Calibration Flow Meter - Weighing Method (5)

International Standard 4185

Measurement of liquid flow in closed conduits -

Weighing method

First edition - 1980-12-15

Ref. No. ISO 4185-1980 (E)

Accuracy of the method

Overall uncertainty on the weighing  measurement

The weighing method gives an absolute measurement of flow which in principle requires only mass and time measurements. Provided that the precautions listed in 2.2.2 are taken, this method may be considered as one of the most accurate of all flow-rate measuring methods, and for this reason it is often used as a calibration method. When the installation is carefully constructed, maintained and used, an uncertainty of ± 0,1 % (with 95 % confidence limits for the random part of that uncertainty) tan be achieved.

 Requirements for accurate measurements

The weighing method gives an accurate measurement of flow rate provided that :

• there is no leak in the flow circuit and unmetere d leaka ge flow acro lss the diverter;
• there is no accumulation ior depletion) of liquid in a part of the circuit by thermal contraction (or expansion) and there is no accumulation (or depletion) by Change of vapour or gas volume contained unknowingly in the fiow circuit;
• necessary corrections for the influence of atmospheric buoyancy are made; this correction may be made when calibrating the weighing apparatus;
• the weighing machine, the timer and means for starting  and stopping it achieve the necessary accuracy;
• the time required by the diverter for traversing is small with respect to the filling time, the timer being started and stopped while the diverter is crossing the hydraulic centre line;
• in the case of the dynamic weighing me thod the effects of the dynamic phenomena are sufficiently small

Calibration Flow Meter - Weighing Method (4)

International Standard 4185

Measurement of liquid flow in closed conduits -

Weighing method

First edition - 1980-12-15

Ref. No. ISO 4185-1980 (E)

Comparison of instantaneous and mean of flow rate

lt should, however, be emphasized that only the mean value of flow-rate for the filling is given by the weighing method. Instantaneous values of flow-rate as obtained on another instrument or meter in the flow circuit tan be compared with the mean rate only if the flow is maintained stable during the measurement interval by a flow-stabilizing System, or if the  instantaneous values are properly time-averaged during the whole filling period.

Calibration Flow Meter - Weighing Method (3)

International Standard 4185

Measurement of liquid flow in closed conduits -

Weighing method

First edition - 1980-12-15

Ref. No. ISO 4185-1980 (E)

Dynamic weighing

The principle of the flow-rate measurement method by dynamic weighing (see figure 1D for a schematic diagram of a typical installation) is :

• to let the liquid collect in the tank to a predetermined initial mass, when the timer is then started;
• to stop the timer when a predetermined final mass of collected liquid is reached.

The flow-rate is then derived from the mass collected, the collection time and other data as discussed in clause 5 and annex A.

Figure 1D - Diagram of an installation for calibration by weighing

 (dynamic method, supply by a constant level head tank)

Calibration Flow Meter - Weighing Method (2)

Static weighing

The principle of the flow-rate measurement method by static weighing (for schematic diagrams of typical installations, see figures 1 A, 1 B, 1 C) is :

ü                     to determine the initial mass of the tank plus any residual liquid;

ü                to divert the flow into the weighing tank (until it is considered to contain a sufficient quantity to                   attain the desired accuracy) by Operation of the diverter, which actuates a timer to measure the filling time;

ü                   to determine the final mass of the tank plus the liquid collected in it.

The flow-rate is then derived from the mass collected, the collection time and other data as discussed in clause 5 and annex A.

Figure IA - Diagram of an installation for calibration by weighing (static method, supply by a constant level head tank)

Figure 1B - Diagram of an installation for flow-rate measure by weighing

 (used for an hydraulic machine test; static method, supply by a constant level head tank) 

Figure 1C - Diagram of an installation for calibration by weighing

(static method, direct pumping supply)

Calibration Flow Meter - Weighing Method (1)

International Standard 4185

Measurement of liquid flow in closed conduits -

Weighing method

First edition - 1980-12-15

Ref. No. ISO 4185-1980 (E)

Scope and field of application

This International Standard specifies a method of liquid flowrate measurement in closed conduits by measuring the mass of liquid delivered into a weighing tank in a known time interval. Lt deals in particular with the measuring apparatus, the procedure, the method for calculating the flow-rate and the uncertainties associated with the measurement.

The method described may be applied to any liquid provided that its vapour pressure is such that any escape of liquid from the weighing tank by vaporization is not sufficient to affect the required measurement accuracy. C!osed weighing tanks and their application to the flow measurement of liquids sf high vapour pressure are not considered in this International Standard.

This International Standard does not cover the cases of corrosive or toxic liquids. Theoretically, there is no limit to the application of this method which is used generally in fixed laboratory installations only. However, for economic reasons, usual hydraulic laboratories using this method tan produce flow-rates of 1.5 m³/s or less.

Owing to its high potential accuracy, this method is often used as a primary method for calibration of other methods or devices for mass flow-rate measurement or volume flow-rate measurement provided that the density of the liquid is known accurately. lt must be ensured that the Pipeline is running full with no air or vapour pockets present in the measuring section.

References

ISO 4006, Measurement of fluid flow in closed conduits -Vocabulary and symbols.

ISO 5168, Measurement of fluid flow - Estimation of uncertain ty of a flow-rate measurement.

OIML, Recommendations Nos. 1, 2, 3, 20, 28, 33.

Definitions

Only terms which are used in a special sense or the meaning of which merits restatement are defined below.

static weighing : The method in which the net mass of liquid collected is deduced from tare and gross weighings made respectively before and after the liquid has been diverted for a measured time interval into the weighing tank.

dynamic weighing : The method in which the net mass of liquid collected is deduced from weighings made while fluid flow is being delivered into the weighing tank. (A diverter is not required with this method.)

diverter : A device which diverts the flow either to the weighing tank or to its by-pass without changing the flow-rate during the measurement interval.

flow stabilizer : A structure forming part of the measuring System, ensuring a stable flow-rate in the conduit being supplied with liquid; for example, a constant level head tank, the level of liquid in which is controlled by a weir of sufficient length.

buoyancy correction : The correction to be made to the readings of a weighing machine to take account of the difference between the upward thrust exerted by the atmosphere, on the liquid being weighed and on the reference weights used during the calibration of the weighing machine.

Units

The units used in this International Standard are the SI units, metre, kilogram, and second; the degree Celsius is used for convenience instead sf the kelvin.

Certification

If the instaltations for flow-rate measurement by the weighing method are used for purposes of legal metrology, they should be certified and registered by the national metrology Service. Such installations are then subject to periodical inspection at stated intervals. If a national metrology Service does not exist, a certified record of the basic measurement Standards (weight and time), and error analysis in accordance with this lnternational Standard and ISO 5168, shall also constitute certification for legal metrology purposes.