Flow Metering Challenges, Considerations

In the aerospace industry, measuring the flow of fluids used onboard aircraft or in component test stands demands superior instrument performance. From measuring the fuel consumption of rotary and fixed-wing aircraft, missiles and drones, to evaluating the performance of hydraulic fluid and lubricants, aerospace applications present difficult flowmetering challenges.

Among today’s common liquid and gas flowmeter technologies, are widely differing designs such as turbine, coriolis, magnetic, differential pressure and vortex meters. Each technology offers individual strengths and weaknesses in relation to a given flow measuring application. This is especially true in the aerospace industry.

A manufacturer prototyping a new helicopter platform, for example, may need to test the performance of an orifice or pump, or monitor engine heating under various flight conditions. Flow measurements must not only be extremely accurate, but data must also be provided in a real-time basis. Once the flow data is collected, the manufacturer makes the necessary design changes, keeping the aircraft within specified performance tolerances.

In addition to initial prototype flight testing, manufacturers often employ flowmeters for a host of recurring tests. For instance, a flowmeter may be mounted along with several other sensors on a panel installed under the helicopter’s engine cover. The helicopter is taken through its flight envelope and the flow of various engine or hydraulic fluids is measured and recorded. The wide range of temperatures encountered during these tests requires that the meter be able to accurately compensate for temperature changes that significantly affect fluid viscosity.

Another use for flowmeters is to measure fuel consumption as part of an aircraft’s on-board fuel management system, or as part of a ground-based engine test stand. These applications often involve aircraft designed for commercial and industrial service.

Flowmeter Selection
Endusers contemplating a flowmeter purchase should take the time to study the characteristics of respective measurement technologies, and analyze their advantages/disadvantages for different environments.

Certain applications have unique requirements for sensor size, weight, material of construction and mounting. For this reason, endusers must not only identify a flow sensing technology meeting their specifications, but also select a flowmeter manufacturer with proven experience in their industry – and the knowledge to provide sound application guidance and advice.

The user should consider size and weight requirements, which typically discounts the use of Coriolis technology, as well as fluid type, which limits the use of electromagnetic flowmeters. The need for fast response, resolution and accuracy many times rules out the use of vortex and differential pressure meters.

One of the most versatile types of flowmeters – and as such, one of the most popular among aerospace engineers – is the turbine flowmeter. Turbine meters are regarded by many as the meter of choice for obtaining precise flow measurements in clean, known liquids and the turbine meter can be designed and manufactured based on custom specification.

Precision turbine meters can be used to measure the flow of fuel, hydraulic fluid, cryogenic fluid, lubricants, coolants and other fluids crucial to aircraft performance. In these applications, the meters can withstand a variety of environmental conditions as specified by TSO C44a, RTCA/DO-160 and a host of military specifications.

Technology, Principles
Turbine flowmeters employ a proven, high-precision measurement technology, which provides exceptionally reliable digital outputs. The meters incorporate a freely-suspended turbine, or rotor, rotated by the flow of the fluid (liquid or gas) through the meter body. Since the flow passage is fixed, the rotor’s rotational speed is a true representation of the volumetric flow rate. The rotation produces a train of electrical pulses, which are sensed by an external pick-off mounted on the surface directly above the rotor. The frequency of the pulses can be converted to an analog current or voltage, or can be displayed as gallons per minute (gpm), pounds per hour (pph), cubic feet per minute, or in other engineering units.

The advent of small, powerful microprocessors that can be easily packaged in an electronic pick-off has resulted in a smart turbine flowmeter. These advanced turbine meters eliminate the need for external temperature sensors, signal conditioners and linearizers by providing real-time compensation for measurement variables (e.g., viscosity and density). This feature increases the meter’s rangeability by extending turndown to as much as a ratio of 100:1 at 0.1% of reading linearity. Total system uncertainty utilizing smart electronics can achieve values of less than 0.15% of reading at 3 Sigma.

This custom-designed turbine flowmeter
provides viscosity and density
corrections over wide flow and temperature ranges.Linearized turbine meters can be configured so that multiple units output an identical K-factor, providing complete interchangeability of the flowmeters. Moreover, the data acquisition device does not have to be rescaled or reconfigured if the meter is changed or recalibrated. The linearized output can be amplified to a 0VDC to 5VDC pulse and scaled in units of volume, such as gpm and liters per minute (lpm).

Coupled with the typical 2ms to 3ms response times of turbine flowmeters, users are able to instantly detect changes in flow. Furthermore, updating measurement variables at microprocessor speeds ensures that calculations are made with current temperature, pressure, viscosity and density values. Utilizing calculated density values, turbine flowmeters can be configured to output flow values in mass units.

Turbine meters have a relatively high turndown ratio, with a linear range of 10:1 and a repeatable range of up to 100:1. This capability often enables a single turbine meter to replace multiple meters with a lesser turndown capability and can significantly reduce cost in applications requiring accurate rate and totalization measurement over a wide flow range.

Depending on size and weight requirements, temperature ranges, fluid characteristics and other variables, optional materials such as Hastelloy C and aluminum can be utilized for the meter’s housing, rotor, bearings and shaft, although stainless steel is used most frequently.

In addition, the turbine’s compact size and packaging flexibility make it well-suited for use on-board aircraft for measuring fuel, coolants, lubricants and hydraulic fluids. In these environments, the flowmeter is durable enough to withstand high levels of vibration, shock and G-force loads encountered during flight.

The high resolution of the turbine meter also makes it ideal for detection of leaks in aircraft fluid systems. With resolutions to 48,000ppg for small turbines, minute fluid flow can be detected.

The relative simplicity of the turbine meter’s required signal processing allows linearizing and temperature conditioning circuits to be adapted to military levels of temperature and EMI noise immunity. All of the signal conditioning electronics can be packaged with the turbine in a compact, rugged housing that is quiet to EMI susceptibility and emissions.

Another key difference between turbine meters and other common flow sensors is their compatibility with remote electronics; paired with a heat-tolerant electronic pick-off and amplifier, turbine meters can be located in areas where extreme temperatures are normal, while data acquisition electronics are safely mounted elsewhere.

Flowmeter Calibration
Aerospace users with critical flow measurement applications must maintain their instrumentation in optimal working order. In order to obtain the best possible accuracy from the device, calibration is a critical component in all flowmeter technologies. This requires periodic meter calibration using a flow calibration system that is traceable to recognized government and industry standards, such as those established by the National Institute for Standards and Technology (NIST).

Turbine flowmeters are capable of measurement accuracies to 0.100% reading. In order to obtain the best possible accuracy from the device, calibration is a critical component in meter configuration. Optimal accuracy is achieved if the flowmeter is calibrated in a like fluid to the one being measured. Moreover, matching a fluid’s kinematic viscosity is vital to performance. Kinematic viscosity defines both absolute viscosity and density for a fluid and is required data for successful turbine meter performance over varying temperature ranges.

For the aforementioned reasons, calibration systems used for turbine flowmeters must be flexible in terms of fluid usage while maintaining extremely low system measurement uncertainty characteristics.

The extent to which a flowmeter calibration is NIST-traceable depends on whether the system used is a primary standard or secondary standard. A primary standard calibration is one that is based on measurements of natural physical parameters (i.e., mass, distance and time). This calibration procedure assures the best possible precision, and through traceability, minimizes bias or systematic error. A secondary standard calibration is not based on natural, physical measurements. It often involves calibrating the user’s flowmeter against another flowmeter, known as a master meter, that has been calibrated itself on a primary standard.

A growing number of flowmeter customers are specifying that their meters be serviced by calibration facilities accredited under the National Voluntary Laboratory Accreditation Program (NVLAP) to perform ISO/IEC 17025 liquid calibrations – the gold standard for absolute accuracy and repeatability.

The NVLAP program, administrated by NIST, provides accreditation services to the U.S. industry and is recognized internationally as a world-class laboratory accreditation program. NVLAP conforms to NIST Handbook 150, ISO/IEC 17025 and Guide 58, as well as ANSI/NCSL Z540.

NVLAP accreditation serves as a vital tool to enhance user confidence that flowmeter calibration data are accurate, traceable and have good reproducibility. This accreditation provides third-party verification of superior calibration proficiency and correlation. It also offers assurance of technically-valid results on every instrument calibration, with enhanced data reports showing measurement uncertainties.

A calibration laboratory must be audited annually to maintain its ISO/IEC 17025 NVLAP accreditation. This audit is not only based on the lab’s calibrator documentation traceability, but also on the process used to maintain guaranteed uncertainty.

As with all sensors, the device is only as good as its last calibration.

Conclusion
Today, popularity of turbine flowmeters is due, in large part, to the demanding flow measurement requirements of today’s high technology aircraft. Like many other sensor technologies that originate in the aerospace industry, these reliable flowmeters are finding acceptance in a host of other applications where high accuracy, extended range, fast response time and rugged construction are required.

Aerospace component manufacturers require the best available test equipment to verify the operation of their flight-critical components. Whether it is a fuel pump, hydraulic pump, heat exchanger or simply a valve, accurate flow measurement is critical to ensure component performance.

Flow Technology Inc.
Tempe, AZ
ftimeters.com