Pressure Calibration

Pressure calibration determines the difference between a pressure instrument's reading and a reference with documented traceability. A deadweight tester (pressure balance) or a high-accuracy digital reference is used as the reference.

The aim is for measurement to be reliable where pressure determines safety and process control. Pressure-vessel safety-valve setting, hydrostatic testing and process control rest directly on this measurement.

• Analogue and digital pressure gauges
• Pressure transmitters and sensors
• Vacuum gauges
• Pressure switches and pressostats
• Differential pressure gauges
• Hydrostatic test pumps and calibrators

• Indication error: with rising and falling pressure
• Hysteresis: the difference between rising and falling direction at the same point
• Repeatability
• Zero drift
• Linearity error
• The set and reset points of pressure switches

The calibration interval is not fixed by any standard and there is no general rule such as "once a year". The interval is set by weighing together the manufacturer's recommendation, how often and under what conditions the instrument is used, how critical the measurement is, the drift shown by the instrument's previous calibration results, and any legal requirement. ILAC-G24 / OIML D10 describe the methods for setting it. The sound approach is to start with a reasonable interval for the first period and then lengthen or shorten it in the light of subsequent results: an instrument that proves stable can have its interval extended, one that drifts has it shortened.

• ISO/IEC 17025
• EURAMET cg-17 (pressure balance calibration)
• EN 837-1 (Bourdon-tube pressure gauges)
• DKD-R 6-1 (pressure measuring instruments)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• EURAMET cg-17 (pressure balance calibration)
• EN 837-1 (Bourdon-tube pressure gauges)
• DKD-R 6-1 (pressure measuring instruments)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe physical condition of the instrument is examined for wear and damage. On dimensional instruments the cleanliness of the measuring faces is also checked.
2. 2Conditioning to Ambient ConditionsThe instrument is brought to equilibrium at laboratory temperature. In dimensional measurement a temperature difference turns directly into measurement error through thermal expansion of the material.
3. 3Preparation of the Reference StandardsWeights, gauge blocks, a dead-weight tester or a reference transducer with documented traceability are prepared.
4. 4Preloading and Zero CheckIn force, torque and weighing calibrations the instrument is taken up to full load and back down several times; the zero point is then checked.
5. 5Readings at the Measurement PointsReadings are taken in increasing and decreasing directions at points representing the measuring range; hysteresis is determined in this way.
6. 6Repeatability MeasurementsThe same point is measured repeatedly; in weighing calibration an eccentricity (off-centre load) check is also performed.
7. 7Application of CorrectionsCorrections for local gravity, air buoyancy and temperature are taken into account; ignoring them produces significant deviation in precision measurements.
8. 8Uncertainty and CertificateThe uncertainty budget is established and the results are entered on the certificate; where requested, a statement of conformity is added with a decision rule.

Every calibration result is given together with a measurement uncertainty; a result reported without its uncertainty is incomplete. The uncertainty is budgeted following the GUM (JCGM 100) approach: the uncertainty of the reference, repeatability, resolution, environmental effects and the stability of the instrument are combined. The certificate states the expanded uncertainty with the coverage factor (usually k = 2) and the approximate level of confidence. The smallest uncertainty a laboratory can report is limited by its calibration and measurement capability (CMC) in the accredited scope; no better uncertainty may be declared.

• The reference instrument's calibration uncertainty and drift
• The local value of gravitational acceleration (for deadweight systems)
• The temperature effect and thermal expansion of the weight material
• The head-height (column) correction
• Instrument resolution and reading error
• Hysteresis and repeatability

• Laboratory details and certificate number
• The customer and identification of the instrument calibrated: make, model, serial number
• Date of calibration and date of issue of the certificate
• The calibration method applied or reference to the procedure
• Reference standards used and traceability information
• Ambient conditions during calibration (temperature, humidity and, where relevant, pressure)
• Measurement points, reference values and instrument readings
• The deviations calculated
• Expanded measurement uncertainty and coverage factor
• The personnel who performed and who approved the calibration
• The pressure unit and the absolute/gauge distinction
• The fluid used: hydraulic oil, water or gas
• Gravitational acceleration and the corrections applied

A calibration certificate is NOT a certificate of conformity. Calibration measures and reports the deviation of the instrument; it does not correct that deviation and does not say that the instrument is "correct". To decide whether an instrument is fit for use, a tolerance (acceptance criterion) must be defined — and it is the user who sets that tolerance, not the laboratory. If a statement of conformity is requested, ISO/IEC 17025 clause 7.8.6 requires a DECISION RULE to be agreed in advance, defining how measurement uncertainty is taken into account in the decision; a conformity statement made without regard to uncertainty is misleading. The certificate also shows the condition AT THE TIME of calibration: later use, transport or shock can change the instrument's condition, and the certificate does not cover that. Where a next calibration date appears on the certificate, it is not the laboratory's decision but a reminder entered according to the interval stated by the customer.

Calibration is carried out under ISO/IEC 17025, which requires the laboratory's technical competence, impartiality and metrological traceability. Accreditation is granted per METHOD and per MEASUREMENT RANGE: a quantity or range that does not appear in the laboratory's scope annex is not accredited, and no certificate bearing the accreditation mark may be issued for that measurement. The scope also states the smallest uncertainty the laboratory can offer (CMC); no better uncertainty may be declared. EUROLAB holds internationally recognised accreditation under ISO/IEC 17025, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; whether your instrument can be calibrated within accredited scope is confirmed in writing at the quotation stage.

• Mass Calibration
• Balance Calibration
• Force Calibration
• Torque Calibration
• Dimensional Calibration