Various measuring principles are available for measuring the flow of compressed air and technical gases. In addition to vortex, differential-pressure, ultrasonic and Coriolis systems, turbine flow meters and thermal mass flow sensors are frequently considered. Both methods can measure a gas flow, but they differ fundamentally in their measuring principle and in their requirements concerning the medium, installation and evaluation.
A turbine flow meter initially measures the gas volume under the actual conditions inside the pipe. Its rotational speed or output frequency depends on the flow velocity. Pressure, temperature and gas density therefore have a particular influence on the usable measuring range and on the conversion to standard volumetric flow.
A thermal mass flow sensor such as the IVA 521, by contrast, operates without a moving rotor. It measures the gas flow through the heat transfer of the flowing medium and can output the consumption directly as a standard volumetric flow. This is particularly useful when the consumption of individual machines, production lines or cost centres must be monitored continuously.
A turbine flow meter is therefore not fundamentally unsuitable for gas. For typical industrial compressed-air consumption measurements, however, a thermal inline sensor is often easier to size, simpler to integrate into an energy-monitoring system and more suitable for use in the lower part of the measuring range.
Table of Contents
- Flow measurement or consumption measurement?
- How does a turbine flow meter measure gas and compressed air?
- How does a thermal mass flow sensor work?
- Comparison of turbine flow meters and thermal sensors
- Operating volume, standard volume and mass flow
- Minimum flow and measuring range
- Gas type, composition and moisture
- Installation and flow profile
- Output signals and energy monitoring
- Accuracy, calibration and plausibility checks
- Practical example: Monitoring the consumption of a production machine
- Which measuring principle is suitable for the application?
- Which products are suitable?
- Conclusion
- Frequently asked questions
Flow measurement or consumption measurement?
Before selecting the measuring instrument, it must first be clarified which information is actually required. The instantaneous flow rate describes how much gas is currently flowing through the pipe. Total consumption adds up this quantity over a defined period. On a test bench, the rapid instantaneous value may be particularly important. For energy monitoring, by contrast, the key information is often how many standard cubic metres a machine consumes per shift, job or operating hour.
A leakage analysis also serves a different purpose from process control. During leakage analysis, small, permanent consumption levels outside production hours must be made visible. The sensor must therefore be capable of measuring a sufficiently low minimum flow reliably. In a pneumatic test machine, by contrast, a rapid response to strong load changes may be more important.
The nominal pipe size alone therefore does not determine the suitable measuring technology. The decisive factors are the smallest relevant flow, maximum consumption, gas type, compressed-air quality and whether operating volume or standard volume is required.
How does a turbine flow meter measure gas and compressed air?
A turbine flow meter contains a rotor that is set in motion by the gas flow. Within the specified measuring range, the rotational speed is approximately proportional to the volumetric flow. A signal pickup detects the passing rotor blades and generates a pulse or frequency signal.
The so-called K-factor defines how many pulses correspond to a specific unit of volume. The instantaneous flow can be calculated from the frequency, while the sum of the pulses represents the total volume that has passed through the meter. This principle can provide a very high-resolution output on test benches and in dynamic measurements.
With gases, however, there is a particular challenge: the rotor requires sufficient flow force to overcome bearing friction and mass inertia. At low gas density or very low flow, the turbine may therefore operate below its start-up threshold. Gas is flowing through the pipe, but the rotor is not yet producing a stable signal.
The operating pressure plays an important role here. Compressed air at 7 bar absolute has a significantly higher density than air at atmospheric pressure. A turbine may therefore exhibit different start-up and measuring behaviour at higher pressure. A turbine flow meter for gas must consequently be expressly designed and calibrated for the medium, pressure range and expected volumetric flow.
In addition, a turbine contains moving parts. Particles, condensate or oil can affect the rotor and bearings. The principle can work very well with clean, dry gases within a defined measuring range. For permanent compressed-air monitoring on many individual machines, however, the maintenance and sizing requirements are often higher than with a thermal sensor.
How does a thermal mass flow sensor work?
A thermal mass flow sensor contains a heated sensing element and a temperature measurement. The gas flowing past the element removes heat from it. The mass flow is determined from the required heating power or the measured temperature difference.
Because the method is based on the transported gas mass and not solely on the current pipe volume, the sensor can output a standard volumetric flow directly. A separate pressure measurement is not fundamentally required for this output. This is a major difference between a thermal sensor and a pure operating-volume measurement.
The IVA 521 is designed as a compact inline flow sensor for compressed air and other suitable gases. The sensor and defined measuring block form a matched measuring section. The sensor displays the current flow, accumulated consumption and gas temperature. Its compact design makes it particularly suitable for installation downstream of a service unit or directly upstream of individual compressed-air consumers.
As there are no moving parts, there is no mechanical start-up threshold caused by a rotor and no conventional bearing wear. The thermal sensor is therefore particularly suitable when small consumption levels must also be measured or when a measuring point is to be operated continuously with low maintenance requirements.
Comparison of turbine flow meters and thermal sensors
| Criterion | Gas turbine flow meter | Thermal sensor such as IVA 521 |
|---|---|---|
| Measuring principle | Gas flow drives a rotor | Heat transfer of the flowing gas |
| Primary measured variable | Operating volumetric flow | Mass flow or standard volumetric flow |
| Moving parts | Rotor and bearings | No moving parts |
| Low flow rates | Limited by the mechanical minimum flow | Often a wide usable measuring range |
| Influence of operating pressure | Important for gas density, start-up behaviour and standardisation | No separate pressure measurement required for standard-volume output |
| Contamination | Particles and condensate can affect the rotor and bearings | Deposits on the sensing element can alter heat transfer |
| Typical output signal | Pulse or frequency via K-factor | 4–20 mA, pulse and Modbus RTU |
| Typical application | Test bench, defined gas flow, rapid pulse evaluation | Machine consumption, cost-centre measurement and energy monitoring |
The table shows that the two methods cannot simply be substituted for one another. A turbine flow meter can be advantageous when a high-resolution frequency signal and highly repeatable volume totalisation are required within a fixed operating range. The thermal sensor is often the stronger choice when a wide consumption range, direct standard-volume output and permanent consumption monitoring are the main priorities.
Operating volume, standard volume and mass flow
With compressible gases, it must always be stated under which conditions a volumetric flow applies. One cubic metre of compressed air in a pipe at 7 bar absolute contains significantly more gas mass than one cubic metre of expanded air at atmospheric pressure.
The operating volumetric flow describes the actual volume flowing at the current pressure and temperature. The standard volumetric flow converts this quantity to defined reference conditions. Depending on the application, these conditions may be 0 °C and 1.01325 bar absolute, or 20 °C and 1,000 mbar, for example.
| Measured variable | Meaning | Typical use |
|---|---|---|
| Operating volumetric flow | Gas volume at the current pressure and temperature | Pipe sizing and flow velocity |
| Standard volumetric flow | Volume converted to defined reference conditions | Consumption comparison, cost centres and energy monitoring |
| Mass flow | Actual gas mass transported per unit of time | Process balancing and quantity measurement independent of volume conditions |
| Total consumption | Standard or operating volumetric flow accumulated over time | Shift, machine and annual consumption |
With a turbine flow meter, the measured operating volumetric flow must additionally be corrected using pressure and temperature in order to obtain a standard-volume output. These measurements should be taken as close to the turbine as possible. If gauge pressure is accidentally used instead of absolute pressure, a significant calculation error results.
A thermal mass flow sensor can derive the standard volumetric flow directly from its measuring principle. Nevertheless, the reference conditions stored in the device must be known and documented. A value in m³/h or Nm³/h is only unambiguous when it is clear to which temperature and pressure it refers.
Correctly assessing minimum flow and measuring range
The maximum flow is often known because it can be derived from the consumer or pipe cross-section. Determining the smallest relevant consumption is usually more difficult. This value is particularly important for energy monitoring and leakage assessment.
A turbine flow meter that is selected too large may stop at the lower end of the range or generate only irregular pulses. The sensor then indicates no flow or a value that is too low even though gas is actually being consumed. A thermal sensor has no mechanical start-up threshold, but it also requires a minimum flow and must be sized so that the relevant operating range lies within its specified characteristic.
For machine monitoring, the maximum consumption at full load should therefore not be the only value considered. Standby consumption, partial load, cleaning cycles and possible leakage flows outside production hours are equally important.
The selected pipe diameter affects the flow velocity. If the measuring block is too large, the velocity may be too low at the bottom of the consumption range. If it is too small, high velocity and unnecessary pressure loss can occur at the upper end of the measuring range. The inline sensor should therefore be selected according to the actual flow range and not merely the existing connection size.
Gas type, composition and moisture
Thermal sensors determine the flow through the heat transfer properties of the gas. These properties depend on the gas type. Air, nitrogen, argon, carbon dioxide and oxygen have different thermal characteristics. The correct gas type or gas constant must therefore be configured in the sensor.
With a turbine flow meter, density and viscosity also influence the characteristic. A K-factor determined using air cannot automatically be transferred to every other gas without verification. For demanding applications, calibration should therefore be performed using the actual gas or under sufficiently comparable conditions.
Gas quality is important for both measuring principles. Water droplets, compressor oil or larger particles can impair the measurement. In a turbine, they can cause mechanical problems in the rotor and bearings. In a thermal sensor, deposits can alter the heat transfer and lead to a gradually increasing measurement deviation.
The IVA 521 is primarily intended for clean compressed air and suitable non-corrosive gases. With permanently wet compressed air, aggressive gases, strongly fluctuating gas mixtures or hazardous-area applications, it must be checked whether another measuring principle or a specially designed device version is required.
Installation and flow profile
The flow profile is an important influencing factor in almost all flow-measurement methods. Bends, valves, T-pieces, pressure regulators and major changes in cross-section can generate swirl and uneven velocity profiles. A sensor may then measure a flow condition that is not representative of the entire pipe cross-section.
Classic turbine flow meters therefore often require straight inlet and outlet sections. The exact requirements depend on the design and manufacturer. Partially closed valves or pressure regulators positioned directly upstream of the measuring instrument are particularly critical.
The IVA 521 has an integrated flow straightener inside the measuring block. As a result, no additional inlet sections are required for this defined inline measuring section. This is particularly advantageous when retrofitting compact machines where only limited space is available between the service unit, valve terminal and consumer.
The integrated measuring block does not, however, prevent every installation error. The direction of flow, connection size, permissible operating pressure and mounting position must still be observed. The measuring point should also be positioned so that the sensor remains accessible and can be removed for cleaning or recalibration.
Output signals and energy monitoring
For local checks, the integrated display is often sufficient. It can show values such as the current flow, total consumption and gas temperature. For systematic consumption analysis, however, the values must be transmitted to a PLC, data logger or energy-monitoring system.
The 4–20 mA output is particularly suitable for the current flow rate. The analogue signal is robust and can be integrated easily into many existing control systems. The measuring range and scaling must be defined unambiguously in both the sensor and the PLC. A UPS4E current-loop calibrator can be used to test the analogue measuring chain.
The pulse output is often more suitable for total consumption. Each pulse represents a defined gas quantity. The controller counts the pulses and calculates the accumulated consumption. Following a power failure or replacement of the controller, however, it must be clear how the meter reading is stored and continued.
Modbus RTU enables several measured values to be transmitted digitally over a shared interface. Depending on the configuration, instantaneous flow, total quantity, temperature, status information and additional parameters can be read. When several machines are involved, a bus connection reduces wiring requirements and simplifies central consumption analysis.
Accuracy, calibration and plausibility checks
The accuracy of a flow sensor applies only within the specified measuring range and under the intended operating conditions. Below the minimum flow, with an incorrectly configured gas type or with severe contamination, the indicated value can become significantly less reliable.
When assessing accuracy, a distinction should also be made between measurement deviation as a percentage of the reading and as a percentage of full scale. A full-scale component has a particularly strong relative effect at the lower end of the measuring range. For assessing small standby consumption levels, the percentage accuracy stated at high flow is therefore not the only relevant figure.
Regular plausibility checks are particularly useful when the measured values are used for cost allocation, evidence of savings or energy management. Changes can result from deposits, incorrect configuration, modifications to the pipework or a change in gas type.
During calibration, it should be defined which flow range actually needs to be tested. Calibration only near the upper range limit says little about the quality of the measurement at low machine consumption. Several calibration points across the relevant operating range are useful for a reliable consumption balance.
Practical example: Monitoring the consumption of a production machine
The compressed-air consumption of an automated assembly line is to be recorded in a production hall. During normal production, the machine operates with strongly varying consumption. A small base consumption remains during short standstill periods. At night, the machine should require almost no compressed air.
A turbine flow meter could measure the high operating flow very dynamically via a frequency output. For correct selection, however, the operating pressure, actual operating volumetric flow, minimum flow and gas density would have to be determined accurately. The low night-time consumption could fall below the reliable start-up threshold.
A thermal inline sensor is therefore selected for the measuring point. The IVA 521 is installed downstream of the service unit in the machine’s supply line. The integrated measuring block and flow straightener allow the measuring point to be implemented despite the limited installation space.
The current flow is shown locally on the display. Total consumption and the instantaneous value are transmitted to the energy-monitoring system via Modbus RTU. The system then evaluates consumption per job, consumption per operating hour and night-time consumption.
After several weeks, the analysis shows that the consumption does not fall completely after production ends. By isolating the system section by section, a leaking valve terminal is identified as the cause. Following the repair, the base consumption falls significantly. The measuring point therefore serves not only for cost allocation but also for continuous system monitoring.
Which measuring principle is suitable for the application?
| Application | More suitable for a turbine flow meter | More suitable for the IVA 521 or another thermal sensor |
|---|---|---|
| Rapid frequency signal on a gas test bench | Well suited within a defined measuring range | Also possible, depending on response time and interface |
| Permanent consumption measurement on a machine | Higher sizing and maintenance requirements | Very well suited |
| Direct output in standard volume | Additional pressure and temperature compensation required | Possible directly through the measuring principle |
| Small standby or leakage flows | Mechanical start-up threshold must be considered | Often advantageous |
| Very dirty or wet gas | Rotor and bearings are critical | Sensing element also critical; consider another principle |
| Limited space for inlet sections | May be problematic depending on the device | IVA 521 with integrated flow straightener is advantageous |
| Energy monitoring of several machines | Additional evaluation electronics required | Modbus, pulse and analogue output directly available |
For a specific selection, at least the gas type, pressure, temperature, moisture, minimum and maximum flow, required reference unit, pipe connection and output signal are needed. It should also be clarified whether the measuring point is intended only to provide a process value or to form the basis of permanent consumption and cost monitoring.
Which products are suitable?
IVA 521 – compact inline flow sensor
The IVA 521 is a thermal inline flow sensor for compressed air and other suitable gases. It is particularly suitable for monitoring individual machines and consumers in compressed-air and energy-monitoring systems.
The integrated measuring block ensures a defined relationship between the sensor and pipe cross-section. The integrated flow straightener reduces the required installation space because no additional inlet sections are needed. Depending on the version, threaded sizes from 1/2″ to 2″ are available.
The device measures the current flow, total consumption and temperature. For integration into control and energy-management systems, available outputs include 4–20 mA, a galvanically isolated pulse output and Modbus RTU. Further communication options are available as optional features.
Consumption meters for gases and compressed air
The consumption meters for gases and compressed air category includes further measuring systems for machine consumption, compressed-air distribution, technical gases and larger pipes.
For wet compressed air, very high flow rates, hazardous areas or special gas mixtures, other thermal, vortex, ultrasonic or differential-pressure systems may be more suitable. The measuring principle should therefore always be matched to the gas quality and specific measuring task.
Flow measurement technology
The higher-level flow measurement technology category provides an overview of additional methods for liquids, gases and steam.
A turbine flow meter remains an interesting solution for dynamic test benches, defined gas flows and applications using pulse or frequency evaluation. For typical compressed-air consumption measurement directly on a machine, however, a compact thermal sensor such as the IVA 521 is often the more suitable and easier-to-integrate solution.
Conclusion: The thermal inline sensor is often more practical for machine consumption
Turbine flow meters can also be used effectively with air and gas. They provide a rapid pulse signal and enable precise volume totalisation when the gas type, operating pressure, minimum flow, bearings and calibration are matched exactly to the application.
Permanent compressed-air consumption measurement, however, introduces additional requirements. Operating volume and standard volume must be distinguished, small flows must be measured reliably and moving parts must be protected from particles, oil and condensate.
The IVA 521 thermal mass flow sensor avoids many of these difficulties. It contains no moving parts, outputs the standard volumetric flow directly and can be integrated into an energy-monitoring system using analogue, digital and pulse-based signals. The integrated flow straightener also makes installation easier where space is limited.
For monitoring individual machines, production lines or cost centres, the IVA 521 is therefore often more suitable than a gas turbine flow meter. Correct sizing according to gas type, flow range, pipe size and measuring objective nevertheless remains essential.
Frequently asked questions about compressed-air flow measurement
Can a turbine flow meter measure compressed air?
Yes, provided that the turbine flow meter is expressly designed for gases. The rotor, bearings, signal pickup, measuring range and calibration must match the gas type, operating pressure and flow range.
Why is a liquid turbine not automatically suitable for gas?
Gas has a significantly lower density than liquid. This changes the driving force, minimum flow and start-up behaviour of the rotor. The bearings and calibration may also be designed for liquids.
What is the main difference compared with the IVA 521?
A turbine measures the gas flow using a moving rotor and initially measures operating volume. The IVA 521 uses a thermal principle, contains no moving parts and can output the standard volumetric flow directly.
Does the IVA 521 require an additional pressure sensor?
No additional pressure measurement is required for direct output of the standard volumetric flow. The permissible operating pressure must nevertheless be considered when selecting the device.
What does the IVA 521 display?
Depending on the configuration, the integrated display shows the current flow, total consumption and gas temperature.
Does the IVA 521 require inlet sections?
The intended inline measuring block has an integrated flow straightener. As a result, no additional inlet sections are required for this version.
Can the IVA 521 measure small leaks?
It can measure small consumption and leakage flows provided they lie within the specified measuring range. A dedicated leak detector is often more suitable for very small individual leaks.
How is total consumption transmitted?
Total consumption can be transmitted to a PLC, data logger or energy-monitoring system via the pulse output or a digital interface such as Modbus RTU.
When is a turbine flow meter still the better choice?
A turbine flow meter can be advantageous when a particularly rapid frequency signal, a defined K-factor or high-resolution pulse counting is required on a test bench within a clearly limited flow range.
Is the IVA 521 suitable for wet compressed air?
A measuring solution specifically designed for the purpose should be selected for permanently wet or heavily contaminated compressed air. Condensate and deposits can also affect thermal sensing elements.
Which information does ICS Schneider require for selection?
The required information includes the gas type, minimum and maximum flow, operating pressure, gas temperature, moisture or compressed-air quality, pipe connection, required standard conditions, output signal and requirements concerning accuracy, calibration and data connection.