The short answer
This glossary covers 60 calibration terms used in Singapore's quality management, instrument engineering, and laboratory contexts. Terms are drawn from VIM (International Vocabulary of Metrology), ISO/IEC 17025:2017, ISO 9001:2015, and common Singapore industry practice. Where the official VIM definition is unclear in industrial context, a plain-English explanation is provided alongside.
Key takeaways
- VIM (Vocabulaire International de Métrologie) is the authoritative source for metrology definitions. When a calibration lab uses a term, it should mean what VIM says it means.
- Uncertainty and error are different concepts. Error is the deviation of a measured value from the true value (unknown in practice); uncertainty is the characterised range within which the true value is estimated to lie (quantifiable).
- Traceability is a property of a measurement result, not a property of a lab or a certificate. The result is traceable if the measurement chain back to SI units is documented and unbroken.
- Accuracy, precision, and resolution are three distinct concepts that are commonly confused. An instrument can be precise (repeatable) without being accurate (close to true value), and can display high resolution (many decimal places) without being either.
- Calibration, verification, and adjustment are different operations. Calibration determines the deviation; verification checks whether the deviation is within specification; adjustment corrects the deviation. Not all calibrations include adjustment.
How to use this glossary
Terms are organised A–Z. Each definition gives the plain-English meaning first, then references the relevant VIM or ISO standard where applicable, and includes a practical Singapore industry example where helpful. Use the letter headings to jump to the section you need. Terms that cross-reference each other are noted in the definition.
A
Accuracy
Accuracy describes how close a measurement result is to the true value (or accepted reference value) of the measurand. VIM 2.13 defines it as "closeness of agreement between a measured quantity value and a true quantity value." In practice, accuracy is never perfect. It is always limited by the instrument's design, calibration quality, and environmental conditions. A thermocouple with an accuracy of ±1°C will read within 1°C of the actual temperature under specified conditions. Accuracy should not be confused with precision (repeatability) or resolution (display digits). A high-accuracy instrument must be supported by a valid calibration certificate from a traceable laboratory.
Adjustment
Adjustment is the set of operations carried out on a measuring instrument to bring it into a state of performance suitable for its use. VIM 3.11. Unlike calibration, which only documents deviation, adjustment actively changes the instrument's output, by zeroing, spanning, or offsetting the reading. Adjustment must always be followed by re-calibration to verify the new state. On a calibration certificate, adjustments are noted as part of the as-found/as-left record. Not all calibrations include adjustment: many labs document the deviation and return the instrument without touching it, leaving the adjustment decision to the owner.
Artifact (measurement)
A measurement artifact (also called a material measure) is a physical object that embodies a quantity value and is used as a reference during calibration. Examples include gauge blocks (dimensional), resistor standards (electrical), and dead weights (pressure). Unlike instruments that measure, artifacts represent a known quantity. Artifacts must themselves be calibrated against higher-level standards to maintain traceability. In SAC-SINGLAS accredited labs, artifacts used as reference standards must be listed in the scope of accreditation with their uncertainty values and recalibration intervals.
As-Found
As-found refers to the condition and measurement error of an instrument at the beginning of calibration, before any adjustment, repair, or corrective action is taken. As-found data is critical for two reasons: it confirms whether the instrument was within tolerance during the period since its last calibration (which may affect the validity of measurements made during that period), and it informs decisions about calibration interval extension or reduction. ISO 9001 and GMP auditors may specifically request as-found records to verify historical measurement validity. If as-found data is not captured, there is no evidence that past measurements were reliable.
As-Left
As-left refers to the condition of an instrument after calibration, including any adjustment performed. The as-left reading confirms the instrument's performance at the point it was returned to service. A certificate reporting only one set of results (without distinguishing as-found from as-left) is ambiguous. The reader cannot tell whether adjustment was performed or what condition the instrument was in beforehand. Best practice, and SAC-SINGLAS expectation for ISO/IEC 17025 compliance, requires both sets of results to be clearly documented whenever adjustment is performed.
B
Bias
Bias is a systematic offset between the average of repeated measurement results and the accepted reference value of a measurand. VIM 2.18. Unlike random error, which varies unpredictably between measurements, bias is consistent and directional. An instrument with a positive bias always reads high by approximately the same amount. Bias can be caused by a miscalibrated reference, incorrect environmental correction, or instrument design limitations. It is corrected by adjustment or mathematically by applying a correction factor. MSA (Measurement System Analysis) studies specifically evaluate bias as one of the key indicators of measurement system performance.
Best Measurement Capability (BMC)
Best Measurement Capability (BMC) is the smallest measurement uncertainty a calibration laboratory can achieve under ideal conditions for a given parameter, using its best available equipment and procedures. BMC values are listed in a laboratory's scope of accreditation as published by SAC-SINGLAS. They represent the lab's capability at the best end of its range. Actual uncertainty for a specific instrument calibration may be larger than the BMC, depending on the instrument's characteristics and the calibration point required. When selecting a calibration lab, comparing BMC values against your instrument's tolerance helps confirm the lab is capable enough for your application.
Budget (uncertainty)
An uncertainty budget is a systematic tabulation of all identified sources of measurement uncertainty in a calibration, along with their individual contributions and the combined result. GUM (Guide to the Expression of Uncertainty in Measurement) defines the method for constructing uncertainty budgets. Typical sources include: the reference standard's own uncertainty, repeatability of the measurement, instrument resolution, temperature effects, and operator factors. ISO/IEC 17025 requires accredited laboratories to have documented uncertainty budgets for each calibration method in their scope. An accredited certificate's stated expanded uncertainty is derived from this budget.
C
Calibration
Calibration is the operation that, under specified conditions, establishes a relationship between the quantity values indicated by a measuring instrument and the corresponding values realised by standards. VIM 2.39. Calibration determines the error of the instrument. It does not necessarily fix it. The result is documented in a calibration certificate. Calibration must be traceable to national standards (via NMC Singapore or another NMI) through an unbroken chain of comparisons. Calibration is not the same as adjustment (correcting the instrument) or verification (confirming the instrument is within a specified tolerance).
Calibration Certificate
A calibration certificate is the formal document that records the results of a calibration. ISO/IEC 17025:2017 clause 7.8 specifies the required contents: unique identification, the laboratory's name and accreditation number, date of calibration, instrument identification, reference standards used (with their own traceability information), environmental conditions, measurement results, measurement uncertainty, and any statement of conformance if requested. An accredited calibration certificate from a SAC-SINGLAS laboratory must bear the laboratory's accreditation number and the SAC-SINGLAS accreditation mark. Certificates from non-accredited labs may look similar but lack independently verified backing for the traceability and uncertainty claims.
Calibration Due Date
The calibration due date is the date after which an instrument should not be used in quality-critical measurements without recalibration. It is calculated from the date of calibration plus the assigned calibration interval. An instrument used beyond its due date creates compliance risk. ISO 9001 auditors will flag instruments with lapsed calibration as a non-conformance, and any measurements made with them after the due date may need to be assessed for validity under clause 7.1.5.3. Due dates should be tracked in a calibration register, and instruments should be recalled for recalibration before (not on), the due date to allow turnaround time.
Calibration Interval
The calibration interval is the period between successive calibrations of an instrument. Intervals are not fixed by standard. They must be set by the instrument owner based on risk assessment, historical drift data, manufacturer recommendation, regulatory requirements, and the consequences of out-of-tolerance measurement. Common intervals range from 3 months (for critical process instruments) to 12 or 24 months (for stable reference instruments). Intervals should be reviewed periodically using as-found data from successive calibrations: if an instrument consistently shows little drift, the interval can often be extended safely. ILAC G24 provides guidance on calibration interval adjustment.
Calibration Register
A calibration register is the controlled record (typically a spreadsheet, CMMS, or dedicated asset management system), that tracks every measurement instrument in a facility, its location, calibration status, due date, and certificate reference. ISO 9001:2015 clause 7.1.5 requires documented information on all monitoring and measuring resources, making a calibration register a mandatory quality system element. A well-maintained register enables proactive recall scheduling, provides auditors with a single-source view of calibration compliance, and supports traceability investigations when suspect measurements occur. Common fields: asset ID, instrument description, range, accuracy class, location, last calibration date, due date, certificate number, and calibration lab.
Coverage Factor (k)
The coverage factor (symbol: k) is a multiplier applied to the combined standard uncertainty to produce the expanded uncertainty at a desired level of confidence. GUM §2.3.6. For a normal distribution, k=2 gives approximately 95% confidence and k=3 gives approximately 99.7% confidence. Accredited calibration certificates in Singapore state expanded uncertainty at k=2 (95% confidence) unless otherwise specified. When comparing uncertainties from different certificates, always check the coverage factor. A small expanded uncertainty calculated at k=1 is not directly comparable to one at k=2. The certificate must state both the expanded uncertainty value and the coverage factor used.
Conformity Assessment
Conformity assessment is the determination of whether a measurement result (or an instrument) meets a specified requirement or tolerance. ISO/IEC 17025:2017 clause 7.8.6 covers statements of conformity. When a calibration certificate includes a pass/fail statement (e.g. "within manufacturer specification"), this is a conformity assessment. The key issue is how measurement uncertainty is handled in making the decision: if uncertainty is not accounted for, the assessment may incorrectly accept an out-of-tolerance instrument or reject a conforming one. ILAC G8 provides guidance on decision rules for conformity assessment, including guardbanding approaches to manage this risk.
D
Deadweight Tester
A deadweight tester (also called a piston gauge) is a primary pressure standard that generates a precise, known pressure by balancing the weight of calibrated masses against the piston area of the instrument. Because it derives pressure directly from mass, gravity, and area (all of which can be measured with high accuracy), a deadweight tester provides pressure reference values traceable to SI units without requiring calibration against another pressure instrument. Unitest uses deadweight testers as primary references for pressure calibration, generating traceability from NMC Singapore through this first-principles measurement rather than through secondary instrument chains.
Drift
Drift is the gradual, unintended change in an instrument's output over time under constant conditions. VIM 4.21. All real instruments drift due to aging components, mechanical wear, thermal cycling, contamination, or chemical change in sensing elements. Drift is the primary reason instruments need periodic recalibration: even a correctly calibrated instrument will eventually read outside its specification as drift accumulates. Historical as-found data from successive calibrations allows engineers to quantify drift rate and adjust calibration intervals accordingly. Shortening them if drift is faster than expected, or extending them if the instrument is demonstrably stable.
Due Date (calibration)
See Calibration Due Date above. In practice, due date is the term most commonly used on calibration certificates and asset registers. It should be clearly distinguished from the calibration date (when calibration was performed) and the issue date (when the certificate was produced, which may be later). An instrument's due date governs when it must be recalled for recalibration. Using it after this date, even by one day, creates a compliance gap that requires documented investigation under ISO 9001:2015 clause 7.1.5.3.
E
Expanded Uncertainty
Expanded uncertainty (symbol: U) is the measurement uncertainty expressed at a specified level of confidence, obtained by multiplying the combined standard uncertainty by a coverage factor (k). GUM §2.3.5. For example, a calibration result might state: "temperature = 25.0°C, expanded uncertainty U = ±0.15°C, k = 2 (approximately 95% confidence)." This means the laboratory is 95% confident that the true temperature lies within 25.0°C ± 0.15°C. Expanded uncertainty is the form of uncertainty most commonly reported on calibration certificates, and is the figure that should be compared against instrument tolerances when assessing fitness for purpose.
Error (measurement)
Measurement error is the difference between the measured value and the true value of the measurand. VIM 2.16. Because the true value is never exactly known, error can never be exactly determined, it can only be estimated. Error has two components: systematic error (bias, consistent offset) and random error (scatter, varying between measurements). Calibration quantifies the error of an instrument relative to a reference standard, documenting how far the instrument's reading deviates from the reference value at each calibration point. Error and uncertainty are related but distinct: error is the actual (unknown) deviation; uncertainty is the characterised estimate of how large that deviation might be.
F
False Accept
A false accept (also called a Type II error or consumer risk) occurs when a measurement or conformity assessment concludes that an instrument or product is within specification when it is actually out of tolerance. False accepts are more common when the Test Uncertainty Ratio (TUR) is low. Meaning the calibration measurement's uncertainty is large relative to the instrument's tolerance. The consequence of a false accept is using an out-of-tolerance instrument in production, potentially shipping non-conforming product or making incorrect regulatory decisions. Guardbanding reduces false accept risk by tightening the acceptance zone to account for measurement uncertainty.
False Reject
A false reject (also called a Type I error or producer risk) occurs when a measurement or conformity assessment concludes that an instrument or product is out of specification when it is actually conforming. False rejects result in unnecessary remediation costs. Recalibrating or adjusting instruments that were within tolerance, or scrapping conforming product. Like false accepts, false rejects become more frequent when TUR is low. Guardbanding, while it reduces false accepts, can increase false reject rate, creating a trade-off that must be managed according to the risk profile of the application.
G
Guardbanding
Guardbanding is the practice of tightening the acceptance zone used in conformity assessment to reduce the risk of making incorrect pass/fail decisions when measurement uncertainty is significant relative to the tolerance. Instead of accepting results anywhere within the full specification limits (T), guardbanding applies an inward offset (commonly equal to the expanded uncertainty U), accepting only results within T − U on each side. ILAC G8:09/2019 provides recommended decision rules for guardbanding. It is especially important in Singapore regulatory and GMP contexts where incorrect acceptance of an out-of-tolerance instrument could have safety or compliance consequences.
Gauge R&R (GRR)
Gauge Repeatability and Reproducibility (GRR) is a statistical study that quantifies the variation introduced by a measurement system. Specifically separating variation due to the instrument (repeatability) from variation due to different operators using the same instrument (reproducibility). AIAG MSA Manual. GRR is typically expressed as a percentage of process tolerance or total variation. A GRR of less than 10% is generally acceptable; 10–30% may be acceptable with justification; above 30% the measurement system requires improvement. GRR studies are mandatory in automotive (IATF 16949), aerospace, and high-volume electronics manufacturing in Singapore before a measurement system is approved for production use.
H
HART (protocol)
HART (Highway Addressable Remote Transducer) is a communication protocol used in process instrumentation that allows digital communication to overlay a conventional 4–20 mA analogue signal. HART-enabled instruments can transmit additional diagnostic, configuration, and measurement data digitally while simultaneously providing the standard analogue output. During calibration of HART instruments, technicians can access the instrument's internal variables, perform digital zero/span adjustments, and retrieve process variable readings via the HART protocol rather than only through the analogue signal. This is particularly relevant for calibrating smart transmitters for pressure, temperature, and flow in Singapore's process industry facilities.
I
Influence Quantity
An influence quantity is any quantity that affects a measurement result but is not the measurand itself. VIM 2.52. Common influence quantities in calibration include ambient temperature, relative humidity, barometric pressure, vibration, electromagnetic interference, and supply voltage. All influence quantities that materially affect the measurement must be identified and either controlled, corrected for, or included in the uncertainty budget. ISO/IEC 17025 requires accredited laboratories to document the environmental conditions during calibration and include the effects of uncontrolled influence quantities in their uncertainty budgets.
In-Situ Calibration
In-situ calibration (also called on-site calibration) is calibration performed at the instrument's installed location rather than in a laboratory. It is used when instruments cannot be removed for calibration (permanently installed pipeline sensors, critical process monitors) or when removing and reinstalling the instrument would introduce significant additional uncertainty. In-situ calibration may have larger uncertainty than laboratory calibration because environmental conditions are less controlled. Unitest provides on-site calibration services in Singapore for instruments that require calibration in their operational context. See the distinction from laboratory calibration in our on-site vs in-lab guide.
Instrument Under Test (IUT)
The Instrument Under Test (IUT) (also called the Device Under Test (DUT) or Unit Under Test (UUT)), is the measuring instrument that is the subject of a calibration. The IUT's output is compared against a reference standard to determine its error, and the results are documented in the calibration certificate. The IUT's characteristics (range, resolution, accuracy class, and drift history) inform the calibration method selected and the uncertainty budget constructed for its calibration. The terms IUT, DUT, and UUT are used interchangeably across different industry sectors.
L
Laboratory Scope
A laboratory scope (or scope of accreditation) is the formal document, published by SAC-SINGLAS, that defines the specific parameters, ranges, methods, and Best Measurement Capabilities for which a laboratory holds accreditation. Accreditation is scope-specific: a lab accredited for electrical calibration is not automatically accredited for pressure or temperature. When submitting instruments for calibration, the customer should verify that the required parameter, range, and method are listed in the lab's current scope. Unitest's scope (LA-2023-0845-C) covering electrical, temperature, and pressure parameters is available on the SAC website and on our accreditation page.
Linearity
Linearity describes how uniformly an instrument's error is distributed across its measurement range. An instrument with perfect linearity would have the same error (or zero error) at every point in its range. In practice, all instruments have some non-linearity. The error varies across the range. Linearity is assessed during calibration by measuring at multiple points across the range and comparing each result to the reference value. Significant non-linearity means a single-point calibration (or a correction factor derived from one point) may not represent the instrument's true error at other points. Multi-point calibrations reveal linearity errors that a single-point check would miss.
M
Measurement
Measurement is the process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity. VIM 2.1. Measurement is not the same as reading an instrument's display. It involves the complete process: selecting the appropriate instrument, controlling environmental conditions, applying the correct method, reading the result, and associating an uncertainty. In quality management, every measured result used to make a conformity decision or control a process is a measurement in this sense, and must be supported by evidence of the instrument's calibration status and uncertainty.
Measurement System Analysis (MSA)
Measurement System Analysis (MSA) is a collection of statistical techniques used to evaluate the performance and fitness of a measurement system for its intended purpose. The AIAG MSA Manual defines the standard approaches. MSA studies include Gauge R&R (repeatability and reproducibility), bias study (systematic offset), linearity study (error across range), and stability study (drift over time). MSA is required in automotive (IATF 16949, AIAG PPAP) and aerospace supply chains, and is increasingly used in Singapore's electronics and medical device manufacturing sectors. Calibration is a prerequisite for MSA. You cannot meaningfully analyse a measurement system that is not calibrated.
Metrological Traceability
Metrological traceability is the property of a measurement result whereby it can be related to a stated reference (typically SI units or a national standard) through a documented, unbroken chain of calibrations, each with stated uncertainties. VIM 2.41. Traceability is a property of a measurement result. Not of a lab, a certificate, or an instrument. A result is traceable only if the entire chain from that result back to the SI unit is documented with uncertainties at each link. In Singapore, the chain passes through NMC A*STAR for most physical parameters. SAC-SINGLAS accreditation independently verifies that this chain exists and is correctly maintained for each accredited lab.
Metrology
Metrology is the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology. VIM 2.2. Metrology encompasses three domains: scientific metrology (development and maintenance of measurement standards), industrial metrology (ensuring that measurement equipment in production and testing is fit for purpose), and legal metrology (measurement requirements in law, such as trade measurement). Calibration sits within industrial metrology. In Singapore, metrology activities are coordinated through NMC A*STAR (national standards) and SAC-SINGLAS (laboratory accreditation).
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N
National Metrology Institute (NMI)
A National Metrology Institute (NMI) is the government-designated body responsible for maintaining a country's primary measurement standards and realising the SI units at the highest level of accuracy. NMIs participate in international comparisons to confirm the equivalence of their standards with those of other countries. Calibration laboratories trace their measurements to the national standard through calibrations performed by (or directly traceable to), the NMI. In Singapore, the NMI is the National Metrology Centre (NMC), a division of A*STAR. In the UK it is NPL; in the US, NIST; in Germany, PTB.
NMC Singapore
The National Metrology Centre (NMC) is Singapore's national metrology institute, a division of A*STAR (Agency for Science, Technology and Research). NMC maintains Singapore's primary measurement standards for physical parameters including mass, temperature, pressure, electrical quantities, time and frequency, and dimensional measurement. Calibration laboratories in Singapore trace their reference standards to NMC, establishing the national traceability chain. Unitest's reference instruments are calibrated by NMC, forming the first link in the traceability chain that reaches every customer's certificate. NMC also conducts proficiency testing programs and supports Singapore's participation in BIPM/CIPM international comparisons.
Non-conformity (calibration)
In the calibration context, a non-conformity refers to a situation where a calibration process, certificate, record, or result does not meet the requirements of ISO/IEC 17025, the lab's own quality system, or a customer's specification. Common calibration non-conformities include: instruments used beyond their due date, calibration certificates without stated measurement uncertainty, reference standards with lapsed traceability, environmental conditions outside specified limits during calibration, or failure to record as-found/as-left data. ISO 9001 non-conformities related to clause 7.1.5 are raised when instruments cannot demonstrate calibration with stated traceability and uncertainty. All non-conformities must be documented, root-caused, and corrected.
O
Out-of-Tolerance (OOT)
Out-of-Tolerance (OOT) refers to the condition where a calibration result reveals that an instrument's error exceeds its specified tolerance or accuracy class. An OOT finding triggers a documented investigation process: how long was the instrument OOT? What measurements were made during that period? Were any products released or decisions made using data from the OOT instrument? This investigation (sometimes called an OOT impact assessment), is required under ISO 9001:2015 clause 7.1.5.3 and GMP regulatory frameworks. The outcome may require product review, customer notification, or batch re-evaluation. Capturing as-found data at each calibration makes OOT investigations significantly easier.
Offset
An offset is a fixed, constant difference between an instrument's indicated value and the reference value across its range. Unlike non-linearity (which varies across the range), an offset is a uniform shift. The instrument reads consistently high or low by the same amount everywhere. Offsets are corrected by zero adjustment or by applying a correction factor. On a calibration certificate, a constant offset across all measurement points suggests the instrument simply needs re-zeroing; varying errors across points suggest linearity or gain errors that require multi-point correction. The term is also used in instrumentation to refer to the deliberate introduction of a bias for engineering purposes (e.g. 4 mA offset in a 4–20 mA signal).
P
Primary Standard
A primary standard is a measurement standard designated to realise a unit of measurement directly from its definition, or from first principles, without reference to a higher-level standard of the same quantity. VIM 5.4. Primary standards are maintained by NMIs and represent the highest level in the traceability chain. In Singapore, NMC maintains primary standards. Reference standards in accredited calibration labs (like Unitest) are calibrated against NMC primary standards, placing them one or two levels below primary in the traceability hierarchy. The closer a lab's reference is to the primary standard, the lower its measurement uncertainty can be.
Proficiency Testing
Proficiency testing (PT) is the evaluation of a laboratory's performance by means of interlaboratory comparisons. Multiple labs measure the same artifact or reference material, and results are compared to reveal systematic errors, bias, or outliers. ISO/IEC 17043 defines the requirements for proficiency testing providers. ISO/IEC 17025:2017 clause 7.7 requires accredited laboratories to participate in proficiency testing as part of quality assurance. SAC-SINGLAS requires proficiency testing participation for all accredited parameters. PT results that reveal a lab as a systematic outlier may trigger accreditation suspension pending investigation.
Precision
Precision describes the closeness of agreement between repeated measurement results under specified conditions. VIM 2.15. Precision is not the same as accuracy: a precise instrument produces consistent readings that may still be systematically offset from the true value. Two types of precision are commonly distinguished: repeatability (same operator, same instrument, short time interval) and reproducibility (different operators, different instruments, or different occasions). Precision is assessed by repeated measurement under controlled conditions and expressed as a standard deviation or range. High resolution (many decimal places) does not imply high precision. A display may show 4 decimal places but the last two may be noise.
R
Reference Standard
A reference standard is a measurement standard designated for the calibration of other standards or instruments of the same quantity in a given organisation. VIM 5.6. In an accredited calibration laboratory, reference standards are the highest-level instruments available within the lab, used to calibrate customer instruments submitted for calibration. Reference standards must themselves be calibrated (by NMC or another accredited laboratory), and their own uncertainty forms part of the calibration uncertainty budget for every customer certificate issued using that reference. Unitest's reference standards are calibrated by NMC Singapore and listed in the SAC-SINGLAS scope of accreditation.
Repeatability
Repeatability is the closeness of agreement between results of successive measurements of the same measurand, carried out under the same conditions (same operator, same instrument, same location, same measurement procedure, short time interval). VIM 2.21. Repeatability is a component of precision. It is evaluated by taking multiple measurements under identical conditions and computing the standard deviation of the results. Poor repeatability contributes to Type A uncertainty in the calibration uncertainty budget. In a GRR study, repeatability is the variation attributable to the instrument itself, as opposed to reproducibility, which captures operator-to-operator variation.
Reproducibility
Reproducibility is the closeness of agreement between repeated measurements of the same measurand made under changed conditions. Different operators, different instruments, different laboratories, different times, or different environments. VIM 2.25. Reproducibility is a broader indicator of measurement system consistency than repeatability. In a GRR study, reproducibility captures operator-to-operator variation when measuring the same part with the same instrument. In the context of proficiency testing, reproducibility is assessed between laboratories. Large reproducibility variation suggests the measurement method, instrument, or operator training is inconsistent and requires investigation.
Resolution
Resolution is the smallest change in a measured quantity that produces a detectable change in the corresponding indication. VIM 4.14. For a digital instrument, resolution is the value of the last digit displayed. A thermometer displaying "25.1°C" has a resolution of 0.1°C. Resolution is often confused with accuracy. A high-resolution display does not imply high accuracy. Resolution limits the uncertainty contribution from reading the display: the reading uncertainty due to resolution is typically ±½ of the last digit. A fine resolution that is better than the instrument's uncertainty is useful; a coarse resolution that is worse than the uncertainty wastes measurement capability.
Risk-Based Calibration
Risk-based calibration is an approach to calibration planning that allocates calibration resources (frequency, method, and level of accreditation), based on the risk associated with each instrument. Instruments whose failure would directly affect product quality, patient safety, or regulatory compliance are treated as critical and receive more frequent accredited calibration. Instruments used only as indicators with no quality-critical function may receive less rigorous calibration or extended intervals. ISO 9001:2015 and ISO/IEC 17025 both support a risk-based approach. ILAC G24 provides practical guidance. The result is a calibration programme that matches investment to risk rather than applying uniform treatment to all instruments.
S
SAC-SINGLAS
SAC-SINGLAS (Singapore Laboratory Accreditation Scheme) is Singapore's national laboratory accreditation scheme, operated by the Singapore Accreditation Council (SAC) under the Ministry of Enterprise and Innovation. It accredits laboratories to ISO/IEC 17025 for calibration and testing activities. SAC is a full signatory to the ILAC Mutual Recognition Arrangement, meaning SAC-SINGLAS accredited certificates are accepted in over 100 economies without re-calibration. Unitest holds SAC-SINGLAS accreditation no. LA-2023-0845-C. Accreditation status is publicly verifiable at sac.gov.sg. SAC-SINGLAS accreditation is the primary mechanism by which Singapore calibration laboratories demonstrate independent, internationally recognised technical competence.
SI Units
The International System of Units (SI) is the modern form of the metric system, defining the seven base units from which all other units are derived: the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). The 2019 SI revision redefined all base units in terms of fixed numerical values of fundamental physical constants, making the SI fully stable and universal. All metrological traceability chains ultimately lead back to SI units, whether directly through first-principles realisations or through NMI-maintained standards that are proven equivalent to the SI definition by international comparison.
Span
Span is the difference between the upper and lower limits of the measurement range of an instrument. For example, a pressure transmitter with a range of 0–10 bar has a span of 10 bar. Span adjustment (or gain adjustment) changes the instrument's sensitivity. How much output change corresponds to a given input change. Span errors appear as increasing deviation from the true value as the measurement moves away from zero, unlike offset errors which are constant across the range. During calibration, span is typically assessed at multiple points and may need adjustment if the error increases linearly with the measured quantity.
Specificity
Specificity in measurement refers to the ability of a measurement method or instrument to measure only the intended measurand without interference from other quantities present in the measurement environment. The term is most commonly used in analytical chemistry and pharmaceutical testing (where it appears in ICH Q2(R1) guidelines for method validation), but the concept applies broadly. In physical calibration, specificity considerations arise when electromagnetic interference, cross-sensitivity between parameters (e.g. a pressure sensor that also responds to temperature), or contamination affects the measurement. These cross-sensitivities must be identified and included as influence quantities in the uncertainty budget.
T
Test Uncertainty Ratio (TUR)
The Test Uncertainty Ratio (TUR) is the ratio of the tolerance of the Unit Under Test to the expanded uncertainty of the calibration measurement: TUR = Tolerance / Uncertainty. A TUR of 4:1 is the traditional minimum recommended in ANSI/NCSL Z540.3 and ILAC G8. The calibration measurement's uncertainty should be no greater than one-quarter of the tolerance being assessed. A higher TUR gives more confidence in conformity decisions. When TUR falls below 4:1, the risk of false accept or false reject increases significantly, and guardbanding should be applied to manage this risk. TUR is a key criterion when selecting a calibration laboratory. The lab's uncertainty must be small enough relative to your instrument's tolerance to give a meaningful TUR.
Traceability
Traceability (metrological traceability) is the property of a measurement result whereby it can be related to a stated reference through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty. VIM 2.41. Traceability is often misunderstood as a property of a lab or a certificate. It is a property of a specific measurement result. The chain runs from the customer's instrument → the lab's reference standard → NMC's primary standard → SI unit definition. Each link in the chain adds uncertainty. Traceability is "unbroken" only if every link is documented with uncertainty, and each instrument in the chain was calibrated before its due date. A gap in the chain (lapsed calibration of any reference) breaks traceability.
True Value
The true value of a measurand is the value consistent with the definition of the quantity being measured. VIM 2.11. In practice, the true value is never exactly known, it can only be estimated. This is the fundamental reason why measurement uncertainty exists: because the true value is inaccessible, every measurement result carries some uncertainty about how close it is to the true value. Calibration compares an instrument's reading against a reference value (the value produced by a traceable reference standard), which is itself an estimate of the true value with its own uncertainty. The better the reference standard's traceability and the lower its uncertainty, the closer its value is to the true value.
U
Uncertainty
Measurement uncertainty is a non-negative parameter characterising the dispersion of the quantity values being attributed to a measurand, based on the information used. VIM 2.26. Uncertainty represents the quantified doubt about a measurement result. A statement of the range within which the true value is believed to lie at a specified confidence level. Uncertainty has two types of component: Type A (evaluated by statistical analysis of repeated measurements) and Type B (evaluated by other means, specifications, calibration certificates, physical limits). Both types are combined using the GUM method to produce the combined standard uncertainty, then multiplied by a coverage factor (typically k=2 for 95% confidence) to give the expanded uncertainty reported on a calibration certificate.
Unit Under Test (UUT)
The Unit Under Test (UUT) is the instrument, device, or component submitted to a calibration laboratory for calibration. The terms UUT, Device Under Test (DUT), and Instrument Under Test (IUT) are used interchangeably across different industries and standards. In military and aerospace metrology, UUT is the most common term. In industrial calibration, DUT or IUT are more typical. The UUT's characteristics (range, accuracy class, resolution, and historical drift data), guide the selection of calibration method, reference standard, number of calibration points, and the uncertainty budget construction. The calibration certificate is issued in the name of the UUT, identified by its serial number and asset ID.
V
VIM (Vocabulaire International de Métrologie)
The VIM (International Vocabulary of Metrology. Basic and General Concepts and Associated Terms) is the internationally agreed vocabulary of metrology terms, published jointly by BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, and OIML. The current edition (VIM3) was published as JCGM 200:2012. VIM definitions are the authoritative source for calibration terminology. When a standard like ISO/IEC 17025 or ISO 9001 uses a metrological term without defining it, the VIM definition applies. The VIM is freely available from the BIPM website. Quality managers and instrument engineers working with calibration documentation benefit from familiarity with VIM definitions, particularly for terms like measurement, uncertainty, traceability, error, and accuracy.
Verification
Verification in the metrology context is the provision of objective evidence that a given item fulfils specified requirements. VIM 2.44. Verification checks whether an instrument is within a specified tolerance. It is a pass/fail determination based on the calibration results. Verification is not the same as calibration: calibration determines the error without necessarily making a conformity judgement; verification applies a decision rule to conclude whether the instrument meets its specification. When ISO/IEC 17025 labs perform a conformity statement, they are performing verification. The verification decision must account for measurement uncertainty. A result must be clearly within tolerance (accounting for uncertainty) to give a valid conforming finding. See also: guardbanding, false accept, false reject.
W
Working Standard
A working standard is a measurement standard used routinely in a laboratory to calibrate or verify measuring instruments. VIM 5.7. Working standards are calibrated against reference standards (which in turn are calibrated against NMC primary standards), placing them lower in the traceability hierarchy. The advantage of working standards is that they can be used frequently and in more varied environments, protecting higher-level reference standards from wear and damage. Working standards must be calibrated at regular intervals appropriate to their drift characteristics, and their uncertainty is larger than the reference standards used to calibrate them. This additional uncertainty propagates to the customer's calibration result.
Z
Zero Point
The zero point of a measurement instrument is the reference point (typically zero of the measurement scale) from which measurements are taken. Zero point calibration (zeroing) is the adjustment of an instrument's output to read zero when the measurand is at its reference condition, for example, setting a pressure gauge to zero at atmospheric pressure, or a balance to zero with no mass on the pan. An unstable zero point (zero drift) is a common cause of measurement error, especially in instruments sensitive to temperature changes. Zero point stability is assessed by monitoring the instrument's zero reading under controlled conditions over time.
Zero Error
Zero error is a specific type of offset error where an instrument reads a non-zero value when the measurand is actually zero. It is the simplest and most common calibration finding. The instrument's display or output is offset by a constant amount because its zero point has drifted. Zero errors are corrected by zero adjustment. On a calibration certificate, a zero error is identified by the reading at the instrument's reference (zero) condition. A large zero error may indicate that the instrument has drifted significantly since its last calibration, or that it was inadvertently knocked or subjected to overload conditions. Zero error differs from span error: zero error is constant across the range, while span error grows proportionally with the measured quantity.
Using this glossary in an audit: When an ISO 9001 or GMP auditor uses calibration terminology during an audit, these definitions (particularly around traceability, measurement uncertainty, and as-found/as-left records), are the reference framework they are working from. Familiarity with these terms helps quality managers respond accurately and confidently to audit questions about their calibration programme.
Frequently asked questions
What is the difference between accuracy and precision in calibration?
Accuracy describes how close a measurement result is to the true value. Precision describes how repeatable the results are. How tightly clustered they are when the measurement is repeated under the same conditions. An instrument can be highly precise (consistent readings) but inaccurate (all readings are offset from the true value). Conversely, results can scatter widely (imprecise) yet average close to the true value. Calibration assesses accuracy; repeatability studies assess precision. Both matter, and they are evaluated separately. Resolution (the number of decimal places displayed) is a third distinct concept: a high-resolution display does not imply either accuracy or precision.
What is measurement uncertainty and why does it matter?
Measurement uncertainty is a quantified estimate of the range within which the true value of a measurement is believed to lie, at a stated confidence level. ISO 9001:2015 clause 7.1.5 requires calibration results to include stated measurement uncertainties. Without this, a calibration certificate does not meet the standard's requirements. Without uncertainty, you cannot determine whether an instrument's deviation is acceptable for its intended tolerance. For example, a temperature instrument reading 0.5°C high may be acceptable, but if the uncertainty is ±1.5°C, the true error could be as large as 2°C, which may exceed your process tolerance. Uncertainty makes measurement results interpretable and defensible.
What is the test uncertainty ratio (TUR) and what should it be?
The Test Uncertainty Ratio (TUR) is the ratio of the tolerance of the Unit Under Test to the expanded uncertainty of the calibration measurement: TUR = Tolerance ÷ Uncertainty. A TUR of 4:1 is the traditional minimum recommended by ANSI/NCSL Z540.3 and ILAC G8. Meaning the calibration system's uncertainty should be no more than one-quarter of the tolerance being verified. A 4:1 TUR gives reasonable confidence that conformity or non-conformity is correctly determined. Where TUR falls below 4:1, guardbanding should be applied to reduce the risk of false accept or false reject decisions. When selecting a calibration laboratory, check that their published uncertainty for your parameter is sufficient to achieve a TUR of at least 4:1 against your instrument's tolerance.
What is the difference between calibration and adjustment?
Calibration is the operation of comparing an instrument's output against a reference standard and documenting the deviation. It determines the error but does not change the instrument. Adjustment is the corrective operation that modifies the instrument's output to bring it into conformance with the reference, it changes the instrument's indication. Not all calibrations include adjustment. Many customers prefer to receive calibration results only, then decide internally whether to adjust based on the magnitude of the error. When adjustment is performed, the certificate must record both as-found (before adjustment) and as-left (after adjustment) readings, so the original condition is permanently documented.
What does "as-found" and "as-left" mean on a calibration certificate?
As-found refers to the instrument's condition and measurement error at the start of calibration, before any corrective action is taken. As-left refers to the condition after any adjustment, repair, or correction has been applied. Both sets of readings are important: as-found data shows whether the instrument was within tolerance during its last service interval. Critical for determining whether any measurements made during that period are valid and whether an OOT (Out-of-Tolerance) investigation is required. As-left data confirms the instrument meets specification before it is returned to service. ISO 9001:2015 clause 7.1.5.3 specifically requires assessment of the validity of previous measurements when an instrument is found to be out of tolerance, making as-found records essential evidence.
What is guardbanding and when is it used?
Guardbanding is the practice of tightening the acceptance zone used during conformity assessment to account for measurement uncertainty. Reducing the risk of incorrectly accepting a non-conforming instrument. Instead of accepting results anywhere within the full specification limits, guardbanding applies an inward offset (commonly equal to the expanded uncertainty U), so an instrument must be clearly within tolerance with room to spare. It is applied when the Test Uncertainty Ratio (TUR) is low (typically below 4:1), when the consequences of a false accept are high, or when regulatory frameworks demand it. ILAC G8:09/2019 provides recommended decision rules. The trade-off is that guardbanding increases the false reject rate. Some conforming instruments will be rejected.
What is GRR (Gauge Repeatability and Reproducibility)?
Gauge R&R (Repeatability and Reproducibility) is a measurement system analysis technique used to quantify how much of the observed variation in measurement results is caused by the measurement system itself, rather than actual variation between parts or products. Repeatability is the variation when the same operator measures the same part multiple times with the same instrument. Reproducibility is the variation between different operators measuring the same part with the same instrument. GRR results are typically expressed as a percentage of tolerance or total variation. A GRR below 10% is generally acceptable; above 30% requires remediation. GRR studies are mandatory in automotive, aerospace, and high-volume electronics manufacturing in Singapore before a measurement system is qualified for production use. Calibration is a prerequisite. GRR cannot be validly conducted on an uncalibrated instrument.
Written by Unitest Instruments
Unitest Instruments Pte. Ltd. is a SAC-SINGLAS accredited calibration laboratory (ISO/IEC 17025, no. LA-2023-0845-C) based in Singapore. We calibrate electrical, temperature, pressure, humidity, and related instruments for manufacturers, service providers, and regulated industries across Singapore and the region. Definitions in this glossary are grounded in VIM3, ISO/IEC 17025:2017, ISO 9001:2015, and GUM.
SAC-SINGLAS accredited calibration for Singapore manufacturers
Electrical, temperature, pressure, dimensional. NMC-traceable, ISO 9001 audit-ready certificates with stated measurement uncertainty.
Verifiable at sac.gov.sg · Acc. No. LA-2023-0845-C