Overview
ISO 387:1977 - Hydrometers: Principles of construction and adjustment - defines the requirements for glass hydrometers of constant mass that do not incorporate a thermometer. The standard specifies the basis of scale, reference temperature, surface tension categories for adjustment, reference reading levels, and essential requirements for shape, materials, calibration conditions, graduations and inscriptions. Annex A tabulates standard surface‑tension categories; Annex B explains why density is the preferred basis of hydrometer scales.
Key topics and technical requirements
- Scope: Glass hydrometers, constant mass, without built‑in thermometers.
- Basis of scale: Preferentially density (kg/m3); relative density (to water) is permitted but less preferred (Annex B explains advantages of density).
- Reference temperature: Standard reference for density hydrometers is 20 °C (alternatives: 15 °C or 27 °C in special cases). For relative density scales the reference temperature is 60 °F (15.56 °C).
- Surface tension: Hydrometers must be adjusted for surface tension. Annex A defines standard surface‑tension categories (low/medium/high) and example liquids (organic liquids, oils, aqueous solutions, acids); precise mN/m values can be marked on the instrument.
- Calibration conditions: Emergent stem dry except near meniscus; stable meniscus behavior when displaced.
- Materials & workmanship:
- Transparent, low‑stress glass with cubic thermal expansion coefficient (25 ± 2) × 10‑6 °C‑1 (per ISO 1768).
- Loading material fixed in the bulb base; mercury is prohibited.
- Stem must remain essentially vertical (recommended max tilt 1.5°) and extend at least 15 mm above top graduation.
- Shape & stem: Symmetrical outer surface; tapered bulb design preferred to avoid trapped air.
- Scale & graduations: Rules for line spacing (centers 0.8–3.0 mm, preferably 1.2–2.0 mm), long/medium/short line sequencing per smallest interval, clear figured numbers and vertical alignment.
- Inscriptions: Must include basis of scale (e.g., “kg/m3”), standard reference temperature, surface tension or category, maker ID, instrument ID and reference to the standard.
Applications and users
ISO 387 is used by:
- Hydrometer manufacturers (design, marking and production).
- Calibration and test laboratories (verification and adjustment).
- Quality control and R&D in industries handling liquids: petroleum, chemicals, oils, alcohols, food & beverage, fermentation and laboratory services.
- Regulatory and trade bodies concerned with accurate density/relative density measurement.
Practical benefits include improved interoperability of instruments, consistent readings across liquids (with surface‑tension adjustment), and traceable manufacture/calibration practices.
Related standards
- ISO 1768 - referenced for the conventional value of the glass cubic thermal expansion coefficient.
- A corresponding ISO for hydrometers with incorporated thermometers is noted (separate document).
Keywords: ISO 387:1977, hydrometers, glass hydrometer, density scale, surface tension, calibration, reference temperature, hydrometer graduations.