Maximize Measurement Accuracy: Understand Absolute, Relative, and Reference Error

In automation and industrial measurement, precision matters. Terms like “±1% FS” or “class 0.5″ frequently appear on instrument datasheets—but what do they really mean? Understanding absolute error, relative error, and reference (full-scale) error is essential to selecting the right measurement tools and ensuring process accuracy. This guide breaks down these key error metrics with simple formulas, real-world examples, and practical tips.

1. Absolute Error: How Far Off Is Your Reading?

Definition:

Absolute error is the difference between the measured value and the true value of a quantity. It reflects the raw deviation—positive or negative—between what’s read and what’s real.

Formula:

Example:

If the actual flow rate is 10.00 m³/s, and a flowmeter reads 10.01 m³/s or 9.99 m³/s, the absolute error is ±0.01 m³/s.

2. Relative Error: Measuring the Error’s Impact

Definition:

Relative error expresses absolute error as a percentage of the measured value, making it easier to compare across different scales.

Formula:

Example:

A 1 kg error on a 50 kg object results in a relative error of 2%, showing how significant the deviation is in context.

3. Reference Error (Full-Scale Error): Industry’s Favorite Metric

Definition:

Reference error, often called full-scale error (FS), is the absolute error as a percentage of the instrument’s full measurable range—not just the measured value. It’s the standard metric manufacturers use to define accuracy.

Formula:

Example:

If a pressure gauge has a 0–100 bar range and a ±2 bar absolute error, its reference error is ±2%FS—independent of the actual pressure reading.

Why It Matters: Choose the Right Instrument with Confidence

These error metrics are not just theoretical—they directly impact process control, product quality, and regulatory compliance. Among them, reference error is the most widely used for instrument accuracy classification.

Pro Tip: Choosing a narrower measurement range on a multi-range instrument reduces the absolute error for the same %FS accuracy—improving precision.

Master Your Measurements. Optimize Your Accuracy.

By understanding and applying these three error concepts, engineers and technicians can select instruments more wisely, interpret results more confidently, and design more accurate systems in automation and control environments.