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ASME MFC-9M-1988 (2011) — Measurement of Liquid Flow in Closed Conduits by Weighing Method
⚖️ ASME MFC-9M-1988 (2011) (originally ASME/ANSI MFC-9M-1988) is the American National Standard for the measurement of liquid flow in closed conduits by the weighing method. It was prepared by the ASME Committee on Measurement of Fluid Flow in Closed Conduits (MFC), approved by the American National Standards Institute (ANSI) on December 15, 1988, and reaffirmed in 2011. The Standard is based on and closely parallels ISO 4185-1980, incorporating U.S. practices and terminology where they differ. The weighing method is the highest-accuracy, directly traceable primary method of flow measurement, referenced to the fundamental quantities of mass and time. It is widely used for meter calibration, water turbine and pump performance tests, and for establishing and comparing liquid flow calibration standards.
💡 The essence of the weighing method: flow = mass ÷ time. It converts “measuring flow” into two more fundamental, more accurate measurements — weighing and timing — making it the reference basis for calibrating other flowmeters.
1. Scope and Principles of the Standard 📌
According to Section 1 (General), the scope and basic principles are:
- Method: determining liquid flow in closed conduits by measuring the mass of liquid delivered into a weighing tank in a known time interval.
- Applicable media: in principle any liquid, provided its vapor pressure and operating conditions permit; for high-vapor-pressure or volatile liquids, anti-vaporization measures are required.
- Content: measuring apparatus, procedure, and method for calculating flow rate and the associated uncertainties.
- Principle: the liquid passes through a diverter into the weighing tank; measuring the collected mass M and the collection time t gives mass flow rate qm = M/t directly; volume flow rate follows by dividing by the liquid density.
⚠️ Scope limitation: the weighing method of this Standard is intended for liquids. If the vapor pressure is so high that vaporization occurs in the pipe or tank, or the liquid is a multi-phase flow (with entrained gas), the weighing result develops significant error and must be evaluated separately.
2. Measuring Apparatus and Requirements 🔧
Section 3 (Apparatus) sets out requirements for each component of the measuring installation:
| Apparatus | Function and Key Requirements | Impact |
|---|---|---|
| Diverter | Rapidly switches between “return to bypass” and “into the weighing tank”; switching must be symmetrical in both directions and the timing determined | Diverter timing error directly affects start/stop of timing — the critical component for high accuracy |
| Time-Measuring Apparatus | Records the instants of diverter switching; accuracy and resolution must match the target uncertainty | Timing error enters flow uncertainty proportionally |
| Weighing Tank | Capacity sized for collection time and expected flow; must hold the full collection without overflow | An undersized tank forces multiple collections, increasing operational error |
| Weighing Device | Balance or load cell with adequate range, sensitivity, and linearity; regularly calibrated | Weighing error is the fundamental uncertainty source of the mass measurement |
| Auxiliary Measurements | Liquid temperature and density measurement (for volume flow); pressure if needed | Density error directly affects the volume flow result |
🚨 The diverter is the most critical component of a weighing installation: if its switching is asymmetric in the two directions or the switching time is uncertain, the start/stop instants of collection cannot be defined accurately. Even with accurate weighing and timing, the flow result may still carry a systematic bias.
3. Procedure: Static and Dynamic Weighing 📊
Section 4 (Procedure) specifies two basic measurement procedures:
- Static Weighing Method: the weighing tank is stationary during collection; after collection ends and the tank is still with a settled liquid surface, the mass is measured. The tank and the weighing device are decoupled during flow, so flow continues through the bypass while weighing; accuracy is highest, suited to laboratory precision measurements. Typical arrangements include static installations supplied by a constant-level head tank and static installations with direct pumping supply.
- Dynamic Weighing Method: the tank is weighed continuously while liquid flows in, deriving the flow rate from the rate of change of the weighing signal. This suits larger flows or continuous recording, but is affected by liquid impact, sloshing, and the dynamic response of the weighing device, so its uncertainty is generally somewhat larger than the static method.
Both methods must follow the common provisions: no leakage from the tank or connecting lines; consistent draining and wetting before collection; stable supply during measurement (e.g., constant-level head tank); and synchronized recording of diverter opening/closing timing and weighing readings.
💡 Selection advice: for maximum accuracy (meter calibration, primary standard installations), prefer the static weighing method; for large-flow online or continuous measurement, use the dynamic method and compensate the uncertainty for impact and sloshing.
4. Flow Calculation and Uncertainty 📈
Section 5 (Calculation of Flow Rate) specifies the flow calculation:
qm = M / t
where qm is the mass flow rate and M is the net liquid mass collected in the timing interval t. The volume flow rate is:
qv = qm / ρ
where ρ is the liquid density at the collection temperature (density of pure water is given in Appendix B). Section 6 (Uncertainties in the Measurement of Flow Rate) requires identifying, combining, and reporting the uncertainty components of the weighing, timing, density (where used), and diverter switching time, and giving the overall uncertainty of the flow result. Appendix A provides “Corrections on the Measurement of Filling Time” to handle systematic bias at the diverter/timing interface.
| Uncertainty Source | Typical Components | Control Measures |
|---|---|---|
| Mass measurement | Balance calibration, reading, tare and residue | Periodic calibration, standardized taring and rinsing procedure |
| Time measurement | Timer resolution, start/stop synchronization, diverter asymmetry | High-resolution timer, diverter symmetry check |
| Density | Temperature measurement error, density table deviation | Precision thermometer, density data per Appendix B |
| Installation and procedure | Leakage, vaporization, tank overflow, supply fluctuation | Tightness checks, constant-level supply, anti-vaporization measures |
⚠️ Uncertainty evaluation is the precondition of any accuracy claim: only after identifying and combining all components per the Standard may the installation be labeled with its accuracy. An unevaluated weighing installation should not claim “primary accuracy.”
5. Engineering Practice Essentials 🛠️
- 🔍 Calibration and traceability: weighing and timing devices should be traceable to national metrology standards (mass standards, time/frequency standards) with maintained calibration records.
- 🔧 Supply stability: use a high constant-level head tank or a regulated pumping station to reduce flow fluctuation effects on the mass/time match.
- 📊 Diverter checks: perform periodic diverter symmetry tests (Appendix A provides the method for plotting the timer actuator curve) to ensure equal switching time in both directions.
- 📋 Records and reporting: reports should include the medium, temperature, density, collection time, net mass, calculated flow, and expanded uncertainty, so the measurement is reproducible and auditable.
✅ A weighing installation can serve as a laboratory “standard flow source”: calibrate turbine, electromagnetic, and ultrasonic meters against it in real flow; the uncertainty of the calibration result is determined jointly by the installation’s uncertainty and the comparison process.
6. Frequently Asked Questions ❓
❓ Q: How does the weighing method relate to ISO 4185?
A: MFC-9M-1988 is based on and closely parallels ISO 4185-1980, localized with U.S. practices and terminology (unit systems, installation conventions); the methodology is identical.
A: MFC-9M-1988 is based on and closely parallels ISO 4185-1980, localized with U.S. practices and terminology (unit systems, installation conventions); the methodology is identical.
❓ Q: Which is more accurate, static or dynamic weighing?
A: Usually the static method is more accurate because weighing occurs with the tank still and the surface settled, unaffected by flow impact; the dynamic method suits large flows and continuous measurement but must additionally evaluate dynamic-response uncertainty.
A: Usually the static method is more accurate because weighing occurs with the tank still and the surface settled, unaffected by flow impact; the dynamic method suits large flows and continuous measurement but must additionally evaluate dynamic-response uncertainty.
❓ Q: Why is diverter switching symmetry so important?
A: Flow = mass/time, and the start/stop of timing is defined by the diverter switching instants. If switching is asymmetric in the two directions, the “timed interval” disagrees with the actual collection interval, producing a systematic timing error that a more accurate timer cannot correct.
A: Flow = mass/time, and the start/stop of timing is defined by the diverter switching instants. If switching is asymmetric in the two directions, the “timed interval” disagrees with the actual collection interval, producing a systematic timing error that a more accurate timer cannot correct.
🔍 Q: Can this method be used for gas or steam?
A: This Standard is for liquids. Gases/steam are not suited to direct weighing collection (low density, high compressibility, difficult condensation); use other MFC-series standards (orifice plate, turbine, etc.) or dedicated gas methods instead.
A: This Standard is for liquids. Gases/steam are not suited to direct weighing collection (low density, high compressibility, difficult condensation); use other MFC-series standards (orifice plate, turbine, etc.) or dedicated gas methods instead.
