there is no menu to program. You thread or flange the device into the pipe and watch the mechanism move when flow passes through it.
Only very roughly. On a paddle wheel model, how fast the wheel spins gives a rough sense of whether flow is low or high, but there is no scale and no flow rate value. If you need an actual number, what you want is a flow meter, which measures the rotation speed to report a rate.
Standard models combine a brass or stainless steel body with glass, and most use a plastic indication element inside the line. High pressure versions use metal, glass, and seals rated for higher pressure plus a tougher mechanism, which is the main reason they cost more. For corrosive liquids like fertilizer, stainless steel construction is usually the recommendation.
Temperature limits depend on the model and its materials. KOBOLD's standard sight flow indicators run from 176 °F (80 °C) for the DIG and DIH up to 248 °F (120 °C) for the DKB and DKF, and the all-metal DA handles 250 °F or 300 °F by model, with an optional 530 °F version for hotter service.
KOBOLD’s DAA sight flow indicator has an integral cleaning mechanism: a twist of the device clears the build up that media contaminants leave on the sight glass over time. No system shut down and no line removal are needed, which matters for media such as liquid fertilizer that tend to coat the glass.
That is a job for a flow meter or flow transmitter, which provides a continuous rate of flow. A meter can spot a leak as a significant unexpected drop in rate and transmit readings to a control center so flow can be monitored continuously. A simple indicator usually needs someone on site to confirm flow visually.
It depends on the model. Some indicators handle particulate matter and higher viscosity liquids poorly, and large particles carried in an air stream can damage the visual mechanism. Before choosing, account for whether the water or liquid is clean, ultra-pure, dirty, aggressive, or viscous, and check the specific model against those conditions using the flow product comparison page.
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