Humidity Calibration

Humidity calibration determines the difference between the reading of a relative-humidity instrument and a reference with documented traceability. Measurement is done by comparison in a controlled environment created with a humidity generator or saturated salt solutions.

The aim is for measurement to be reliable in processes where humidity directly affects product quality. In pharmaceutical stability studies, food storage, paper and textile production, and electronic assembly, humidity is as decisive as temperature.

• Digital hygrometers and humidity probes
• Thermohygrometers and data loggers
• Climate and stability chambers
• Clean-room and warehouse humidity-monitoring systems
• Humidity transmitters

• Relative-humidity error: the difference between the instrument's reading and the reference
• Repeatability and settling behaviour
• Humidity distribution within the chamber volume
• Temperature dependence: at the same absolute humidity, a change in temperature changes the relative humidity
• Hysteresis: the difference in reading between rising and falling humidity

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-19 (humidity calibration)
• ISO 17714 (meteorological measurements)
• OIML R121 (saturated salt solutions)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• EURAMET cg-19 (humidity calibration)
• ISO 17714 (meteorological measurements)
• OIML R121 (saturated salt solutions)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe instrument is received and its physical condition and functioning are checked. Any damage or fault is reported before calibration; calibrating a faulty instrument yields no meaningful result.
2. 2Conditioning to Ambient ConditionsThe instrument is left until it reaches equilibrium with the laboratory environment. A measurement taken before the temperature difference from transport has settled is erroneous.
3. 3Preparation of the ReferenceA reference thermometer, dry block or climatic chamber with documented traceability is brought to a stable condition.
4. 4Selection of the Measurement PointsPoints representing the instrument's working range are selected; where the customer has stated a critical operating point, that point is always included.
5. 5Reading at Steady StateAt each point, thermal equilibrium between reference and instrument is awaited and repeated readings are taken. This waiting time is the most critical step in thermal calibration.
6. 6Calculation of the DeviationThe difference between the instrument's indication and the reference value is calculated for each point.
7. 7Building the Uncertainty BudgetThe expanded uncertainty is calculated by combining the contributions of the reference uncertainty, repeatability, resolution, homogeneity and stability.
8. 8Issuing the CertificateMeasurement results, deviations and uncertainty values are entered on the certificate; where requested, a statement of conformity is added using an agreed 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 hygrometer's calibration uncertainty and drift
• The humidity generator's stability and homogeneity
• Temperature stability — the largest contributor in humidity measurement
• Settling time and hysteresis
• Instrument resolution 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 temperature values at the measurement points (inseparable from the humidity result)
• The method used: a humidity generator or a saturated salt solution
• Settling times

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.

• Temperature Calibration