The turndown ratio (also called rangeability) of a flowmeter is the ratio between its maximum and minimum accurately measurable flow rates. It indicates the meter's ability to handle flow rate variations while maintaining its specified accuracy. A meter with a high turndown ratio can accurately measure over a wider range of flow conditions, making it more versatile for applications where the flow rate varies significantly. For example, a flowmeter with a maximum flow of 100 L/min and a minimum accurate flow of 5 L/min has a turndown ratio of 20:1.
In oil applications, the required turndown ratio depends on the variability of the flow conditions in service. A pipeline that normally operates at 80% of its maximum capacity but occasionally drops to 10% during low-demand periods needs a flowmeter with a turndown ratio of at least 8:1. Positive displacement meters typically offer turndown ratios of 10:1 to 50:1, making them very suitable for variable-flow oil applications. Turbine meters have turndown ratios of approximately 10:1 to 15:1 for oil within their rated viscosity range. Coriolis and ultrasonic meters can achieve turndown ratios of 100:1 or more, making them among the most versatile technologies for applications with highly variable flow.
When a single meter cannot cover the required flow range, two meters of different sizes can be installed in parallel—a large meter for high flows and a small meter for low flows—with automatic switching logic controlling which meter is in service based on the current flow rate. This configuration, known as a duplex or cascade metering system, effectively combines the strengths of each meter to achieve high accuracy across a very wide flow range. Understanding the expected flow range and selecting a meter with an adequate turndown ratio for the application is an important step in the flowmeter selection process and directly impacts the accuracy and usefulness of the measurement throughout the full operating envelope.