BIZOMAT MAKEOS · Quality
Instrument Calibration Check (Errors, Accuracy Class, Interpolated Correction)
From my calibration readings against a reference, is this gauge within its accuracy class, and what is the corrected value of a reading taken in service?
Calibration check of a pressure gauge, thermometer or other instrument at five points across its range: the error at each point and as a percentage of span against the accuracy class, the worst point, and the corrected (true) value of an in-service reading by linear interpolation between the two calibration points that bracket it (as the linear interpolation tool).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Range low end | Lowest value of the instrument range. | |
| Range high end | Highest value of the instrument range. | |
| Accuracy class (% of span) | % | Permitted error as a percentage of span. |
| Reference values and readings | ||
| Reference 1 | Reference (true) value at calibration point 1. | |
| Reading 1 | Instrument reading at point 1. | |
| Reference 2 | Reference value at point 2. | |
| Reading 2 | Reading at point 2. | |
| Reference 3 | Reference value at point 3. | |
| Reading 3 | Reading at point 3. | |
| Reference 4 | Reference value at point 4. | |
| Reading 4 | Reading at point 4. | |
| Reference 5 | Reference value at point 5. | |
| Reading 5 | Reading at point 5. | |
| In service | ||
| Reading taken in service | A reading taken in use that you want corrected. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Range low end
- What it is
- The lowest value the instrument is designed to indicate, at the bottom of its scale, in the instrument’s own units.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the scale range, not the first calibration point; the span (high end minus low end) sets the allowed error in units.
What this means: Range high end
- What it is
- The top of the instrument’s scale, in the same units as the low end; together they define the span used for the accuracy class.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A wider span allows a larger error in units and makes each error a smaller percentage of span.
What this means: Accuracy class (% of span)
- What it is
- The largest permitted error of the instrument as a percentage of its span, as stated on the dial, data sheet or calibration procedure.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.001% to 50%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is a percentage of span, not of reading; a class quoted as a percentage of reading needs converting first.
Reference values and readings
What this means: Reference 1
- What it is
- The true value applied at calibration point 1 by the reference standard, such as a dead-weight tester, reference thermometer or calibrator.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- References and readings must both rise from point 1 to point 5, or the tool rejects them.
What this means: Reading 1
- What it is
- What the instrument under test indicated when reference value 1 was applied, in the same units as the reference.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.02 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Record the reading as found, before any adjustment; readings taken after adjustment hide the error you are checking.
What this means: Reference 2
- What it is
- The true value applied at calibration point 2 by the reference standard, such as a dead-weight tester, reference thermometer or calibrator.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- References and readings must both rise from point 1 to point 5, or the tool rejects them.
What this means: Reading 2
- What it is
- What the instrument under test indicated when reference value 2 was applied, in the same units as the reference.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.53 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Record the reading as found, before any adjustment; readings taken after adjustment hide the error you are checking.
What this means: Reference 3
- What it is
- The true value applied at calibration point 3 by the reference standard, such as a dead-weight tester, reference thermometer or calibrator.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- References and readings must both rise from point 1 to point 5, or the tool rejects them.
What this means: Reading 3
- What it is
- What the instrument under test indicated when reference value 3 was applied, in the same units as the reference.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5.04 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Record the reading as found, before any adjustment; readings taken after adjustment hide the error you are checking.
What this means: Reference 4
- What it is
- The true value applied at calibration point 4 by the reference standard, such as a dead-weight tester, reference thermometer or calibrator.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- References and readings must both rise from point 1 to point 5, or the tool rejects them.
What this means: Reading 4
- What it is
- What the instrument under test indicated when reference value 4 was applied, in the same units as the reference.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.52 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Record the reading as found, before any adjustment; readings taken after adjustment hide the error you are checking.
What this means: Reference 5
- What it is
- The true value applied at calibration point 5 by the reference standard, such as a dead-weight tester, reference thermometer or calibrator.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- References and readings must both rise from point 1 to point 5, or the tool rejects them.
What this means: Reading 5
- What it is
- What the instrument under test indicated when reference value 5 was applied, in the same units as the reference.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 9.98 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Record the reading as found, before any adjustment; readings taken after adjustment hide the error you are checking.
In service
What this means: Reading taken in service
- What it is
- A reading taken from the instrument in normal use that you want converted to the true value, using the two calibration points either side of it.
- Units and format
- Enter a plain number with no unit. Negative values are accepted. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A reading below the first or above the last calibration reading is extrapolated and flagged; treat that correction with caution.
Method and formulas
- Error = reading − reference; % of span = error ÷ (high − low) × 100.
- Within class if every |error| ≤ class × span ÷ 100.
- Corrected value: find the true value for the reading by linear interpolation between the bracketing calibration points (as the linear interpolation tool, finding x for a given y).
- Example: the worst error is 0.4% of span against 1%; a reading of 6 corresponds to 5.9677.
Limitations
- Rising readings only; hysteresis needs falling readings as well.
- Readings must increase with the reference for interpolation.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Record and judge a gauge calibration
- Correct readings from a gauge with a known error
- Decide whether to adjust or replace an instrument
Who it is for
- Instrument technicians
- Quality technicians
- Maintenance planners
What you get
- Error at each point and as % of span
- Worst error against the class
- Corrected value of the service reading
Interpreting the result
- If the errors are all one way, a zero or span adjustment usually brings the instrument back into class.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 121, 709–735. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Measuring humidity and proving the fix
In Condensation Control by the Numbers
What it covers: Humidity sensors and their errors, dew point uncertainty, checking a logger against salt solutions, measuring a heater's real performance, and the evidence that shows a fix works.
Method explained here · BIZOMAT original book
Coating thickness by lift-off
In Eddy-Current Inspection by the Numbers
What it covers: Measuring anodising on aluminium as lift-off, why coating thickness is not dimensional growth, calibrating with foils, a wrong zero that drove the process off target, and tolerance intervals.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
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