Laboratory hydrometers: how can the density of a liquid be measured and the different scales interpreted?

Indispensable for quality control and R&D, the hydrometer — also called an areometer — measures the density (relative density/specific gravity) of a liquid via Archimedes’ principle. In formulation, fermentation, chemistry or agri-food, reading a SG, Brix, Baumé, Plato or API scale informs your decisions in real time. Specialised models include the milk hydrometer (lactometer) and the wine hydrometer (oenological). Choose the right range, scale and accuracy — the repeatability of your measurements depends on it.

What are the different types of hydrometer, and how do you choose the right one?

A hydrometer floats higher or lower depending on the liquid’s density; the equilibrium position indicates the value on a graduated scale. Universal models read relative density (SG) or kg/m³. Others convert directly to Brix/Plato (sugar content) or to Baumé/Oechsle (oenology). Alcoholmeters are used in sugar-free hydroalcoholic solutions (spirits), while the lactometer targets milk (adulteration detection, quality monitoring). Thermohydrometers incorporate a thermometer to apply the correction to 20 °C.

Key selection criteria:
• Scale (SG, Brix/Plato, Baumé/Oechsle, % vol, API) to suit the matrix and industry standard.
• Measurement range and resolution: a low-range hydrometer provides higher accuracy.
• Reference temperature (typically 20 °C) and compensation method (built-in or table-based).
• Compliance (e.g. ASTM E100), length and meniscus readability.
• Materials: laboratory glass, sometimes a protective sleeve, lead-free ballast, chemical compatibility.

Best practice: use a clean tall cylinder, degas fermenting samples, read at the bottom of the meniscus (unless stated otherwise), rinse and dry without leaving a surfactant film.

Type	Principle / scale	Applications
Lactometer (milk hydrometer)	Relative density at 20 °C	Milk quality, detection of added water
Oenological hydrometer	Oechsle / Baumé / Brix	Fermentation monitoring, potential alcohol of the must
Alcoholmeter (spirits)	% vol alcohol (no sugars)	Distilleries, cut and blend control
Hydrometer SG / API	SG 20/20, API	Petroleum, solvents, process baths
Thermohydrometer	Scale + thermometer	Built-in temperature correction

In which fields is a hydrometer used?

• Life sciences / microbiology: fermentation monitoring (density, Brix), media preparation.
• Food & beverage: brewing, cider making, sugar processing, oenology (wine hydrometer), quality control of syrups and juices.
• Pharmaceutical: verification of density for solvents, excipients and oral syrups.
• Chemistry / materials: electroplating baths, acids/bases, solvents; SG/API hydrometers for process control.
• Cosmetics: formulation of lotions and surfactants, in-process control.
• Environment: approximate salinity, density of brackish waters, leachates.
• Quality control: batch conformity, inter-site comparability, traceability with ASTM-compliant hydrometers.
• Dairies: milk hydrometer (lactometer) for goods-in and supplier checks.

Available hydrometer types

Frequently asked questions about hydrometers

How to choose the right scale?
Base your choice on the sample matrix and industry convention: Brix/Plato for sugars, Baumé/Oechsle in oenology, % vol for spirits, SG/API for chemical applications.

Do I need to apply a temperature correction?
Yes. Hydrometers are calibrated at 20 °C. Use a thermo-hydrometer or apply the manufacturer’s correction table.

How do I read the meniscus correctly?
Read at the bottom of the meniscus for clear liquids (unless stated otherwise). Avoid bubbles and wait until buoyancy has stabilised.

Will a spirit hydrometer work in wine?
No, dissolved sugars distort the result. Use an oenological hydrometer (Brix/Baumé/Oechsle) for musts or dedicated methods.

What accuracy should I expect?
Low-range models offer the best resolution. Check ASTM compliance and the tolerance stated on the certificate.

What handling precautions?
Use a clean hydrometer jar (tall cylinder), rinse with alcohol/water, and dry without leaving a film. Handle with care: fragile glass.

Milk hydrometer (lactometer): what are the limits?
Sensitive to temperature and fat content. Measure at 20 °C and use the dedicated tables for reliable results.