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Flowmeter Basics

By Dan Herring

Fig. 1. Typical flow control panel (multiple-zone continuous furnace)

Everyone knows what a flowmeter is, and, yet, few of us really understand them the way we should. The sad reality is that once flowmeters are installed and operating we tend to take them for granted. This can often lead to serious flow errors and potential process or safety issues that compound themselves over time. Let’s learn more.

What is a flowmeter, really?

Fig. 2. Flowmeter types (a) Endothermic gas FurnaceMeter (Courtesy of Atmosphere Engineering Company); (b) Mass-flow controller (Courtesy of MKS Instruments)

A flowmeter is a device used for measuring the flow of gases or liquids. There are actually two different ways to measure flow – by volumetric means (Fig. 2a) and by mass-flow techniques (Fig. 2b). As heat treaters, we are probably more familiar with the volumetric-flow measurement of gases. The principle involves the displacement of the gas volume over time. Atmosphere furnaces, gas generators and combustion systems typically use these types of devices. Mass flow involves measuring the weight of a gas, and these are commonly found on vacuum furnaces that meter in gases for partial-pressure operation.

What types of flowmeters are there?

As heat treaters, we typically focus on the measurement of gases. Anyone who has seen a nitrogen/methanol system, however, is aware that liquid methanol must be metered into the furnace. Flowmeters are also used for measuring liquid flow. In addition to flowscopes and mass flowmeters, other common types include: orifices, rotameters, positive-displacement meters, electromagnetic meters, ultrasonic (Doppler-effect) devices, turbine meters, wedge flow devices, impact meters and turbine meters.

What are the features and advantages of the most common flowmeter types?

Variable-area flowmeters offer:

• Mechanical flow measurement with only a single moving part, ensuring measurement reliability
• Application versatility and availability of a variety of construction materials, inlet and outlet sizes and types
• Easy installation with generally no straight pipe requirements
• Low pressure drops
• Linear scales, allowing easy flow measurement interpretation
• Electronic output availability, preserving the benefits of mechanical flow measurement

Tapered-tube rotameters offer:

• Low instrument cost (when glass or plastic metering tube is used)
• Accuracy at very low flow rates

Slotted-cylinder flowmeters offer:

• A flow range of 25:1 since flow-measurement accuracy is determined by the precision of the slot manufacturing operation
• Instrument specifications can be changed by field replacement of the slotted tube and float, without having to repipe the flowmeter body
• Ability to handle high flows and pressures
• Improved immunity to the effects of pulsating flows, with no minimum back-pressure

Limitations common to both tapered-tube and slotted-cylinder variable-area flowmeters are that vertical mounting is required and that they contain moving parts.

The user should also be aware that the accuracy of mass flowmeters and mass-flow controllers is determined by two factors: flow calibration and repeatability. Proper instrument calibration ensures starting-point accuracy. Repeatability is the measure of continuous performance-to-specification over the lifetime of the device. Most mass flowmeters and mass-flow controllers have an accuracy of ±1% of full scale and a repeatability of ±0.25% of full scale.

Is it easier to control a gas or a liquid?

Interestingly, liquids are easier to measure and control because of their small compressibility. For most volumetric-flow applications, the incoming pressure in liquid systems does not need to be closely controlled. By their very nature, liquids can be captured easily and measured to a high degree of accuracy. Gases on the other hand, because of their compressibility, require more complex sensing and control methods.

What is the accuracy range of a gas flowmeter?

When measuring gas flows in heat-treating applications there is an important distinction between the operating range of a flowmeter and the design range when purchasing a new meter. Plan to operate a flowmeter in a range not below 25% and not above 90% of the flowscope’s scale capacity. In other words, if your flowmeter is rated for 0-2,000 cubic feet per hour (cfh), you can be assured of accuracy when the flow is between 500 cfh and 1,800 cfh. Flows outside these limits are not considered accurate.

A good “rule of thumb” for sizing a flowmeter is to purchase your meter “in the middle third.” The flowmeter should be chosen so that the actual flow will be no less than 1/3 and no higher than 2/3 of the scale you select. This gives you the ability in actual operation to compensate for unexpected changes in flow requirements that may occur. Often over the life of a heat-treating furnace, process requirements and operating conditions change – sometimes dramatically – and you want your gas measurement to remain accurate.

Should I have my flowmeters recalibrated?

If a change of operating conditions is permanent, such as the desire to constantly operate at a different pressure, then recalibration of the flow measurement device is strongly recommended. As a rule, flowmeters used in heat-treating applications are designed for a maximum temperature of 150°F (66°C) and an operating pressure up to 50 psig (345 kPa). However, application-specific flowmeters have maximum operating pressures outside these ranges.

What affects my gas measurements?

If knowing the proper flowrate is important to you, be aware that a change in temperature, pressure or specific gravity of the gas from that for which the meter was calibrated will cause a serious error in the indicated scale reading. (These topics are covered in detail in the online portion of this column.) It is quite common in a heat-treat shop to find flowmeters operating at different pressures and temperatures than they have been calibrated for.

Do I need to maintain my flow devices?

All flowmeters eventually require maintenance. It is a sad truth that some units require more maintenance than others, so this factor should be considered when a unit is selected. However, in most heat-treating operations the equipment manufacturer has already made that choice for you, so understanding what maintenance is required and when it should be performed is of paramount importance.

Flowmeters have moving parts and require internal inspection, especially if the fluid is dirty or viscous. For example, in furnaces using endothermic gas, the flowmeters often become contaminated with soot (carbon) and must be cleaned by CAREFULLY disassembling the flowmeter and cleaning all internal moving parts as well as replacing the dirty fluid in the flowmeter tube. Caution: This involves isolating the flowmeter, or performing maintenance when the unit is shut down, and must be done in a safe manner as many of the gases involved are asphyxiants as well as being flammable, toxic and possibly life-threatening.

Remember also that electromagnetic flowmeters and all flow measurement devices that use secondary instruments such as pressure sensors to actuate a control valve or send a signal to a remote source must be periodically inspected, calibrated, repaired and/or replaced. Improper location of the flowmeter itself, the secondary sensor or readout devices can result in measurement errors and hidden costs.

Do I really need to learn about my flowmeters to be in control, stay in control, operate safely and keep the cost of my operation as low as possible?

Simply stated, yes. Hopefully, this discussion has helped reinforce this idea. Now go out today and check your flow devices!

What do I need to consider if I want to use a flowmeter for one gas, but it is calibrated for another?

One of the most common problems seen in heat-treat shops is that operating personnel and supervisors are unaware of the consequences of a flowmeter that has been calibrated for use with one gas while having another gas flowing through it. When switching gases, the change in specific gravity of the two gases is the principle factor that must be taken into account. Specific gravity is the ratio of the density of the gas under consideration to the density of dry air (at standard temperature and pressure, 77°F (25°C) and 14.7 psi).

Table 1 provides a quick conversion chart. To calculate the actual flow rate of gas being metered, multiply the indicated scale reading of the flowmeter by the factor shown in the table.

Equation (1) allows us to calculate the actual flow when a change in specific gravity occurs.

How do I compensate for changes in pressure and temperature?

A change in temperature and/or pressure of the gas from for which the meter was calibrated will cause a serious error in the indicated scale reading. However, there is an easy way to calculate the effect of these changes.

Temperature Compensation

Equation (2) allows us to calculate the actual flow when a change in temperature occurs.

A summary for some common values is shown in Table 2.

Pressure Compensation

Equation (3) allows us to calculate the actual flow when a change in pressure occurs.

A summary for some common values is shown in Table 3. If it is necessary to compensate for both temperature and pressure, Equation (4) should be used.

Figure 3

Mass Flowmeters

Thermal-mass flowmeters are also used by many heat treaters. In most industrial-grade devices, gas enters the flow body and divides into two flow paths. Most of it goes through the laminar-flow bypass, creating a pressure drop that forces a known fraction of the flow through the sensor tube (Fig. 3). A power supply is used to direct a constant amount of heat into the gas stream. Resistance temperature detector (RTD) coils are placed around the bypass sensor tube at its upstream and downstream ends. Heat is transferred to the molecules of the flowing gas, independent of pressure or temperature fluctuations.

The gas flow carries heat from the upstream coil to the downstream coil. The downstream coil, therefore, has a higher temperature and more resistance than the upstream coil. The coils are legs of a bridge circuit with the resultant output voltage proportional to the difference between the coils’ resistance, which in turn is proportional to the mass-flow rate. The two other parameters — heat input and coefficient of specific heat — are constant.

Another type of mass flowmeter uses one flow channel with a temperature sensor located in the path of the flow. This technology is simpler, but often less accurate, and is limited to higher flow rates.

Flowmeter Accuracy and Repeatability

Thermal-mass flowmeters are gas-specific devices and must be calibrated with either the actual gas or a reference gas. This “inconvenience” has led to the development of many “fixes” and is driving the development of smarter devices. For now, however, primary calibration with the actual gas or a gas of similar molecular characteristics is the only way to ensure accuracy.

The accuracy of mass flowmeters and mass-flow controllers is determined by two factors: flow calibration and repeatability. Proper instrument calibration ensures starting-point accuracy. Repeatability is the measure of continuous performance to specification over the lifetime of the device. Most mass flowmeters and mass-flow controllers have an accuracy of ±1% of full scale and a repeatability of ±0.25% of full scale.

Several factors affect repeatability. To compensate, highly stable materials and electronic components, as well as precise internal voltage and current regulation, are used. Sensor and bypass design also play a major role in preventing errors caused by contamination and clogging. For example, U-type sensor tubes exhibit residual stresses from bending, which can cause long-term strains and unraveling of sensor coils. These sensors are also more likely to develop drift due to contaminant deposits.

Consideration should also be given to the bypass element. Accuracy can be degraded by changes in temperature if the bypass is an orifice (or venturi) as opposed to a pure laminar-flow element. With an orifice bypass, the pressure drop is proportional to the square of the bypass flow. In this case, the ratio of bypass flow to sensed flow is not a constant but instead is a complex nonlinear function having temperature-dependent terms such as gas viscosity. Both the nonlinearity and temperature dependence of the orifice bypass can seriously degrade the accuracy of a mass-flow controller.

Mass Flowmeters for Vacuum Applications

A heat-processing application of thermal-mass flowmeters and mass-flow controllers is maintaining a specified gas-flow rate into a vacuum chamber when the process requires a partial pressure of additive gas. Typically, a throttle valve or an orifice-limiting device is used to control the output of a vacuum pump. This is an extremely pressure-sensitive method and can result in inefficient gas delivery and poor product quality. Mass-flow controllers automatically compensate for changes in system pressure caused by vacuum-pump fluctuations and deliver a precisely controlled gas-flow rate to the chamber.

Flowmeter Selection Basics

For a thermal-mass flowmeter, it is important to understand its principle of operation and calibration requirements. As is true for most instrumentation, application conditions must be completely and accurately known to avoid costly mistakes that can delay a start-up or even damage the device. Information required to properly specify a mass flowmeter includes gas type, estimated flow rate, pressure range, temperature range and the allowable pressure drop for the system.

Once flow parameters have been defined, the proper flow-body size can be determined by following these steps:

1. Consult manufacturer product specifications for the acceptable flow range of each flow body.
2. Convert the estimated flow rate to the corresponding units listed in the product specifications.
3. If the gas to be measured is not the manufacturer’s specified standard gas (typically nitrogen), convert the flow rate to the equivalent nitrogen flow rate. This is done with a correlation factor, K, defined as the ratio of the actual gas flow rate to the equivalent nitrogen flow rate. To obtain the equivalent nitrogen flow rate, divide the actual gas flow rate by the K factor.
4. Select the flow-body size that will accommodate the estimated flow rate.
5. Ensure that the differential pressure falls within the acceptable range for the selected flow-body size and flow rate. (Note: As mentioned previously, primary calibration with the actual gas or a gas of similar molecular characteristics is the only way to confirm proper accuracy.)

Flow variations with correspondingly large pressure drops require a limitation on turndown to maintain accuracy. Sizing, turndown and accuracy must therefore be considered when selecting a thermal-mass flowmeter.

Remember, too, that mass flowmeters are not sensitive to inlet pressure and temperature. In other words, they “auto compensate” for changes in these variables.

All flowmeters, regardless of type, should be checked periodically for calibration and accuracy before flow-measurement errors start affecting your process. Thermal-mass flowmeters use electronic measurement. Therefore, sensor failure modes are not always known. A failed sensor may give readings that are either too high or too low. Users can visually inspect most variable-area flowmeters by following the manufacturer’s instructions.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring (at) heat-treat-doctor.com

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

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