ISO 27894:2009

Overview

ISO 27894:2009 - Vacuum technology: Specifications for hot cathode ionization gauges defines standardized terms and the information manufacturers must provide for hot cathode ionization vacuum gauges. The standard clarifies which gauge parameters and measurement uncertainties are relevant when using these gauges for high and ultra‑high vacuum pressure measurement and for disseminating the pressure scale.

ISO 27894:2009 complements related guidance (e.g., ISO/TS 3567) and references the GUM (ISO/IEC Guide 98-3) for uncertainty expression. It also includes informative annexes (typical Bayard‑Alpert gauge, electrical connections, common problems) to support practical use.

Key topics

• Terms and definitions for gauge components (gauge head, control unit, integrated/separated types, single/combined gauge) and physical parameters (sensitivity, ionization sensitivity, relative sensitivity factor, correction factor, residual current, warm‑up time).
• Principle of operation: electrons emitted by a hot cathode ionize gas molecules; collected ion current is proportional to gas density/pressure.
• Manufacturer specifications required to be supplied (sections 6.1–6.20), including:
  • Gauge type (e.g., Bayard‑Alpert, triode, extractor)
  • Display/output (SI unit Pa, or mapping of electrical signal to pressure)
  • Measurement range and measurement uncertainty (relative standard uncertainty per GUM)
  • Residual current‑equivalent pressure
  • Mechanical fittings, envelope type, maximum bake‑out temperature
  • Filament material, emission current, and warm‑up time
• Optional specifications and additional recommended data for users and calibrators.
• Sources of measurement uncertainty and guidance on evaluating significant influences when operating hot cathode ionization gauges.

Applications and users

ISO 27894 is practical for:

• Gauge manufacturers - to standardize datasheets and user instructions so devices are usable for traceable pressure measurement.
• Calibration laboratories - to perform correct calibrations and to estimate uncertainties when using ionization gauges as reference instruments.
• Vacuum engineers and technicians in semiconductor manufacturing, surface science, thin‑film deposition, accelerator facilities, and research labs who require reliable pressure measurement in high/ultra‑high vacuum conditions.
• Quality and metrology teams that need to document uncertainty budgets and ensure compliance with international measurement best practices.

Related standards

• ISO/TS 3567 - Vacuum gauges: calibration by direct comparison with a reference gauge
• ISO/IEC Guide 98‑3:2008 (GUM) - Guide for expression of measurement uncertainty
• ISO/TS 27893 - Complementary guidance when using ionization gauges as reference standards

By following ISO 27894:2009, organizations can improve traceability, reduce measurement uncertainty, and ensure consistent documentation for hot cathode ionization gauges used in critical vacuum applications.