Step by Step Guide for Gas Flow Meter Calibration

Gas flow meters are essential for monitoring volumetric or mass flow rates of various process gases in industrial plants and machinery. Flow rate denotes the speed at which fluids move through closed pipes or ducts at any given time. In modern instrumentation and control engineering, precise monitoring of fluid flow rates optimizes gas usage and enhances process and equipment efficiency. Therefore, gas flow meter devices require calibration against a reference or master calibration system.

Due to variations in process and environmental conditions, flow meters may experience drift in flow rate readings, necessitating periodic recalibration within their specified flow range to maintain desired accuracy and ensure smooth process operations. Many organizations implement ISO audit systems that mandate recalibration at scheduled intervals.

Types of Gas Flow Meters Available

Several flow meter technologies are currently available in the market, each requiring calibration tailored to its design:

- Turbine flow meter

- Vortex flow meter

- Coriolis flow meter

- Thermal mass flow meter

- Ultrasonic flow meter

Calibration methods vary based on the specific product design and application requirements.

Types of Gas Flow Meter Calibration Procedures

Gas flow meter calibration procedures depend largely on the technology of the product being calibrated. Some widely used calibration procedures include:

- Reference Master Meter calibration

- Primary Standard Calibration using Bell or Piston provers

- Critical Flow Venturi (Sonic nozzle) calibration

- Wind Tunnel Air Velocity calibration

- Onsite validation using calibrated reference gas flow meters

Gas flow meter calibration adheres to various calibration systems that involve comparing and adjusting the flow meter under test to comply with national and international standards, primarily following ISO 17025 standards for calibration methods.

Reference Master Meter Calibration Procedures

Reference Master meter calibration compares with duly calibrated master gas flowmeter traceable to national accredited calibration laboratories with a gas flowmeter under test at desired flow ranges and adjusts its calibration as per desired accuracy tolerances.

Reference Master meter calibration steps :

- Place the reference master meter in series with the flow meter under test.

- Compare the readings of the master flow meter and flow meter using air or standard gas flow rate with a close loop calibrated volumetric tank storage system.

- Calibrate the flowmeter under test to conform with the master flow meter calibration.

Piston / Bell Prover Gravimetric Calibration Procedures

Gravimetric calibration is among the most accurate and cost-effective methods for calibrating volumetric and gas mass flow meters at low pressures. The piston prover-based method is ideal for low-flow-rate gases, while Bell Prover calibration is used for higher flow rates of air. Calibration flow laboratories maintain controlled ambient conditions such as temperature and humidity to optimize the performance of master flow meters.

During piston or bell prover flow meter calibration, a known volume of fluid is passed through the flow meter under test. The piston or bell prover, a cylindrical device with a known internal diameter, uses positive displacement to generate volumetric flow. The piston prover method ensures high accuracy when calibrating low-flow-rate gas or liquid thermal mass flow controllers, ultrasonic flow meters, and turbine flow meters.

Steps in Piston / Bell Prover Gravimetric Calibration:

1. Secure the flow meter in a piston or bell prover system for calibration.

2. Direct the gas flow through the test flow meter into the piston or bell prover, causing the piston or bell to move upward while sensors measure the known volume between intervals.

3. Utilize software systems to calculate flow rate based on measured travel time, pressure, temperature, and flow rate at reference conditions.

4. Compare the calculated flow rate with that of the flow meter under test and make necessary adjustments to align them.

Critical Flow Venturi (Sonic Nozzle) Calibration Procedures

Critical Flow Venturi Nozzles, also known as Sonic Nozzles, are designed for various flow rate capacities according to ISO 9300:2015 standards. Sonic nozzles serve as the premier reference standard for calibrating precision flow meters used in custody transfer applications. In these nozzles, gas flow accelerates to critical velocity at the throat (known as sonic velocity), achieving a steady flow state of single-phase gases. This sonic velocity represents a specific point velocity at a precise pressure, serving as the accurate reference velocity for calibrating the flow meter under test. Sonic nozzles are employed across various fixed-type flow meters, including vortex, turbine, thermal mass, and Coriolis flow meters.

Steps for Sonic Nozzle Calibration:

1. Install the flow meter in series with the pipeline containing the calibrated sonic nozzle.

2. Activate the air compressor within the testing system.

3. Adjust the calibration of the flow meter under test once the sonic velocity reaches the measurement section. Repeat this process for different sonic velocity points corresponding to specific flow rates.

4. Compare the flow rate measured by the sonic nozzle with that indicated by the flow meter under calibration.

Onsite Validation & Calibration Procedures

Onsite validation and calibration of gas flow meters are feasible in certain industrial applications. While this method may not match the accuracy of other calibration methods, it serves to validate gas flow meters in use that exhibit high errors or lack repeatability.

Two primary methods are typically employed for onsite validation:

1. Pump-up Test with Pressure Vessel:

- Introduce the pressure vessel into the gas flow meter series.

- Initiate gas flow and record the readings on the gas flow meter and the time taken to fill the pressure vessel.

- Compare the pressure vessel's pressure relative to its volume capacity with the totalizer reading on the gas flow meter.

- Validate by comparing both sets of results and identifying any discrepancies.

2. Calibrated Reference Gas Flowmeter:

- Install a calibrated reference gas flowmeter in series with the gas flowmeter undergoing validation.

- Compare the results from both flowmeters and calculate the margin of error.