Investigations on pressure dependence of Coriolis Mass Flow Meters used at Hydrogen Refueling Stations

In the framework of the ongoing EMPIR JRP 16ENG01 “Metrology for Hydrogen Vehicles” a main task is to investigate the influence of pressure on the measurement accuracy of Coriolis Mass Flow Meters (CFM) used at Hydrogen Refueling Stations (HRS). At a HRS hydrogen is transferred at very high and changing pressures with simultaneously varying flow rates and temperatures. It is clearly very difficult for CFMs to achieve the current legal requirements with respect to mass flow measurement accuracy at these measurement conditions. As a result of the very dynamic filling process it was observed that the accuracy of mass flow measurement at different pressure ranges is not sufficient. At higher pressures it was found that particularly short refueling times cause significant measurement deviations. On this background it may be concluded that pressure has a great impact on the accuracy of mass flow measurement. To gain a deeper understanding of this matter RISE has built a unique high-pressure test facility. With the aid of this newly developed test rig it is possible to calibrate CFMs over a wide pressure and flow range with water or base oils as test medium. The test rig allows calibration measurements under the conditions prevailing at a 70 MPa HRS regarding mass flows (up to 3.6 kg min⁻¹) and pressures (up to 87.5 MPa).     

On-site calibration system for trace humidity sensors

A portable device for calibration of trace humidity sensors and an adopted calibration procedure have been developed. The calibration device is based on humidity generation by permeating water through polymeric membrane tubes. Water vapour transmission rates for various polymers were experimentally determined in order to select the most suitable polymeric material. The developed trace humidity generator consists of a gas-flow polymeric hose immersed in a water reservoir thermostated by a sensor-controlled heater. Mole fractions of water vapour between 1 μmol mol⁻¹ and 350 μmol mol⁻¹ (equivalent to frost-point temperatures from −76 °C to −31 °C) were generated by varying either the operating temperature or gas flow. The operating temperature can be varied from 20 °C to 60 °C and kept stable within 0.1 K. Uncertainty analysis indicated that the trace humidity generator produces gas flows of constant humidity amounts with a relative expanded uncertainty less than 3.4% (k = 2) of the generated value.     

An ultra high-pressure test rig for measurements of small flow rates with different viscosities

Within the framework of a research project regarding investigations on a high-pressure Coriolis mass flow meter (CMF) a portable flow test rig for traceable calibration measurements of the flow rate (mass - and volume flow) in a range of 5 g min⁻¹ to 500 g min⁻¹ and in a pressure range of 0.1 MPa to 85 MPa was developed. The measurement principle of the flow test rig is based on the gravimetrical measuring procedure with flying-start-and-stop operating mode. Particular attention has been paid to the challenges of temperature stability during the measurements since the temperature has a direct influence on the viscosity and flow rate of the test medium. For that reason the pipes on the high-pressure side are double-walled and insulated and the device under test (DUT) has an enclosure with a separate temperature control. From the analysis of the first measurement with tap water at a temperature of 20 °C and a pressure of 82.7 MPa an extensive uncertainty analysis has been carried out. It was found that the diverter (mainly due to its asymmetric behaviour) is the largest influence factor on the total uncertainty budget. After a number of improvements, especially concerning the diverter, the flow test rig has currently an expanded measurement uncertainty of around 1.0% in the lower flow rate range (25 g min⁻¹) and 0.25% in the higher flow rate range (400 g min⁻¹) for the measurement of mass flow. Additional calibration measurements with the new, redesigned flow test rig and highly viscous base oils also indicated a good agreement with the theoretical behaviour of the flow meter according to the manufacturers׳ specifications with water as test medium. Further improvements are envisaged in the future in order to focus also on other areas of interest.     

Micro-flow facility for traceability in steady and pulsating flow

This paper describes a primary standard for liquid micro-flow, which covers the flow rate range from 1 ml/min down to 100 nl/min with uncertainties in the range from 0.1% to 0.6% (coverage factor 95%). To realize stable flow rates, METAS applies the principle of generating flow by means of a constant pressure drop over a capillary tube according to the law of Hagen–Poiseuille. The constant pressure drop is mainly possible due to the fact that the relative pressure at the outlet needle remains constant as the outlet needle is positioned just above the beaker collecting the water. The special beaker and the adjustments for the weighing zone to control evaporation will be discussed in the paper as well as measurement results from flow sensors and flow generators, which highlight the repeatability and the reproducibility of the facility.     

Development and evaluation of the calibration facility for high-pressure hydrogen gas flow meters

The calibration facility with the multi-nozzle calibrator was developed for the calibration of flow meters to be used with high-pressure, high-flow-rate hydrogen gas. The critical nozzles installed in the multi-nozzle calibrator were calibrated with traceability to the national standard. The relative standard uncertainty of the mass flow rates produced from the calibration facility is 0.09% when the flow rate is between 150 g/min and 550 g/min. In this study, the Coriolis flow meter was calibrated for a pressure range of 15–35 MPa. The relative standard uncertainty of the flow rates obtained from the Coriolis flow meter was 0.44% for the case of the worst fluctuations in the output of the flow meter; based on the calibration curve, this is 0.91%. The present result shows that there is a maximum 3% difference between the output of the Coriolis flow meter and the mass flow rates of the multi-nozzle calibrator, even though the Coriolis flow meter was calibrated using water. Therefore, for the development of a calibration facility that can calibrate a flow meter under the same conditions as those encountered in actual use, it will be important to develop a new flow meter.     

A new approach to detection of vortices using ultrasound

The measurement principle of vortex flowmeter is based on von Karman vortex shedding phenomenon. Frequency of vortices, behind the bluff body, is proportional to the mean flow velocity. There are different ways of detection of vortices, and different sensors are used (presser sensors, capacitive sensors, thermo-resistance sensors, ultrasonic sensors, etc.). Proposed method to vortex identification, presented in this paper is based on simultaneous detection of pair of vortices with opposite circulation, by means of two pairs of ultrasonic transducers. A beam of ultrasound, from ultrasonic transmitter to ultrasonic receiver is transmitted perpendicularly to the vortex street. The received ultrasonic signal is amplitude and phase modulated. Frequency of demodulated signal is equal to the frequency of vortices. This technique allows a number of advantages comparing to conventional solutions: reduction, or elimination of noises caused by installation vibration and disturbances in the flow, higher sensor sensitivity, which as a result leads to a possibility of a reduction of the bluff body size, i.e. reduction of the pressure drop on the flow meter, increase of the measurement range in the low flow region, the possibility of redundant operation of the flow meter, reduced measurement uncertainty, instrument technology improvements, improved reliability of the instrument, assured improved statement of complete uncertainty contributions, improved metrology of the equipment as such and calibration procedures that contribute to measuring capabilities etc. For experimental testing a prototype vortex flowmeter of a nominal inner diameter (ID) 50 mm is developed. A cylindrical bluff body for vortex shedding is used. Ultrasonic transducers based on piezo-crystal PZT-5A, inserted in the wall of the vortex meter casing are utilized. The testing of prototype ultrasonic vortex flowmeter is realized on the calibration station on the water. The results at the testing point to the possibility of measuring flow of liquid fluids at velocities less than 0.5 m/s, with an uncertainty better than ±1%.     

Performance evaluation of a cheap,open source,digital environmental monitor based on the Raspberry Pi

We report on the design, construction and evaluation of a low-cost digital environmental monitoring system based on a popular micro-computer board and mass market digital sensors. The system is based around the use of open source software and readily available digital sensors, providing key parameters required for environmentally-controlled calibration laboratories: air temperature, pressure and humidity. Each system logs data at set intervals with front-panel display, web page graphical display and email alerting when exceeding set tolerances. The sensors have been calibrated at the National Physical Laboratory using standards traceable to the SI. Long term stability of the system is estimated and in addition to monitoring of laboratory environments for regulatory purposes, the systems can also be used to provide on-demand values for local refractive index with an expanded (k = 2) uncertainty of 1.1 × 10⁻⁷ as required for many optical-based measuring systems.     

Uncertainty analysis for a phase-detector based phase noise measurement system

Phase noise is an important parameter to characterise the frequency stability of oscillators and synthesised signal generators. Accurate measurement of phase noise is required for various applications in radar, communication and navigation systems. A single-channel phase-detector based phase noise measurement system is described. The system’s measurement errors and uncertainties have been analysed in details. The expanded uncertainty is about 2.7 dB for calibrating phase noise of a signal generator at 0.001–1.6 GHz for frequency offsets from 1 Hz to 100 kHz. The uncertainty budget for measuring a signal generator’s phase noise at 640 MHz is also presented.     

Calibration procedures and uncertainty analysis for a thermal mass gas flowmeter of a new generation

This paper deals with the differences between traditional and new technology gas meters, and focuses specifically on the calibration procedure and uncertainty evaluation of CTTMFs (Capillary Type Thermal Mass Flow Meter). In particular, measurements performed on a sample set of commercial CTTMFs for natural gas in domestic/residential (G4) applications allowed to evaluate the modifications to calibration procedures required by the new generation, digital, gas flow meters. Indeed, traditionally natural gas is metered by means of volumetric measurement techniques, while the modern, static gas flow meters (thermal and ultrasonic ones) are based on electronic flow sensors. This implies that the gas volume through the meter is measured by sampling the flow rate at selected time points and integrating the flow rate in time. The measurement time becomes therefore an important parameter, thus requiring a thorough rethinking of the calibration procedure. In order to analyse the effects of the various parameters, a series of ad-hoc calibrations were performed. Specifically, one set of calibrations was performed with constant totalized volume, while the other required a constant measurement time. In order to highlight the novelties that will have to be implemented in ordinary calibration procedures to get the best of the new technologies, the two procedures as performed on a sample set CTTMFs will be compared; the theoretical (generic) evaluation of the associated uncertainty will also be presented. Measurements were carried out at the test facility of INRIM, the Italian National Metrology Institute.     

Improved critical flow factors and representative equations for four calibration gases

The critical flow nozzle is widely used to calibrate flowmeters in gas flow measurement. Its use requires the critical flow factor, C*, a parameter dependent upon the thermophysical properties of the gas at the nozzle throat, and the upstream temperature and pressure. This paper presents C* values for four calibration gases (air, argon, nitrogen and methane), calculated from the most recent reference quality equations of state, over a wider range of temperature and pressure than previously available, 200–600 K and up to 20 MPa. In addition, a new empirical equation has been developed to represent the calculated values accurately, thus eliminating the need for complex calculations or interpolations from tables.     

Constant pressure primary flow standard for gas flows from 0.01 cm3/min to 100 cm3/min (0.007–74 μmol/s)

We describe a flow standard for gas flows in the range from 0.01 sccm to 100 sccm with a relative standard uncertainty (68% confidence) of 0.03% at 1 sccm (1 sccm≡1 cm³/min of an ideal gas at 101325 Pa and 0 °C ≈ 0.74358 μmol/s). The flow standard calibrates a secondary meter by withdrawing a piston from a cylinder held at constant pressure P while gas flows from the secondary meter into the cylinder. The flow standard can operate anywhere in the range 10 kPa<P<300 kPa, and it can act as a flow source as well as a flow receiver. The flow standard incorporated features that improved its convenience and lowered its cost without sacrificing accuracy, specifically (1) dry sliding seals made with commercially available, easily replaced, o-rings, (2) a compact design based on a commercially available, hollow piston, and (3) a linear encoder with a small Abbe error.     

Reaffirmation of measurement uncertainty in pressure sensitivity determination of LS2P microphones by reciprocity method

The paper presents the accuracy and precision associated with realization of primary standard of sound using the reciprocity method. An experimental determination of the front cavity volume on Universal Measuring Machine has lead to reaffirmation of measurement uncertainty in pressure sensitivity determination to 0.04–0.15 dB in frequency range 31.5 Hz to 25 kHz. The reduced measurement uncertainty has also been validated from the results of the recent APMP Key comparison and also by comparison to the manufacturer’s value for LS2P microphones. The use of optical method for measuring the front cavity volume has refined the measurement methodology followed with adaptation of a self reliant, traceable and systematic measurement procedure in comparison to the earlier use of nominal values for sensitivity fitting exercise conducted on MP.EXE program. Consequently, the measurement uncertainty associated with the calibration of working standard microphones, multifunction acoustic calibrator and A-weighted sound pressure level measurements is also reduced.     

Coriolis mass flow metering for three-phase flow: A case study

Previous work has described the use of Coriolis mass flow metering for two-phase (gas/liquid) flow. As the Coriolis meter provides both mass flow and density measurements, it is possible to resolve the mass flows of the gas and liquid in a two-phase mixture if their respective densities are known. To apply Coriolis metering to a three-phase (oil/water/gas) mixture, an additional measurement is required. In the work described in this paper, a water cut meter is used to indicate what proportion of the liquid flow is water. This provides sufficient information to calculate the mass flows of the water, oil and gas components. This paper is believed to be the first to detail an implementation of three-phase flow metering using Coriolis technology where phase separation is not applied.Trials have taken place at the UK National Flow Standards Laboratory three-phase facility, on a commercial three-phase meter based on the Coriolis meter/ water cut measurement principle. For the 50 mm metering system, the total liquid flow rate ranged from 2.4 kg/s up to 11 kg/s, the water cut ranged from 0% to 100%, and the gas volume fraction (GVF) from 0 to 50%. In a formally observed trial, 75 test points were taken at a temperature of approximately 40 °C and with a skid inlet pressure of approximately 350 kPa. Over 95% of the test results fell within the desired specification, defined as follows: the total (oil+water) liquid mass flow error should fall within ±2.5%, and the gas mass flow error within ±5.0%. The oil mass flow error limit is ±6.0% for water cuts less than 70%, while for water cuts between 70% and 95% the oil mass flow error limit is ±15.0%.These results demonstrate the potential for using Coriolis mass flow metering combined with water cut metering for three-phase (oil/water/gas) measurement.     

Approach to monitor large two-phase LNG flows

A new version of a system to monitor the average void fraction, φ, the quality, x, and the mass flow rate, G, of two-phase liquefied natural gas (LNG) flows is offered. It is based on a combination of a gamma-densitometer with a Cs-137 radioactive source and a narrowing device. The metrological characteristics of this system are estimated and its practical realization is substantiated. A model of ID =100 mm has been manufactured and tested at the State Primary Special Standard of the Unit of Mass Flow Rate of Gas-Liquid Mixtures GET 195-2011 (Kazan, Russia) with simulated two-phase flows “Exxsol – compressed air”. The offered system can be used for pipelines up to ID =500 mm by applying a gamma source with the necessary activity. The experiments on GET 195–2011 have shown that the void fraction error and the relative mass flow rate error for Exxsol do not exceed 5% and 2%, respectively, at φ<50%. It appears to be suitable for practical application. If one adds a second gamma-source (for example, Am-241) to the proposed system, it can serve as a separationless three-phase flow-meter for mixtures “oil-gas-formation water”.     

Design,development and metrological characterization of a low capacity precision industrial force transducer

The paper discusses the development of the ring shaped force transducers for measurement of force in lower capacity to meet the industrial requirements with the increasing technological developments. A 50 N ring shaped force transducer for tension mode has been developed by studying the analytical and computational methods. The force transducer developed has been metrologically studied according to the calibration procedure based on the standard ISO 376 and uncertainty of measurement of the force transducer is found to be±0.10% (k=2), while taking into account the relative uncertainty contribution due to necessary factors like repeatability, reproducibility, zero offset, interpolation, resolution and reversibility. The force transducer developed may further be studied for improvement of metrological performance and may suitably be developed for other lower capacities like 10 N, 20 N etc. The force transducer developed offers very economical alternative of complex shaped force transducers with simple design and manufacturing features. The force transducer developed may be proved very helpful in providing traceability to the user industries and calibration laboratories in the lower range of force measurement and serve as force transfer standard.     

Design and experimental validation of an ultra-precision Abbe-compliant linear encoder-based position measurement system

The design and development of an Abbe-compliant linear encoder-based measurement system for position measurement with a targeted 20 nm uncertainty (k = 2) in machine tools and CMMs is presented. It consists of a linear scale and a capacitive sensor, mounted in line on an interface which is guided in the scale's measurement direction and driven by a linear motor based on the output signal of the capacitive sensor. The capacitive sensor measures the displacement of a target surface on the workpiece table. The functional point, which is the center of a tool or touch probe, is always aligned with the scale and capacitive sensor such that this configuration is compliant with the Abbe principle. Thermal stability is achieved by the application of a thermal center between the scale and capacitive sensor at the tip of the latter, which prevents both components to drift apart. Based on this concept, a prototype of a one-DOF measurement system was developed for a measurement range of 120 mm, together with an experimental setup aimed at verifying the reproducibility of the system for changing ambient conditions of ±0.5 °C and ±5%rh and the repeatability during tracking of a target surface over a short period of time. These experiments have shown that the measurement uncertainty of the one-DOF system is below 29 nm with a 95% confidence level.     

Research on signal amplitude of the Kármán vortex street in gas–liquid two-phase flow with high void fraction

Based on Biot–Savart law and single-phase flow Kármán vortex characteristics, flow field has been analyzed when gas–liquid flow past a fixed bluff body with high void fraction. Vortex signal characteristics have been studied for stratified two-phase flow on atmospheric conditions in a horizontal pipe. To discuss the relation between void fraction and vortex signal amplitude spectrum, this paper sets up the vortex-induced pressure field model for gas–liquid two-phase flow and gives the relationship between void fraction and relative amplitude spectrum of two-phase flow to single-phase flow. An algorithm is proposed for predicting the two-phase flow parameters. Experiments were performed using air–water as working fluid along with a test tube diameter of 50 mm, at gas volume flow rate of 20–68 m³/h, and void fraction of 0.9–1. The results indicate that calculations by the vortex-induced pressure field model on the amplitude spectrum of vortex signal are in good agreement with the experimental data, and relative errors of the algorithm predictions on gas volume flow rate and liquid volume flow rate are 0.08 and 0.56, respectively.     

High-accuracy displacement metrology and control using a dual Fabry-Perot cavity with an optical frequency comb generator

A displacement metrology and control system using an optical frequency comb generator and a dual Fabry-Perot cavity is developed with sub-nm accuracy. The optical frequency comb generator has expanded the displacement measurement range and the dual cavity system has suppressed the environmental fluctuation. We evaluated the absolute uncertainty of the developed displacement measurement system to be approximately 190 pm for the displacement of 14 μm and the accurate displacement control using a phase-locked loop was demonstrated with a resolution of approximately 24 pm.     

Development of a gas micro-flow transfer standard

LNE has ability to calibrate gas micro-flow rates using the dilution method for nitrogen flow rates in the range from 2 µg/s to 200 µg/s or helium ones in the range 0.75–30 µg/s. In addition, a primary constant pressure flowmeter for leak rate measurements from 0.05 µg/s to 35 µg/s is also available. This equipment will be used to validate the dilution method below 30 µg/s. In order to compare these reference facilities, LNE is developing a micro-flow transfer standard (µFTS) in collaboration with ATEQ France, a manufacturer of control equipment for leak testing. The flowmeter consists mainly of an array of three stainless steel capillaries designed to cover the ranges from 0.035 µg/s to 0.35 µg/s, 0.35 µg/s to 3.5 µg/s and 3.5 µg/s to 35 µg/s for nitrogen (0.1–100 ml/h). A dynamic model of the µFTS determines the mass flow rate from the input pressure, the differential pressure of the capillary, the gas temperature, viscosity and density and the length and radius of the capillary. A comparison of both reference methods is carried out with the µFTS from 0.35 µg/s to 35 µg/s.     

Fast Coriolis mass flow metering for monitoring diesel fuel injection

Prism signal processing is a new recursive FIR technique that facilitates the rapid tracking of sinusoidal signals, such as those used in a Coriolis Mass Flow Meter (CMFM). A Prism-based CMFM prototype has been developed using a commercial flowtube and a dual ARM processor-based transmitter, which is capable of generating flow measurement updates at 48 kHz. This has been applied in a feasibility study to the tracking of fast (e.g. 1.5 ms) injections of diesel fuel on a laboratory rig at engine speeds of up to 4000 rpm equivalent and at fuel pressures of up to 100 MPa. Due to the high level of vibration in the system, Prism-based notch filtering is used to suppress undesired modes of flowtube vibration in the sensor signal. Individual flow pulses can be detected by the system, but the relatively long period of oscillation of the flowtube compared to the fuel injection duration results in a spreading out over time of each flow pulse measurement. More precise measurement results may be obtained using a higher frequency resonant flowtube.