Help Guide: Understanding Linear Accuracy and Addressing a Common Misconception

In food production, accurate measurement is essential for ensuring product quality, safety, and regulatory compliance.

To achieve this, instruments such as pH meters, thermometers, and dry block calibrators are carefully calibrated at specific reference points.

While calibration ensures accuracy at those points, it also establishes a predictable relationship across the instrument’s working range. This is understood as linear accuracy.

What is Linear Accuracy?

When an instrument is calibrated, it is adjusted so that its readings match known reference values at specific points.

When more than one calibration point is used, a defined relationship is established between those points, typically represented as a straight line.

Linear accuracy describes how closely the instrument follows this expected straight-line relationship, both between the calibration points and across the wider measurement range.

How Reliable is Linear Accuracy?

When an instrument is properly designed, maintained, and calibrated, linear accuracy provides a dependable way to predict consistent measurements across a defined range.

In most instruments, measurements between calibration points behave in a stable and predictable way. This means you can generally trust the results within the calibrated range, provided the instrument is performing correctly and used under appropriate conditions.

Example

If a pH meter is calibrated at pH 1.68 and pH 4.00, it would typically be expected to measure accurately at pH 2.

Similarly, if a thermometer is calibrated at 0°C and 40°C, a reading at 4°C would usually be considered reliable.

HOWEVER: This assumes the instrument demonstrates good linearity and is being used under appropriate conditions.

It is sometimes possible to estimate performance slightly outside the calibrated range, although this becomes less reliable depending on the instrument type and application.

Linearity Varies by Instrument

It is important to note that linear accuracy can vary depending on the type of instrument used. For example, contact thermometers typically demonstrate strong linearity across their measurement range, making their performance more predictable between calibration points.

In contrast, infrared thermometers are more sensitive to external factors. As a result, their measurements may not follow a consistent linear relationship across the same range, and accuracy can vary outside the calibrated conditions.

Calibration Requirements in Food Production

In food production environments, calibration is not always based solely on achieving linear accuracy across a range. In many cases, regulatory requirements will call for calibration at Critical Control Points (CCPs), where accurate measurement is essential for ensuring food safety.

For example, a process step involving cooking, chilling, or storage may require verification at a defined temperature threshold. In these situations, calibration at that specific point becomes a priority, regardless of the instrument’s broader linear performance.

Best Practice Guidance for Calibration

Contact Thermometers

  • Use a minimum of two-point (or multi-point) calibration to establish linearity.
  • Include application-specific checks at Critical Control Points (CCPs).

Infrared Thermometers

  • Calibrate at temperatures that closely represent actual conditions of use.
  • Avoid assuming accuracy outside of these calibrated conditions.
  • Use as a screening tool rather than a primary CCP validation device.

Addressing a Common Misconception About Linear Accuracy

A common misunderstanding is that if an instrument is accurate between two calibration points, it can also be calibrated at any point within that range. This is not the case.

While an instrument may measure accurately between two calibration points, this does not mean it can be calibrated at any value within that range. The available calibration points are determined by the instrument’s design, software, and intended application.

Understanding this distinction helps ensure instruments are selected and used correctly, and that measurements remain reliable within the expected operating range.

In food production environments, calibration strategies should prioritise real-world application and food safety requirements. Ensuring accuracy at critical control points is often more important than relying solely on theoretical linear performance across a range.

By understanding how different instruments behave and applying appropriate calibration practices, you can ensure reliable measurements that support both product quality and compliance.

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