Range, Distance or Span of Values in Measurement

1. Range

Range in measurement is the full interval between the lowest and highest values an instrument or sensor can reliably detect, display, or output. This is a foundational specification in all fields involving quantitative measurement, such as temperature, pressure, voltage, force, displacement, or flow.

Range is typically split into:

• Input Range: The minimum and maximum physical or electrical signals an instrument can accept (e.g., 0–10 bar, 0–100°C, –20 V to +20 V).
• Output Range: The corresponding minimum and maximum output signals (e.g., 4–20 mA, 0–10 V).

Upper Range Value (URV) and Lower Range Value (LRV) define the limits of this interval.

A correct understanding and application of range ensures:

• Instrument readings stay within the device’s functional capability.
• Safety and data integrity in regulated industries (e.g., ICAO Annex 5 for aviation).
• Prevention of under-ranging (below LRV) and over-ranging (above URV), which can cause inaccurate data or hardware damage.

Practical Example

A jet engine test cell uses a digital temperature indicator with a range of –50°C to +1500°C. If the process exceeds these limits, the indicator may deliver erroneous results or fail. Quality instruments often include over-range warnings or automatic shutdowns, but these features are no substitute for proper selection and use.

Manufacturers specify ranges according to international standards (such as IEC 61298). Always consult calibration certificates and datasheets for verified range values to ensure compliance and traceability.

2. Span

Span is the difference between the upper and lower range values:

[ \text{Span} = \text{URV} - \text{LRV} ]

• Span is always positive and quantifies the “width” of the measurement window.
• Span impacts an instrument’s resolution and sensitivity; narrower spans increase sensitivity and precision.
• Instruments may allow span adjustment to match process requirements (e.g., configuring a 0–100 bar transmitter to operate between 40–60 bar for improved accuracy).

During calibration, span is set using traceable standards. Incorrect span settings can introduce errors (e.g., non-linearity, offset) and compromise measurement reliability.

Typical Use Cases

• An aircraft pressure transducer with a 0–1000 hPa range may be set for a 900–1100 hPa span to improve granularity for altitude computation.
• Industrial flow meters or weighing scales often use spans that do not start at zero (suppressed zero) to focus on relevant process values.

3. Measuring Range

Measuring range is the part of the total range where the instrument’s accuracy and repeatability are guaranteed, as certified by the manufacturer or calibration authority.

• Only within the measuring range are error limits and uncertainties controlled.
• An instrument may display values outside the measuring range, but these readings are not reliable.

Example:
A digital voltmeter may have a range of –20 V to +20 V but a certified measuring range of –10 V to +10 V. Readings outside the certified range lack guaranteed accuracy.

Regulatory standards (e.g., IEC 61298, ISO 10012, ICAO Annex 5) require that measuring range is clearly defined and traceable, especially for safety-critical or quality-related applications.

Selection tip: Always choose instruments whose measuring range fully covers your expected process values with a safety margin.

4. Related Terminology and Concepts

4.1. Scale and Scale Range

• Scale: The graduated markings or digital values an instrument displays.
• Scale Range: The interval between the lowest and highest values shown on the scale.

The scale range may exceed the certified measuring range to provide over- or under-range warnings. Always distinguish between what is displayed (scale range) and what is certified for accuracy (measuring range).

4.2. Lower Range Value (LRV) and Upper Range Value (URV)

• LRV: The lowest value in the instrument’s range or measuring range.
• URV: The highest value in the range or measuring range.

LRV and URV are used to calculate span and configure analog/digital transmitters (e.g., mapping 0 bar to 4 mA and 10 bar to 20 mA).

4.3. Suppressed Zero Range

Used when the LRV is above zero. This focuses measurement on a specific process window, improving resolution and avoiding irrelevant low-value readings (e.g., weighing scales measuring only 100–2000 kg).

4.4. Output Range

Defines the electrical or digital signal range corresponding to the input or measuring range (e.g., 4–20 mA, 0–10 V). Correct scaling ensures accurate data transfer to controllers or data loggers.

4.5. Indicator Range

The full set of values the display can show. May be wider than the measuring range, but only readings within the measuring range are certified as accurate.

5. Practical Examples and Case Studies

Case Study 1: Pressure Transducer

A transducer with 0–100 bar input range, 4–20 mA output, monitors pipeline pressure varying from 10–80 bar. Occasional surges reach 95 bar. The device’s full span is used, and calibration is performed within this range.

Case Study 2: Temperature Control in HVAC

An RTD sensor with a measuring range of –50°C to +150°C (span: 200°C) regulates supply air between 15–30°C. Calibration occurs at multiple points within this interval for verified performance.

Case Study 3: Digital Multimeter

A multimeter with a 0–600 V range is used for electrical maintenance. Measuring voltages above 600 V risks instrument damage and unsafe conditions. Calibration uses certified voltage standards within the specified range.

Case Study 4: Suppressed Zero Weighing Scale

A scale for packaging only measures 50–150 kg (span: 100 kg). Loads below 50 kg are not displayed, focusing attention and resolution on the relevant range.

Case Study 5: Flow Meter Calibration

A flow meter with a 5–100 L/min measuring range is calibrated at several flow rates. System alarms are triggered outside this range to ensure accuracy and process integrity.

6. Significance of Correct Range and Span Selection

6.1. Accuracy and Resolution

Accuracy depends on matching the instrument’s range and span to the process. Too wide a range reduces resolution; too narrow risks frequent out-of-range errors.

6.2. Instrument Safety

Operating beyond range can damage sensors (e.g., electrical overload, mechanical failure). Over-range protection should not replace proper selection.

6.3. Process Control and Compliance

Accurate measurement is vital for safe and efficient process control. Regulatory standards (e.g., ISO 9001, FDA cGMP) require evidence that instruments are used within their certified ranges.

7. Selection Criteria for Range and Span

1. Understand the Process Variable: Analyze normal/abnormal process values.
2. Choose a Suitable Measuring Range: Ensure coverage with margin for excursions.
3. Optimize Span for Resolution: Use the narrowest practical span for better precision.
4. Account for Environment and Overload: Consider temperature, vibration, and accidental overloads.
5. Review Manufacturer’s Specs: Check datasheets and calibration certificates for all relevant parameters.

8. Potential Errors and Consequences

• Using Outside Measuring Range: Leads to unreliable or spurious data.
• Exceeding Range or Span: Can cause catastrophic instrument failure.
• Improper Calibration: Invalidates traceability and compliance, risking loss of certification.

9. Summary Table: Key Terms and Distinctions

Conclusion

Understanding and correctly applying the concepts of range, span, and measuring range is crucial for accurate, safe, and compliant measurements in science, engineering, and industry. Always select instruments and configure their ranges and spans to match your process needs, verify calibration, and consult authoritative standards for guidance.

For expert advice on instrument selection, calibration, and process optimization, contact our team or schedule a demo .

References:

• IEC 61298: Process measurement and control instruments
• ISO 10012: Measurement management systems
• ICAO Annex 5: Units of Measurement to be Used in Air and Ground Operations
• Instrument manufacturer datasheets and calibration certificates