Overview
ISO/TR 18931:2001 - "Imaging materials - Recommendations for humidity measurement and control" provides guidance for measuring and controlling relative humidity (RH) in photographic test chambers and storage areas. The Technical Report addresses hygrometers and humidistats used in photographic standardization, with special attention to cases where standards require control to ±2 % RH or better. It reviews measurement principles, sensor types, calibration approaches and practical installation issues relevant to imaging materials and photographic testing.
Key topics
- Humidity metrics and terms - definitions of relative humidity, dew‑point, wet‑bulb/dry‑bulb, absolute humidity, precision and accuracy.
- Measurement methods - evaluation of principal techniques:
- Dew‑point (condensation) hygrometers: high accuracy; useful as a calibration reference or primary sensor when combined with ambient temperature to compute RH.
- Electric hygrometers (capacitive/resistive): recommended for precision, low cost and responsiveness; suitable for measurement and control when periodically calibrated.
- Wet‑bulb/dry‑bulb (psychrometers): inexpensive but typically limited in accuracy (often not adequate where ±2 % RH is required).
- Gravimetric (primary standard): ultimate accuracy for calibration but time‑consuming and lab‑based.
- Saturated salt solutions and divided‑flow generators: methods for generating known RH environments for testing or equilibration.
- Hair (mechanical) hygrometers: not recommended for applications requiring tight RH tolerances.
- Calibration and traceability - dew‑point calibration (vendor or in‑house) is emphasized; sensors may drift and require periodic verification. Where budget permits, dew‑point combined with temperature measurements may be the preferred sensor system.
- Sensor placement - correct sensor location (avoid intake, dead‑air pockets; consider downstream of equipment or return air ducts) is critical for meaningful RH control.
Practical applications
- Ensuring test conditions for photographic standards (e.g., 23 °C ±1 °C and 50 % RH ±2 %).
- Environmental control and monitoring in film/paper testing laboratories, conservation vaults, museum storage, and manufacturing quality control.
- Selecting and configuring hygrometers and humidistats for climate chambers, data logging, and facility HVAC integration.
Who should use this report
- Photographic standards laboratories and test houses
- Conservation and archiving professionals responsible for imaging materials
- Quality engineers in photographic materials manufacturing
- Facilities managers of storage vaults, museums and imaging archives
Related standards and references
- ISO photographic standards that specify test conditions (temperature and RH)
- Calibration references such as NIST traceable dew‑point certification (not a standard but commonly used for traceability)
Keywords: ISO/TR 18931:2001, humidity measurement and control, imaging materials, relative humidity (RH), hygrometers, humidistats, dew‑point hygrometer, electric hygrometer, storage areas, test chambers, calibration.
