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With all of the flow meter technology available on the market today, selecting a flow meter can be a daunting task. Believe it or not, incorrect selection happens more often than you might think. In fact, studies report 90% of flow meter measurement problems are due to incorrect meter selection! Keep reading our flow meter buyers guide to learn more.

Choosing the wrong meter can result in serious problems such as constant maintenance, expensive repairs, safety concerns, and other issues that can effect overall operations and profit. In addition to choosing the right meter for the job, proper use, maintenance, calibration, and care are all huge factors in flow meter longevity and ensuring you are getting accurate readings.

This guide outlines best practices in selecting the right flow meter, caring for your flow meter, how to go about regular maintenance without having to shut down operations, and other factors that help to extend the life of your flow meter.

Your Application Comes First

There is no such thing as a “one size fits all” flow meter, and it is important to understand that you should not be choosing a meter based on the technology, you need to choose a meter based on your application. It is much easier to first analyze your application’s requirements and then choose the technology that fits rather than buying the meter first and forcing it to work in your application.

There are a number of things to consider as you evaluate your application and technology options including:

  • Size and measuring range
  • Chemical compatibility
  • Processes and accuracy requirements
  • Acceptable pressure drop
  • Cleaning requirements
  • Desired measurement units (velocity, mass, volume, etc)
  • Uni-direction vs. bi-directional measurement
  • Fluid viscosity limitations
  • Necessary approvals for applications such as sanitary or hazardous areas
  • Custody-transfer approvals
  • Data output requirements (relay, digital, simple, etc.)
  • Calibration and recalibration requirements
  • Maintenance issues
  • Operating costs
  • Connection styles (flanged, wafer, threaded, welded, etc.)

Things to keep in mind during flow meter selection:

  • Every fluid or gas behaves differently due to viscosity
  • You must figure out how the material will behave in the pipeline to help narrow your selection
  • Flow profiles change depending on whether or not your material is Newtonian or non-Newtonian
  • Calculate and match the Reynolds number for your application
  • For the lowest uncertainty of measurement, positive displacement meters are generally the best option
  • Electromagnetic meters give the widest flow range
  • Turbine meters have the highest short-term repeatability
  • Orifice plate meters are the most common metering device
  • Coriolis mass flow meters lend themselves well to measuring particularly viscous substances and for the measurement of mass

Questions to Ask Before Flow Meter Selection

The answers to the questions below will help you narrow down the many choices you have at the beginning of your flow meter selection process.

1. What type of fluid is to be metered?

  • Liquid, gas or vapor
  • Clean or dirty
  • Corrosive or noncorrosive
  • Newtonian or non-Newtonian
  • Other

2. What are the flow conditions?

  • Measuring flow rate, total flow or both
  • Normal flow rate
  • Minimum and maximum flow rate
  • Minimum and maximum temperature
  • Minimum and maximum pressure

3. What are the installation conditions?

  • Pipe size
  • Reynolds Number
  • Is there room for adequate length of piping?
  • Are flow conditioners needed?
  • Is pipe vibration a problem?
  • Is flow steady or pulsating?
  • Are there any other considerations?

4. What are the performance requirements?

  • How critical is accuracy to the application?
  • Is accuracy required under all conditions?
  • Other?

Installation Cautions for Flow Meters

The next step we will cover in the flow meter buyers guide is application. When selecting a flow meter, it is important to consider exactly where and how the device will be installed. The installation location can significantly affect both accuracy and efficiency. Obstructions in the pipeline can all cause distortions in flow, affecting flow meter accuracy and repeatability. To ensure best results, flow meters should be installed in locations where there are several straight-lengths of unobstructed pipeline both upstream and downstream. Also consider the following:

  • In liquid service, be sure the meter is installed in a way that allows it to always be full of liquid so gas/vapor to not alter the accuracy of the meter.
  • In gas/vapor service ensure the opposite, that the meter is always full of gas/vapor so liquid does not change the accuracy of the meter.
  • Be careful in two-phase flow measurement such as liquid gas/ or liquid/solid flow. It is easy for accuracy to be affected and plugging can easily occur in liquid/solid flow streams.

There are a number of things that can go wrong with the actual installation. Common issues include:

  • putting the meter in backwards
  • not putting it in the center of the pipe
  • the placement of the meter in relation to the control valve
  • improper installation range
  • irregular upstream piping configurations
  • gaskets or other protrusions into the line
  • incorrect orientation of a flow sampling sensor into the flow stream

Be sure to inspect your installation carefully. Research the manufacturer installation recommendations before buying a flow meter, particularly where installation space is limited.

Three Major Requirements for Proper Use

The proper use of your flow meter is one of the best ways you can cut down on the need for maintenance, calibration and repair. Rules for use will change from meter to meter. Ask the manufacturer and refer to your owner’s manual to ensure proper use of your meter in your specific application. The top three most important things you can do to ensure proper use and quality measurement include:

  1. Ensure your meter possesses a valid calibration at the time of use
  2. Calibration conditions should mirror the same conditions in which the meter will be used – compensate if necessary
  3. Make sure your calibration laboratory can meet accuracy requirements

What Causes Meter Performance to Change?

Flow meter calibration and maintenance frequency vary based on  industry, application, and flow meter operating environment. The flow meter buyers guide outlines considerations below. What causes the changes in meter performance that require service?

  • Deposits on internal surfaces of the meter, from salt or other minerals. This can occur on meters with and without moving parts. Even if everything seems to be working properly, this can severely impact your meter’s ability to perform as it should.
  • Chemical wearing on any level can affect your flow meter calibration, especially flow meters with moving parts. Wear can greatly affect the movements and overall geometry of the meter.
  • Meter damage or misuse can result in problems. This can be either an accidental drop of the meter, over speeding, etc. The outside of the meter may not show any damage, but performance could be compromised.
  • How long has the meter been in use or service? Older meters will have changed from their original performance due to use, wearing, electrical changes and other factors over time.
  • Fluid changes can negatively impact flow meter performance. If the meter was calibrated in one type of fluid and used in another type of fluid, you may see a difference in performance. A change in fluid property will affect all types of flow to some extent. It is possible to make corrections to compensate for these changes.
  • Has the installation caused any issues? Flow meter installation can be difficult, depending on the application. Incorrectly installed flow meters can cause errors.
  • External influences will always effect your flow meter calibration and performance regardless of what kind of meter you have. Factors such as vibration, temperature, pressure, electromagnetic interference, etc.

Why Certain Meters May Require Maintenance

Of course every situation is different, but flow meters have been around for a long time and we all know history has a tendency of repeating itself. Below is a table of common flow meter types and common reasons they require maintenance, calibration, or repair.

TypeReason for Service
Differential Pressure
  • The wearing of orifice plates, cones, and venturies change the geometry of the meter and behavior of flow
  • Dirty pilot tubes
  • Transmitter failure
  • Electrode coating
  • Linear damage
  • Electronic failure
  • Wear and coating of flow tubes
  • Electronic failure of the transmitter
Open Channel
  • Accumulation of debris
  • Level transmitter calibration
  • Electronic failure of transmitter
  • Volume changed by dirty liquids, corrosion, and abrasion
  • Bearing wear affects accuracy
  • Solids cause plugging in the meter
  • Gear service is affecting calibration
Thermal Mass
  • Wearing of the sensor
  • Sensor failure
  • Leaks
  • Calibrate before putting in service
  • Changes in fluid sonic properties
  • Electronic failure
  • Dirty bearings
  • Bearings are affected by chemicals or rotors wear
  • Bearing service affects calibration
  • Electronic failure
Variable Area
  • Plugging
  • Material buildup
  • Metering tube failure
  • Vibration
  • Expansion due to temp. variation
  • Meter off-center in pipe
  • Electronic failure

Flow Meter Calibration Frequency

Regular flow meter calibration is necessary. But how should you calibrate a flow meter? Time in use and maintenance schedules are a major factor in the performance a flow meter. Gradual and imperceptible changes occur in the meter due to environment, use, and other factors. Flow meters issues aren’t visible externally. To identify possible issues, the flow meter must be taken apart. This process will happen if issues are found during flow meter calibration testing.

The flow meter buyers guide includes manufacturer guidance. In general, the manufacturer recommendation is that new flow meters should have a six month calibration cycle. Flow meters that are used in unfamiliar applications are also recommended to have calibration after six months. As those calibrations are performed you can analyze the results to adjust your schedule accordingly. Recommendations may differ by manufacturer. Check the flow meter manual, contact the manufacturer, or ask your ISO/IEC 17025 accredited calibration lab for help establishing calibration intervals.

Tips for Scheduling Regular Maintenance without Slowing Operations

Flow meter calibration, repair, and maintenance should not interfere with work. To verify equipment reliability and keep operations running smoothly at the same time, consider the following:

  • Use an onsite calibration provider, especially if you have over 20 units
  • Choose a calibration lab that can service all equipment
  • Plan maintenance schedules, repairs, and calibration to include as many instruments as possible
  • If your facility has annual shutdowns or slow periods, try to schedule calibration during that time
  • Utilize pickup and delivery options if available

What to Look for in a Flow Meter Maintenance & Repair Service Provider

Your maintenance, repairs, and calibration are only as good as your service provider, right? Depending on your company, you may require certain certifications, documentation, and proof that your equipment is being properly maintained. Choosing the right service provider is a significant part of being able to comply with regulations. How do you know you are getting the best possible service? Look for the following:

NIST Traceability

NIST (National Institute of Standards and Technology) is one of the oldest physical science laboratories in the US. NIST calibration services are the highest quality services available. According to the NIST website, “Traceability can be defined as an unbroken record of documentation or an unbroken chain of measurements and associated uncertainties.”  A NIST Certificate of Calibration means that the instrument has been tested and approved to be on a certain level of accuracy. Having NIST calibrate your equipment can be extremely expensive and time intensive. Working with a calibration lab that can provide NIST traceability is your best bet.  The following criteria must be met in order to meet the qualifications of being completely NIST traceable:

  1. The instrument’s calibrated indications have to be traced along an unbroken chain, going back to a national or international standard.
  2. The measurement uncertainty for each link in the traceability chain must be calculated or estimated, in a way that is quantifiable. Allowing the overall uncertainty for the whole chain to be calculated or estimated based on the initial test.
  3. Each link in the traceable chain must be preformed according to previously documented and agreed upon procedures.
  4. All those who are participating in the traceable chain must supply proof of their technical competence.
  5. By the end of the comparison chain, there must be principal standards for realization of the International System of Units.
  6. Calibration must take place at regularly repeated intervals.
ISO/IEC 17025 Accreditation

ISO stands for International Organization for Standardization. The ISO website states, “ISO is the world’s largest developer of voluntary International Standards. International Standards give state of the art specifications for products, services and good practice, helping to make industry more efficient and effective. Developed through global consensus, they help to break down barriers to international trade.” Although ISO is the body that develops the rules by which accreditation is granted, they do not perform certification auditing. Companies become ISO accredited after completing an audit, performed by an external ISO accreditation body. A calibration laboratory that holds an ISO 17025 accreditation has successfully demonstrated valid calibration testing procedures, adhered to quality principles, and provided accurate documentation. You can feel confident that you are getting the best possible service for your instruments..

ISO 17025 Accredited vs ISO 17025 Compliant

An ISO 17025 accredited calibration lab goes through a strict annual audit. An auditor observes technicians as they perform procedures, reviews laboratory documentation for standards and customer certifications, and verifies that the lab meets all requirements necessary. The auditor also ensures that proper test equipment is being used to perform calibration. Any lab can be compliant, but that does not mean they are accredited. Labs that are compliant do not go through this auditing process and are not recognized by the ISO accreditation body. This means the lab has not proven they have the competence, expertise, equipment required, or the proper procedures to perform valid calibrations. Any lab can claim compliance, an ISO 17025 accredited lab has actually demonstrated it.

Onsite Calibration Service

Onsite calibration is a cost effective option for companies that have a large quantity of flow meters, and other equipment, to calibrate. Not all service providers have this option. Be sure to ask about this while searching for a provider. Using onsite calibration can help you get your flow meters services quickly, efficiently, and save you money on shipping.

We hope that the flow meter buyers guide has answered all of your questions. Feel free to share the flow meter buyers guide with a friend or coworker that may need help choosing flow meters.

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