Physikalisch-Technische Bundesanstalt

The Working Group "Traceability of Liquid Flow Measurement" deals with all fundamental topics in the field of liquid volume and liquid flow measurement. This includes the fundamental and conceptional questions as to how to realise the corresponding units and how to disseminate them, as well as the global issue of the appurtenant considerations with regard to measurement uncertainties. The task area of this working group can be summarised by the definition "ensuring that all liquid flow measurements are traceable to the national standard".

Specific characteristics for realisation and dissemination in the field of liquid flow measurement

It is in general true that, for ensuring uniform and correct measuring, the measurement must be traced back to a national or international standard for the corresponding measurand. This means that the reading of the measuring instrument used can be compared with the primary standard which stands at the top of the corresponding calibration hierarchy in an uninterrupted chain of successive comparisons.

In the field of liquid flow measurements, PTB has a series of standard measuring facilities at its disposal whose core is the hydrodynamic test field (HTF). The following characteristics, which are specific to the field of volume and flow measurement of liquids, have to be pointed out:

• In total, four measurands occur – volume, mass, volume flowrate and mass flowrate – whereby both volume-flow- and mass-flow-related measurands, within the traceability process, may be converted into one another by incorporating the measurand "density".

• Before the HTF was commissioned for liquid flow measurands in 2003, there was no uninterrupted chain of traceability to a national standard available. Each measuring facility used for testing and calibration was traced back to the required standards of mass, length, density, temperature and time, element by element, according to its active principle of measurement (volumetric or gravimetric). In such a case, the measured value which is defined to be the stimulating reference input to the meter under test is "statically" composed of the corresponding above-mentioned individual measurands. Since such a traceability is not based upon the measurand to be determined, influences on the measurement result which originate from the dynamics of the measuring process are not suitably taken into account. Among these are, for example, disturbed velocity profiles or high degrees of turbulence of the flow at the place of installation of the meter under test, fluctuating fluid flows during the measurements, instable operation of flow control devices, effects due to pump vibrations, to mention only some.

• All methods which have been known up to now and on which liquid flow measurements are based display, besides the fact that they can be dynamically influenced, also a - sometimes very strong - dependence on the material parameters of the liquid to be measured. It is therefore mandatory to test or calibrate each measuring instrument solely with the fluid it is dedicated to.  This means that, for each metrologically relevant liquid, it would be necessary to establish a specific traceability chain, which, for economic as well as for metrological reasons, does not make sense.

Establishing a uniform system for the traceability of all liquid flow measurements to the national standard – the Hydrodynamic Test Field (HTF)

PTB has therefore elaborated an alternative, long-term concept. The plan is that all liquid measurements can be traced back to one single national standard – the HTF – by means of medium-independent transfer standards and methods which still have to be developed. In this way, the measurands which can be realised with the HTF with the fluid "water" can be disseminated to all subordinate testing and calibration facilities by means of such transfer standards – independent of the liquid to be measured with which these facilities work. Figure 1 shows this concept.

Figure 1:  Long-term concept to create a single, closed system of traceability of all liquid flow measurements to the Hydrodynamic Test Field

The Hydrodynamic Test Field (HTF)

The hydrodynamic test field realises all four measurands which are relevant for the total flow and flowrate measurement of liquids: mass, volume, mass flowrate and volume flowrate of a streaming liquid. It is operated with the fluid "water" in a flowrate range from 0.3 m³/h to 2100 m³/h and with an expanded relative measurement uncertainty of 0.02 %.

The core component of the HTF is a gravimetric reference standard which consists of three weighing systems with a 30-ton, a 3-ton and a 300-kilogram balance. The mass which is determined in this way – if necessary using the density of the fluid to be measured, as well as the measuring time – is compared with the readings of the meter under test. The following photograph shows a part of the measuring hall in which the HTF is located. In the foreground, the two measuring sections are to be seen, each of which can be operated alternatively and in which flowmeters can be mounted with nominal diameters between 20 mm and 400 mm. In the background, one can see two of the three weighing systems – the 30-ton balance and the 3-ton balance.

The HTF's three weighing systems basically work according to the same operating principle: they combine a "classic" lever scale with an electromagnetic force-compensating load cell and strain-gauge force transducers as sensor elements. In addition, each weighing system is equipped with an integrated calibration facility. In order to isolate them from dynamic disturbances, the weighing systems were set up on a vibration-isolated concrete foundation.

Flow is generated and stabilised by a system of electronically regulated pumps and a constant-head tank with an overflow weir. The constant-head tank has a capacity of 6 m³, is located 35 m above ground level and serves, in addition, as a high-precision flow stabiliser.

The measuring and operating mode in which the highest calibration accuracy is achieved – and which is utilised at PTB's measuring facility – is static weighing with flying-start-and-finish operation. In this operation mode, the test liquid is kept in a constant circulating motion through the system's pipework prior to and during a calibration run. The actual measuring operation is started by switching the diverter's diverting edge from the bypass position into the diverting position in which the liquid flow is directed towards the weighing tank. When passing the centre of the diverter's liquid jet, a gate signal is initiated that launches data acquisition from the test meter's signal output and starts an electronic counter which determines the diversion time. Upon reaching a pre-defined quantity of water in the weighing tank, the diverter is switched back into its initial position. And when passing the jet's centre again, the signal acquisition of the test meter and the time measurement are stopped.

From the very beginning, all components of the test field were designed and built in such a way that their individual uncertainty components realise pre-defined projected values which, in turn, ensure for the total expanded measurement uncertainty of the facility a value which is better than the above-mentioned value of 0.02 %. When a flowmeter with a pulse frequency signal output f (e.g. a turbine meter, an electromagnetic or Coriolis flowmeter) is calibrated, the equations for the determination of the standard measurement uncertainty of the meter K-factor K_meter have the following form:

where u_{K_meter}     is the standard measurement uncertainty of the measuring instrument's K-factor:

u_f              is the standard measurement uncertainty of the measurement of the pulse signal frequency f

u_m            is the standard measurement uncertainty of the determination of the mass m of the liquid to be measured

u_ρ             is the standard measurement uncertainty of the determination of the density ρ of the test liquid

u_ΔV           is the standard measurement uncertainty of the determination of the connecting pipe's volume

ΔV            (pipe section between the meter under test and the standard device)

u_{T_div}       is the standard measurement uncertainty of the diverter's time error

u_T             is the standard measurement uncertainty of the determination of the measuring time T_MEAS

On the basis of this decisive criterion for the design of this water flow facility, all constructive, functional and uncertainty-determining metrological requirements have been correspondingly derived.

Furthermore, for the function of the entire flow standard facility, the optimal functional integration of all individual components into the overall system of the measuring installation is an indispensable precondition. This was realised by correspondingly designing the process control system of the entire facility. For the weighing system, e.g., this means that also the measurement values of ambient temperature, atmospheric pressure and relative humidity required for the correction of the air buoyancy are acquired and made available to the supervising process control system for further processing of the measurement analysing and processing of the measurement data. Furthermore, the balance calibrations, which are performed each time a high-precision flow calibration is carried out, also provide data for the purposes of a qualified quality management. Thus, it can be continuously checked whether the limit values stated as well as their long-term stability are complied with. On the basis of the regularly recorded data and their permanent surveillance, it is possible to detect deviations from the "normal" functionality in due time and to take, if necessary, maintenance or repair measures. Only this can reliably and credibly guarantee the measurement uncertainty of the hydrodynamic test field for each of the measurements performed.

It should be especially pointed out that also internationally, the high metrological level of the HTF is highly acknowledged and was clearly proved by the BIPM key comparison which was completed in 2006.