Selecting The Right Flowmeter - Top 7 Considerations

So you’ve identified the need for a flow meter, whether it’s to understand, control or monitor a process, a flow meter is required to see what your fluid is doing and translate it to information you can base decisions on.

There are a host of considerations when selecting a flowmeter such as cost, brand, technology, installation requirements, application etc.

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To help narrow down the answer to these questions, we’ve compiled a list of the Top 7 Considerations for Choosing a Flowmeter. Once you understand these requirements, the considerations mentioned above will become clearer and the potential field of products narrowed.

1. Fluid – What are you measuring? 

In order to identify what type of flowmeter is suitable to a certain application, it is important to know what state the fluid being measured is in; fluid or gas. Gases compress and can’t be measured with a liquid meter. This is a vital information to know right from the start. This article focuses on selecting a meter for liquid measurement. 

Once fluid has been identified, it is vital to assess if it is clean. A dirty fluid contains solid particles, typically called a slurry, while a clean fluid will be particle free. For example, flowmeters that have wetted moving parts, such as the positive displacement (volumetric flowmeter) or turbine (velocity flowmeter) would not be suitable to dirty fluids, as it will be more susceptible to mechanical wear, plugging or erosion due to the presence of solid particles. Hence, the flow meters that have wetted moving parts are substantially applicable to clean fluids only. On the other hand, dirty fluids would be appropriate to run in an non-contact meters such as electromagnetic (velocity meter), ultrasonic (velocity meter) or Coriolis (mass meter). These also have limitations on them, but handle particles better.

Another factor to be considered is the fluid compatibility with the material composition of the wetted parts, such as the body, seals and gears/rotors/paddle of the flowmeter. Acids and bases are corrosive to metals and would more likely be compatible with thermoplastics, whereas some organic compounds may not be suitable to thermoplastics and would perhaps be compatible with metals instead. For more information on material compatibility, you can download our free Chemical Compatibility Chart.

2. Viscosity and Flow Profile – How thick or thin is the liquid?

One of the principal parameters to consider when selecting a meter is the fluid’s viscosity, or how thick the fluid is. Since the fluid to be measured has been identified, it is now possible to look at its properties related to flow such as the viscosity. This is defined as the measurement of resistance to flow or alternatively, it is the internal friction of a fluid, the amount of friction the molecules create as they flow over each other. The importance of this parameter in flow measurement is that it determines how well mixed a fluid is and thus how repeatable the reading may be.

For instance, a positive displacement meter, such as oval gear flowmeter, is preferable over a turbine meter to be used in a very viscous (high consistency) fluid. This is because, most high viscosity fluids would have a laminar flow and it is characterised as a smooth, and constant motion. As you can see on below diagram, the velocity profile of a laminar flow is parabolic. What does it tell us? It means that the velocity of the flow inside the pipe is not uniform, the fluid flowing close to the pipe wall is slower due to the friction between the fluid and the pipe wall, while the fluid flowing at the centre of the pipe is travelling at a faster rate.

In turbulent flow, which is characterised as chaotic and occurs mostly on less viscous or thin fluids. Its velocity profile is “fully developed”, in other words, the flow inside a pipeline is moving at the same speed or velocity at all points. A Turbine meter is a velocity meter, as it directly measures the speed of the fluid by measuring the angular velocity of the rotor which is directly proportional to the fluid velocity. A volumetric flow meter is more applicable for high viscosity fluids at low flow rates like honey, treacle or thick oils. A velocity flow meter will be a good option for a low viscous or thin fluid like solvent or water.

In order to determine whether a fluid will undergo a laminar or turbulent flow, it is good to know how to calculate the Reynolds Number. You can find a Reynolds Number Calculator here. It is a dimensionless number that helps in determining the flow characteristic or pattern of a fluid. It is a function of fluid’s density and viscosity. A laminar flow would have a Re< and turbulent flow would have Re>.

It is also worth highlighting that viscosity is a function of temperature. In liquids, viscosity is inversely proportional to temperature, i.e. as the temperature increases the viscosity decreases. Hence, it is important to consider the operating temperature of the system or application to be able to understand how the fluid flow will behave in relation to its viscosity.

3. Flowrate Information – What is the maximum and minimum flow rate?

This parameter is equally important as the prior parameters to determine the right size of the meter that will suit the application. Flowrate is the volume or mass of a fluid flowing/moving per unit time. You can convert from mass to volume through the density (the amount of volume a fluid takes up per unit mass) or specific gravity ( the ratio of the density of the substance to the density of water or how much a litre of the fluid would weigh divided by the weight of the same volume of water).

Knowing the flowrate range, it is now possible to evaluate if the flowmeter in the selected list has the capacity to handle the required flow rate. This stage is equally crucial with the earlier steps of meter selection, as this point determines if the meter will function as designed. For instance, selecting an undersized meter (it means the meter exceeds the flowmeter’s capacity or close to the maximum capacity) would result to damage or failure of internal components of the meter or worst case, would lead to failure of the entire meter. On the other hand, an oversized meter (it means the system’s flow is below or close to the minimum range of the meter) would lead to poor accuracy or inability to read/measure a flow.

At FLOMEC, when performing flowmeter sizing, we follow a rule of thumb, and that is to get between 20% and 80% of the maximum flow rate to gauge the right size of the meter. That should correspond to the minimum and maximum flowrates of the application. For example, a meter with a 1-40 L/min flowrate range, by following the rule of thumb, it is recommended to operate the meter at 8-32L/min to be able to optimise meter’s performance and maximize its durability and longevity. This allows the meter to cope with both peak flows that may damage the meter and lower than normal flows due to an obstruction in the line or blockage that may not register if the meter is at its limits.

4. Temperature and Pressure Rating – What is the maximum allowable?

Other key parameters in sizing of flowmeters are the temperature and pressure. Similar to the flowrate, which represents the size capacity of the meter, the temperature and pressure parameter classifies the meter’s material capability to withstand the effect of thermal energy and forces exerted by the flowing fluid.

In the viscosity section of this article, it discussed the relationship of temperature with viscosity of fluids. Since viscosity is a function of temperature, it is indeed important to take this parameter into consideration when performing the sizing, in the same way as the viscosity. Furthermore, the operating temperature is crucial to the wetted components of the meter, particularly the sealing, as seals have temperature limits and some materials are not able to withstand extreme temperatures or extended periods of time. Lastly, temperature helps to decide if an electronic instrument can be mounted directly onto the meter or it needs to be remote mounted, because the electronic components have a temperature limitation.

Pressure defines the capacity of a flowmeter to withstand the force exerted by the liquid in motion. It's necessary that the operating pressure of the application must not exceed the maximum allowable operating pressure of the selected flowmeter, otherwise it has the potential to create a hazard.

The pressure ratings of meters are calculated with a factor of safety so that small pressure spikes will not cause the meter to fail. Over pressuring the meter will result in deforming the meter and likely inaccuracy over time as the meter material reaches the limits of its elasticity.

It is important that the system’s temperature and pressure application should not exceed the allowable limits of the flowmeter, to prevent inaccuracies in measurement and potential hazards. High temperatures will affect the pressure capacity of the meter, causing metals to become more ductile and likely to stretch. Maximum pressure ratings allow for the maximum temperature rating of a meter.

5. Accuracy/Repeatability/Linearity – How accurate and precise?

Some applications may specify and require high accuracy meters, like for those being used in dosing applications or custody transfer (applications where a consumer is being charged based on the reading). Inaccurate readings result in financial loss or quality issues on a product being manufactured. It is important to select the meter to meet the desired accuracy of the process.

Accuracy relative to flow metering is the measurement of how close the measured value is generated by the device/instrument to the actual flow rate. It can be expressed as percent of Full Scale or percent of Reading. Accuracy over range or Full Scale accuracy implies that the error is consistent over the full range of the flow rate for the meter. For example, a meter with flow capacity of 100L/min and 1% Full Scale accuracy would have an error of 1 L/min no matter whether the reading is 10 or 100 L/min. On the other hand, percentage of reading accuracy calculates from the actual reading. A meter with flow rate range of 10-100L/min and 1 % reading accuracy would have 1L/min error at 100L/min and 0.5L/min at 50L/min. Hence, it is apparent that a meter with accuracy calculated over reading will be more accurate at low range readings than a meter with full scale accuracy specified.

Repeatability measures the ability of the device to produce the same result or reading given the same condition, regardless what the accuracy of the meter is. It is said that “you can have high repeatability without high accuracy, but it is not possible to have high accuracy without high repeatability”. Repeatability is like the grouping of arrows on a target, they may be all together, but they are better if they are close to the bullseye rather than near the edge.

Additionally, Linearity is another important factor that describes meter’s performance. It is measures the ability of the meter to maintain within the specified accuracy all throughout the specified flow range. It is expressed typically as percent error within the meter’s flow range and if the actual flow rate versus indicated flow rate are plotted in a graph, a straight line is expected in a meter with good linearity. An ideal meter provides a linear output across the flow range while in the real world hydraulics mean there are friction, slippage and pressure differences that cause the meter to slow or not measure fluid flow depending on the speed of the fluid and the nature of the flow.

6. Installation - What are the installation parameters?

At this point, your choice of flow meters should have been narrowed down or perhaps the right meter has decided upon. Now, to be able to obtain optimum performance and achieve the desired accuracy of the meter, it is necessary to ensure that proper installation of the meter is well understood and installed correctly.

The piping configuration is one of the key things to take into consideration in the installation of flowmeter. It is crucial mainly because it must be constructed in the way the flow meter is always full of liquid to provide accurate measurement. Also, pipe direction is another factor, which suggests if the meter will be installed horizontally or vertically? For vertical mounting, it is necessary that the flow should be from bottom to top to ensure meter is always full of liquid and will prevent air entrapment in the meter.

Velocity meters, require straight run pipe in the upstream and downstream to get a stable flow profile. This is essential because irregular velocity profile have an impact in the accuracy and repeatability of the meter. Existing installations may not have enough space or provisions to accommodate the straight run pipe necessary and flow conditioning may an alternative to stabilise the flow profile by removing swirl and disturbances.

Lastly, it is also important that the meter’s orientation should be strictly followed. For example, oval gears must be installed where the rotor’s shafts are in the horizontal plane, otherwise the weight of the rotor will come down on the small thrust bearing that keeps the bottom of the rotor off the base of the meter chamber. This will result in the bearing wearing prematurely and the rotor rubbing on the chamber floor. Another good example would be electromagnetic meters that should be installed slightly tilted (1 or 2 ‘clock) to prevent sediments settling on the lower set of the sensing electrodes. Some flow meters are unidirectional, just like our oval gear mechanical meter, and the arrow of the flow must be followed accordingly, whereas our electronic oval gear meters and turbine meters are bidirectional and can be installed in the pipeline in either direction. For detailed installation guideline of a meter, it is necessary to read the Instruction Manual prior installation.

7. Output/Indication – Do you require a display or a signal output?

The final option to select to get a functional meter is how the meter will translate the flow rate into a usable data form. This is determined by what the flow data will be used for; process control, billing, regulatory reporting or monitoring. Is flow rate, batch or accumulated volume required to be recorded manually or electronically on a data logger or control system

Initially we need to decide if the register requires local mounting and if so the temperature of the application must be considered and should coincide with the temperature limit of the electronics. For remote mounting, the key thing is to ascertain whether the transmission is analogue or digital, as some instrument may not have both options. Alongside this, verify power supply’s availability in the installation site and assess if the display being offered can be self-powered, loop powered or externally DC powered etc.  In case power supply is unavailable at site, an alternative solution perhaps would be a mechanical flowmeter or electronic meter that can be operated with batteries.

In searching for an electronic display to couple with a flow meter, ensure that the display’s input signal requirement matches with the flowmeter’s signal specification. For instance, the frequency or number of pulses per second from the meter must be able to be received by the register, otherwise a converter or an additional accessory may be required. Such consideration is crucial during the selection process to avoid unnecessary costly modification.

Some fluid applications may require a device that has a relevant certification. For instance, an electronic flowmeter located in an atmosphere of flammable vapour needs to be certified that is safe to operate. It is a regulatory requirement to meet the hazardous certification for the area the meter is used in. For Europe this is ATEX, for North America this may be FM or CSA, outside of both IEC may be required. The responsibility lies with the installer and operator to ensure that the meter and register comply with national hazardous area regulations. Other certifications may be a Weights and Measures approval where the output from the meter is used for billing or industry specific certifications such as suitability to use with food and beverage.

Understanding not only what the key considerations are when selecting a flow meter but why they are important will help you achieve success with your flow objectives. For assistance with anything discussed in this article or flow measurement in general, we have plenty of resources to assist you. Feel free to contact us.

Mom and Dad always said to do your homework. That was true then for school and true now as process measurement and control professional responsible for specifying flow instrumentation. 

Whether you are working on a plant upgrade, a process improvement or an expansion project, doing your homework on the application and installation will save you time and expense, and ensure first-time right success. 

To specify a thermal mass flow meter correctly, there are 10 key questions you will want to understand, consider and then be able to answer. Being ready with the answers to these 10 questions will help you communicate effectively with consulting engineers, manufacturers, and/or their local sales engineering team.

1) What Installed Accuracy Is Needed?

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Reviewing the general accuracy statement in the manufacturer’s product literature is not enough. The installed accuracy must take into consideration the instrument’s basic accuracy capability, plus calibration (refer to actual gas or equivalency section that follows), plus flow profile disturbances, and both the gas’ and installation temperatures and the instrument’s ability to compensate for it.

Is an actual gas calibration needed or is equivalency acceptable?

In all cases, an actual gas calibration performed at process temperature and pressure conditions will always result in best possible accuracy for thermal mass flow meters. 1 When best possible accuracy and repeatability is required, then an actual gas calibration is the solution. 

In some situations, however, an actual gas calibration might not be practical, achievable, or economical, and then an equivalency calibration is the only practical answer. These situations might include, but are not limited to, complex gas mixtures or for various safety reasons.

Furthermore, in applications where less accuracy and repeatability are acceptable, an equivalency calibration, using a surrogate gas (typically air), might be an acceptable, lower cost alternative. Equivalency calibrations are theoretical and their accuracy is the subject of much debate. When done only for a purchase price savings, buyers should beware of equivalency methods. Reputable manufacturers will provide you with an expected accuracy per your specific installation conditions and the calibration process they will apply before you commit to purchase.

1 Reference ISO :, section 8.2; measurement of fluid flow in closed conduits – thermal mass flowmeters

2) What Is the Gas Type To Be Measured?

Is the gas type air, inert gas, or hydrocarbon based gas (Figure 1)? Is it a single gas or a mixture? If a mixture, what are the proportions of which gases? Could the gas mixture change, and, if so, by what proportions? Is the gas clean or dirty? If dirty, can you qualify and/or quantify it? Is the gas dry, moist or wet? Can you quantify the amount of moisture? Is the fluid corrosive?

Dry, clean gases can be processed by all manufacturers. If it is a moist gas, then constant power technology has been proven to be superior. If liquid droplets or condensation conditions exist within the flow stream, then two manufacturers currently have solutions. One offers a super-heated, 300 °C [572 °F] flow element to flash-off the droplets, while another manufacture provides a mechanical shunt which prevents liquid droplets or condensate from reaching the sensors.

The measuring principle of thermal mass flow meters involves heat transfer caused by gas flow. Any moisture or condensate in the gas stream that contacts the heated sensor can cause a sudden, momentary change in the heat transfer that can result in a spiked or fluctuating reading, creating inaccurate or unstable flow measurement. Thermal flow meters using the constant ∆T (CT) method are particularly reactive to moisture droplets, while constant power (CP) method meters, because their slightly heated sensor’s temperature is elevated above the dew point of the gas are resistant to moisture’s effects. 

    Figure 1
 

3) What Is the Required Flow Range?

One of the compelling features of thermal flow meters is their wide turndown capability (Figure 2). Typical turndown for most manufacturers is 100:1. Flow range capabilities are a big differentiator between suppliers and technology. Typical CT type technology meters have less range than CP type devices due to sensor power limitations. However, some manufacturers have special techniques to extend their measuring ranges up to fps [300 mps].

  Figure 2

4) What Is the Needed Response Time?

While it might seem like “the faster the response the better” is the correct choice, in flow metering this might not be true at all. If the thermal flow meters will be part of a PID control loop, too fast of a response can create excessive valve responses (chatter) resulting in an inability to achieve stable flow control or premature valve failure. Conversely, if the response is too slow, the control valve action might lag by too much and desired control is not achieved. Furthermore if the air/gas flow stream has any entrained moisture (e.g., condensation droplets), a fast responding thermal flow meter will produce erratic, unstable readings as water droplets hit the sensors. (Refer to previous section on gas type to be measured.)

5) In What Type and Size Pipe Will The Meter Be Installed?

Will the installation be in a round pipe or a rectangular duct? What is the diameter, both OD and ID, of the pipe or dimensions of the duct? If an insertion style meter, what is the dimension of the socket (e.g. thread-o-let) and will it be installed through a ball valve? These are important considerations for three reasons: 1) Smaller diameter pipes require use of an inline or spool-piece design, rather than an insertion type; 2) If an insertion-type, whether a single-point or multi-point averaging solution is recommended; and 3) to ensure the probe length is correct to achieve the proper insertion depth into the pipe. (In single point types, the center of the pipe is the required installation depth) (Figure 3).

   Figure 3

6) How Much Straight-Run Is Available?

To meet their laboratory calibrated performance specifications in their actual field installation, thermal mass flow meters require a repeatable flow profile (Figure 4). This will naturally occur with 15d to 20d of upstream straight run and 5d to 10d of downstream straight run. These are laws of flow dynamics physics, not subject to debate. If you do not have enough straight run available, reputable manufacturers will provide information and quantification of the accuracy degradation you could expect. Furthermore, all reputable manufacturers offer some type of flow conditioning technology to produce an accurate, repeatable measurement in installations with inadequate straight run.

   Figure 4

7) What Are The Ambient Conditions and Requirements Of The Meter’s Installation Area?

Will the instrument be installed indoors, outdoors but under a protective roof, or outdoors completely exposed to all weather conditions (Figure 5)? Would the installation benefit by remotely locating the electronics from the sensor element? Would a sun shield help shade the transmitter and readout? Does the instrument enclosure’s IP or NEMA-type rating meet or exceed the installation condition requirements? 

Will the instrument be exposed to corrosive elements (e.g. seawater) or erosive (e.g. high pressure or steam wash downs). Will a plastic enclosure survive? Will the paint come off or will the aluminum enclosure exhibit a patina? Will a carbon steel enclosure rust? Would service life be worth the extra investment in a stainless steel enclosure?

Is the process itself running at high temperature where the instrument could be exposed to radiated heat, or does the pipe have a layer of insulation to consider? Should electronics be remotely located from the sensor element to avoid exposure to excessive heat radiating from the process? If insulated, be sure to add its depth in determining the length of the probe and the process connection.

Is the installation subject to explosive gases such that Ex class/zone approvals are required? If yes, what levels? Is the location a Div.1/Zone 1, Class I, Div.2/Zone 2, etc.? If yes, what country’s approval standards are required (e.g. FM, ATEX). Does the full instrument (sensor, electronics, and enclosure) carry the matching required approval?

 Figure 5

8) What Type Outputs Are Needed and How Many? 

Is a single analog output (e.g. 4-20 mA) of the flow rate adequate? If an output of the temperature is also desired, many manufacturers also provide a second analog output channel for this. Some manufacturers also offer pulse or frequency outputs to send to remote readouts or totalizers.

Or, is your process tied to a bus communications based control network requiring HART, Modbus, Foundation Fieldbus, Profibus, BACnet, EtherNetIP or other protocol? Do you require evidence of these bus comms being registered and certified to better ensure successful integration? 

Is there a chance your output needs could change in the future? For example, is the plant considering migrating from traditional analog 4-20 mA signals to a digital bus? If yes, ask the flow meter manufacturer if its meter can be upgraded and, if yes, how. For the few manufacturers who offer some migration path, the means will be much different. For some it might mean returning the meter to the factory, while others might have a field upgrade kit available, or, still others will have both analog and digital buses already embedded and selectable by the user in the field.

9) What Type of Process Connection Will Be Used? 

How will the flow meter be installed into the pipe and held in place (Figure 6) ? Will the meter be installed in or ever need to be retracted under pressure, and if so, how much? Some manufacturers offer only a limited choice while others offer an extensive selection. 

What type of fitting is required: threaded, flanged, compression type, NPT or metric? What about the required ratings? Will you need a packing gland or need to hot tap the line for installation? Would adding a ball valve be helpful for maintenance? Consider also that non-standard or special order process connections will increase the cost and extend delivery times. 

 Figure 6

10) Are Specific Pedigrees, Certifications and/or Documentation Required?

Often overlooked during initial considerations and application suitability are requirements for certifications and qualifications beyond the basic meter performance. This might include such things as pressure tests, certified materials and traceability, positive material identification report, and/or welding pedigree and certificates. If the thermal flow meter is to be used in a safety instrumented system (SIS), is there proof, and preferably independent verification, of SIL compliance (Figure 7). If the flow meter will be used in emissions monitoring (CEMS), does it need to have special functions or features added (e.g. calibration check routines) to comply with local regulations (e.g. US EPA, European QAL1, etc.)?

 Figure 7

Conclusions

The proper selection of any flow meter requires the specifying engineer to consider several variables. Thermal mass gas flow meters are no different. Thermal mass gas flow meters are a main-stream technology growing in popularity due to continued improvements in the technology, cost effectiveness, and education on best practices. Specifying engineers prepared with answers to the 10 variables presented here will take less time to identify the best suited product as well as ensure first-time right installation success. 
 

Thermal Flow Meter Success Checklist

Learn more about FCI's thermal mass flow meters for your air and gas flow measurement needs here.