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Gravimetric Flow Meter Proving: Building Automated Weigh Scale Benches for Factory Calibrations
Quick Answer: Gravimetric proving on an automated weigh scale bench is the most repeatable factory method for calibrating mass flow meters and positive displacement meters. Silver Automation Instruments uses this method for Coriolis meters from DN10 to DN150 and oval gear meters from DN10 to DN80. The bench returns traceable calibration certificates with mass uncertainty better than 0.05 percent of reading.
Why Gravimetric Proving Matters for Factory Calibration
Factory calibration is only as good as the reference. A weigh scale bench reads mass directly. There is no secondary flow computer. There is no master meter drift. There is no density estimate. That is why gravimetric proving is the base reference for many ISO and OIML procedures.
In practice, many engineers assume a master meter is enough. Here is the thing. A master meter needs its own traceable reference. Without gravimetric proving, you compare a meter to another meter and hope the second one did not drift.
Silver Instruments has seen this on customer sites many times. A water plant in the Philippines ran a master flow meter for two years without recalibration. The master meter had shifted by 1.2 percent. Every field meter calibrated against it inherited the same offset. A gravimetric bench would have caught the drift at the first weighing cycle.
How an Automated Weigh Scale Bench Works
A gravimetric bench collects fluid from the meter under test into a weigh tank. The control system starts the collection at a stable flow. It stops collection at a set mass target. The scale reports the mass collected. The flow computer compares that mass to the meter pulse output. The difference is the meter error.
Automation matters more than people expect. Manual weigh testing has timing scatter, parallel path errors, and valve leaks. An automated bench controls valve sequence, line pressure, pump speed, and temperature. That gives repeatability in the range of 0.02 percent to 0.05 percent of reading in a controlled bay.
We use PT100 sensors for fluid temperature and 4-20 mA HART pressure transmitters on the bench. The flow computer logs every cycle. It corrects volume to mass based on water or oil density tables. For Coriolis meters, the output is already mass flow. For oval gear meters, the bench converts the collected mass back to volume using density at the measured temperature.
Key Components of a Factory Weigh Scale Bench
The bench is not only a scale. A proper automated rig includes a diverter valve, a constant head tank or pump skid, a weigh tank with load cells, PT100 temperature probes, pressure transmitters, and a flow computer. The diverter valve is the most critical part. A slow valve adds mass error during the start and stop cycle. We specify fast actuation valves with a repeatable open to close time below 80 ms for small line sizes.
Load cell selection depends on the target flow range. A DN15 meter bench may use a 30 kg scale. A DN150 line may need a 2,000 kg scale. The scale is calibrated with certified weights before every production batch. This is not optional in our Nanjing calibration bay.
Calibration Sequence for Coriolis and Oval Gear Meters
A Coriolis meter calibration starts with a warm-up run. Water or oil runs through the meter for 15 minutes at the expected process temperature. Then the bench runs five collection cycles. The first cycle is a repeatability check. Three middle cycles are used for the calibration factor. The last cycle verifies the new factor. The same sequence applies to oval gear meters, but we use a light oil with known density instead of water when the meter will run on diesel or gear oil.
For a paint manufacturer in Southeast Asia, we ran a DN25 Coriolis meter from 20 kg/h to 2,000 kg/h. The factory standard was 0.2 percent of rate. The bench result came in at 0.11 percent of rate after a five point adjustment. That is the kind of data you can put in a calibration certificate without hesitation.
For an oval gear meter on a diesel transfer skid in Kenya, the bench test used lo
Real Numbers from Silver Instruments Calibration Bays
Our automated weigh scale benches run in Nanjing. We currently operate three bays for small line sizes from DN10 to DN50 and one bay for DN80 to DN150. The small bench covers flow ranges from 0.5 kg/h to 20,000 kg/h depending on the meter type. The large bench handles flows up to 300 t/h for water and 120 t/h for oil.
Reference uncertainty stays below 0.05 percent of reading for mass. Temperature correction uses PT100 probes with a standard accuracy of 0.1 degree C. Pressure is measured with 4-20 mA HART transmitters with 0.075 percent span accuracy. The flow computer samples at 100 Hz to catch fast valve events.
We have seen many customer cases where factory gravimetric proving reduced startup time. A chemical batch plant in Ecuador avoided two weeks of field commissioning because the Coriolis meters arrived with traceable factory certificates and verified factor curves.
Recommended Meter Pairings for Your Application
For chemical batching and high viscosity fluids, Silver Instruments specifies a Coriolis mass flow meter. A DN25 SIL-CMF025 is a common fit for 20 kg/h to 2,000 kg/h in paints and resins. The same series covers DN10 to DN150. Factory gravimetric proving is included when the process data is provided before production.
For diesel, lube oil, and low flow fuel transfer, an oval gear meter is more practical. A DN20 SIL-OG020 works in the 50 L/h to 600 L/h range. We calibrate it on a gravimetric bench with light oil and adjust the factor at five flow points. The certificate shows density, temperature, flow points, and final error.
For water and wastewater lines, electromagnetic meters still need repeatable factory verification. We run gravimetric checks on DN50 to DN150 electromagnetic flow meters before leaving the plant. The bench data confirms the meter is within 0.2 percent of reading from 0.5 m/s to 10 m/s.
Five Questions on Gravimetric Flow Meter Proving
Q: Why use a gravimetric weigh scale bench instead of a master meter?
A: A weigh scale bench reads mass directly. A master meter reads flow and needs its own traceable reference. Gravimetric proving removes the master meter drift path and gives a primary reference result.
Q: Which flow meters should be calibrated on a weigh scale bench?
A: Mass flow meters and positive displacement meters benefit the most. Coriolis meters use mass output directly. Oval gear meters need a density correction from the bench. Electromagnetic flow meters can also be verified with gravimetric cycles.
Q: What data does a factory gravimetric calibration certificate show?
A: The certificate shows flow points, reference mass, meter output, density, temperature, pressure, final error, and the calibration factor. Silver Instruments adds the PT100 and scale identification numbers for traceability.
Q: How many flow points are required for a good calibration?
A: Five flow points is standard for Coriolis meters. More points give a better curve but increase bench time. For oval gear meters, five points from minimum to maximum rated flow is enough for most transfer applications.
Q: Can the weigh scale bench handle high viscosity fluids?
A: Yes. We use heated oil with viscosity up to 1,000 cP for select bench runs. The plumbing and pump system are sized for the pressure drop at temperature. A separate gear pump skid is used for high viscosity test fluids.
Request a Factory Calibration Quote
Send us the pipe size in DN, flow range in kg/h or L/h, fluid name, viscosity in cP, and required accuracy. We will return a quote with the calibration certificate data and lead time.
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