Flowmeters Accuaracy

Reference data and engineering information about flowmeters accuaracy for process control applications.

flowmetersaccuaracy

Overview

Engineering reference data for Flowmeters Accuaracy in process control.

Key Formulas

PID Controller

$u(t) = K_p e(t) + K_i \int e(t)dt + K_d \frac{de}{dt}$

Proportional-Integral-Derivative control.

Transfer Function

$G(s) = \frac{K}{\tau s + 1}$

First-order system.

Variables

Symbol	Description	Unit
$K_p$	Proportional gain	—
$K_i$	Integral gain	1/s
$K_d$	Derivative gain	s
$\tau$	Time constant	s

Measurement System Components

A flow measurement system consists of two primary components that both influence total accuracy:

Primary Element: The device that creates a measurable physical change in the fluid (e.g., an orifice plate, venturi tube, or turbine)
Secondary Element: The instrument that converts the physical signal into a readable measurement (e.g., a pressure transmitter or flow computer)

Understanding this distinction is critical because accuracy errors can originate from either component and compound in the total system.

Types of Flow Meter Accuracy

Flow meter accuracy can be expressed in two fundamentally different ways:

Accuracy referenced to the maximum scale reading. This method provides a constant absolute error across the entire measurement range, which means relative error increases significantly at lower flow rates.

Key Insight: A flow meter with ±1% full span accuracy at 1000 kg/h scale will have ±10% error when measuring at 100 kg/h — making it unsuitable for applications requiring accurate low-flow measurement.

Percent of Rate (Percent of Reading)

Accuracy referenced to the actual measured value. This method maintains consistent relative accuracy across the measurement range, with absolute error decreasing proportionally with flow rate.

Key Insight: ±1% of rate accuracy means the absolute error is ±10 kg/h at 1000 kg/h but only ±1 kg/h at 100 kg/h, providing much better performance at lower flows.

Hysteresis

Hysteresis is the maximum difference between measurement readings at the same mechanical set point when:

The set point is approached from above (decreasing value)
The set point is approached from below (increasing value)

This phenomenon is caused by mechanical friction, magnetic effects, or material deformation in the sensor components. Hysteresis typically increases over time due to wear and tear of mechanical parts, making periodic calibration essential for maintaining accuracy.

Original Source

Source: engineeringtoolbox.com