How to Determine the Accuracy Class of Instruments and Meters (With Examples)

How to Determine the Accuracy Class of Instruments and Meters (With Examples)

The accuracy class of an instrument is a core indicator of its measurement precision. The determination process should consider national standards, error calculations, and practical application scenarios. Below is a systematic approach to evaluating accuracy classes:

I. Definition and Standards of Accuracy Class

Basic Concept‌

The accuracy class represents the maximum allowable ‌relative error‌ (percentage of absolute error to full-scale value). The smaller the number, the higher the precision. For example:

Class 0.5 instrument: Maximum allowable error of ±0.5% of full scale.

Class 1.0 instrument: Allowable error of ±1.0%, which is less precise than Class 0.5.

National Standard Classes‌

Common industrial instrument classes in China include 0.1, 0.2, 0.5, 1.0, 1.5, 2.5, and 4.0. Some specialized instruments can reach Class 0.005.

Note: Non-recommended classes (e.g., 0.25, 0.3) are marked in parentheses.

II. Methods for Determining Accuracy Class

Reference Error Method‌

Calculate the maximum reference error:

Reference Error

=

(

Maximum Absolute Error

Full Scale

)

×

100

%

Reference Error=(

Full Scale

Maximum Absolute Error

)×100%

Round the result to the nearest standard class (e.g., 1.2% error corresponds to Class 1.5).

Verification by Calibration Standards‌

Follow national standards (e.g., ‌GB/T 13283-2008‌) to verify if the instrument's error meets the maximum allowable error for its claimed class.

Example: A spring-type precision pressure gauge must have ≤±0.05% error to qualify as Class 0.05.

Multi-Range/Multi-Parameter Instruments‌

For instruments with multiple ranges or parameters, evaluate each range/parameter separately.

III. Practical Considerations

Relationship Between Accuracy and Range‌

The same absolute error results in a smaller relative error for larger ranges (e.g., 0.5A error is 10% of 5A but only 1% of 50A).

Calibration and Maintenance‌

High-precision instruments (e.g., Class 0.1) require more frequent calibration (e.g., annually for electronic blood pressure monitors).

Harsh environments (e.g., toxic gas exposure) may require shorter calibration intervals.

Selection Guidelines‌

Industrial settings typically use Class 1.0–2.5, while laboratories require Class 0.1–0.5.

Avoid over-specifying precision, which increases costs unnecessarily.

IV. Common Misconceptions

Accuracy ≠ Precision‌

Precision refers to repeatability, while accuracy reflects closeness to the true value.

Error Labeling Variations‌

Some digital instruments label errors as "±X" or "X% of reading" (check the manual for interpretation).

Example Applications

Example 1: Temperature Gauge‌

Scenario‌: A thermometer with a range of 200–700°C shows a maximum absolute error of +4°C.

Calculation‌:

Full scale‌: 700°C - 200°C = 500°C

Reference error‌:

(

4

500

)

×

100

%

=

0.8

%

(

500

4

)×100%=0.8%

Class determination‌: 0.8% falls between Class 0.5 (±0.5%) and Class 1.0 (±1.0%). By rounding up, the class is ‌1.0‌.

Example 2: Pressure Gauge‌

Scenario‌: A pressure gauge (0–1.6 MPa) has a basic error of ±0.016 MPa.

Calculation‌:

Reference error‌:

(

0.016

1.6

)

×

100

%

=

1.0

%

(

1.6

0.016

)×100%=1.0%

Class determination‌: ‌1.0‌.

Example 3: Multi-Range Flowmeter‌

Scenario‌: A flowmeter has:

0–100 m³/h range: ±0.5 m³/h error

0–50 m³/h range: ±0.3 m³/h error

Calculation‌:

Reference errors‌:

Large range: 0.5/100×100% = 0.5% → ‌Class 0.5‌

Small range: 0.3/50×100% = 0.6% → ‌Class 1.0‌

Final class‌: The stricter requirement (‌Class 0.5‌) applies.

Key Notes

Error Calculation Rules‌:

Use the larger absolute error (positive or negative).

Non-standard classes (e.g., 0.3) are discouraged.

Range Impact‌:

Smaller ranges yield higher apparent accuracy for the same absolute error.

Calibration Frequency‌:

High-precision instruments (e.g., Class 0.1) need more frequent calibration.

This systematic approach ensures compliance with national standards (e.g., ‌GB/T 13283-2008‌) and practical accuracy requirements.