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How to Calibrate a Mass Flow Meter: Gravimetric Weigh Scale Proving Calibration Procedures
Quick Answer: Gravimetric proving is the most direct way to check a mass flow meter. You collect fluid in a weigh vessel over a known period and compare the scale weight to the meter total. If the difference is outside your limit, zero the meter first, then repeat the test.
Silver Automation Instruments supplies Coriolis mass flow meters, oval gear flow meters, and thermal mass flow meters to plants in Southeast Asia, Oceania, the Middle East, Africa, and Latin America. The guide below explains the gravimetric weigh scale proving procedure we recommend for clean liquids, oils, solvents, and light chemical blends.
Why Gravimetric Proving Matters
Gravimetric proving uses a calibrated weigh scale as the reference. It is common in oil and gas terminals, chemical batch reactors, and food and beverage plants. In practice, a palm oil exporter in Indonesia proves a DN40 Coriolis meter before every ship loading. A dairy plant in New Zealand runs a monthly check on a DN25 Coriolis meter because the local authority requires traceable mass records.
Gravimetric proving is heavy but accurate. You need space for a weigh tank or a vessel on load cells. The method gives you a direct mass reference. That is why it fits Coriolis mass flow meters in applications where volume conversion error is not acceptable.
Equipment You Need
You need a calibrated weigh scale or load cell system with a resolution better than 0.1 percent of the expected batch weight. For a 100 kg test batch, the scale should resolve at least 10 g. You also need a collection vessel with a vent and a drain. You need a timer or the flow computer totalizer. The meter should have a pulse output, 4-20 mA HART output, or a digital output. A pressure transmitter downstream of the meter and a PT100 temperature sensor help confirm stable process conditions during the run.
Here is the thing: most engineers skip the line purge and vessel drying step. Trapped air or a wet vessel adds error that looks like meter drift. We have seen this on customer sites many times. A paint manufacturer in Vietnam found a 0.4 kg water residue in the vessel caused a false failure on a DN15 Coriolis meter. The meter was fine.
Step by Step Gravimetric Proving Procedure
First, install the meter in the test line and fill the downstream pipe completely. Vent air at the high point. Place the empty collection vessel on the scale and set the scale to zero. Record the meter totalizer reading before the run. Start the pump and divert flow into the collection vessel at the normal operating flow rate. Keep the flow steady. Stop flow and close the valve. Record the meter totalizer reading and the scale weight. Measure liquid temperature during the run. Run the test at least three times for repeatability.
Do not use a short run. The batch time should be at least 60 seconds or the flow computer total may have poor resolution. For low flow rates under 20 kg/h, use a longer batch, maybe 5 minutes. In practice, a chemical plant in Saudi Arabia testing a DN15 Coriolis meter at 30 kg/h u
Calculate the meter error with this formula: error in percent equals meter total minus scale weight divided by scale weight multiplied by 100. A positive error means the meter reads high. A negative error means the meter reads low.
Acceptance Limits and Correction
For a Coriolis mass flow meter, a good gravimetric proving result under stable conditions is within plus or minus 0.2 percent of reading. For custody transfer in oil and gas, many operators tighten the limit to plus or minus 0.1 percent. For an oval gear flow meter on diesel or light oil, plus or minus 0.5 percent is common. If the meter fails, check zero first. Then check for two phase flow, air pockets, and scale drift. Then verify the density setting in the flow computer if the meter reports mass from a volume signal.
Because gravimetric proving gives a direct mass reference, it often finds installation errors instead of meter faults. A bent inlet pipe or a partially closed valve can create swirl or aeration. Fix the installation and repeat the run before you adjust calibration coefficients.
Proving vs Calibration
Proving is a check against a reference. Calibration is the adjustment of meter coefficients or the zero point. Silver Instruments ships each Coriolis mass flow meter with a factory calibration certificate. Field gravimetric proving tells you if the factory calibration still fits your liquid and your installation. If the average error is stable and outside your limit, send us the proving data and we can advise a zero trim or a span trim.
FAQ
Q: How often should I prove a mass flow meter?
For billing applications, prove before each batch or shipment. For internal batching, a monthly or quarterly check is enough. High viscosity or abrasive fluids may need more frequent checks.
Q: Can I use gravimetric proving for gas or steam?
No. Gravimetric proving is for liquids with low vapor pressure. Gas and steam need a master meter or an air flow standard.
Q: What is the minimum scale accuracy for a 0.1 percent meter test?
The scale resolution and repeatability should be better than 0.03 percent of the batch weight. For a 200 kg batch, that means at least 60 g resolution.
Q: Does Silver Instruments provide calibration service?
We supply meters with factory calibration certificates. We can also help you design a proving skid and review your gravimetric proving data.
Q: Which mass flow meters can be proved by gravimetric weighing?
Coriolis mass flow meters, oval gear flow meters, and thermal mass flow meters for liquid service can be proved by gravimetric weighing. The method also works for some positive displacement meters.
Request a Quote from Silver Instruments
Tell us your liquid, viscosity in cP, flow range in kg/h, pressure in bar, temperature in Celsius, and pipe size such as DN15 or DN50. Send the details by Tel: +86-25-68650347, WhatsApp: +86-25-52155837, or WeChat: +86 15365082610. We will recommend a Coriolis mass flow meter or an oval gear flow meter for your proving system.
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