Réaumur (°Ré) - Unit Information & Conversion
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What is a Réaumur?
The Réaumur scale (symbol: °Ré, °Re, or °R) is a temperature scale where the freezing point of water is defined as 0 degrees and the boiling point of water is 80 degrees under standard atmospheric pressure. Developed by French scientist René-Antoine Ferchault de Réaumur in 1730, this scale was based on the expansion properties of alcohol in glass thermometers rather than mercury. The Réaumur scale gained widespread adoption across continental Europe throughout the 18th and 19th centuries, particularly in France, Germany, Russia, and parts of Italy, where it was used for scientific research, meteorology, industrial processes, and daily temperature measurement. Each Réaumur degree equals 1.25 degrees Celsius (5/4 °C), making it a larger unit than Celsius degrees. While largely replaced by the Celsius scale during the late 19th and 20th centuries, the Réaumur scale persists in specialized applications including traditional Italian Parmigiano-Reggiano cheesemaking, some European confectionery practices, and historical scientific literature.
History of the Réaumur
René-Antoine Ferchault de Réaumur (1683-1757), a French entomologist and physicist, developed his temperature scale in 1730 while investigating the properties of alcohol-based thermometers at the French Academy of Sciences. Unlike Gabriel Fahrenheit's mercury thermometers (1714) or Anders Celsius's later centigrade scale (1742), Réaumur chose diluted ethanol as his thermometric fluid due to its consistent expansion rate and visibility in glass tubes. He fixed the freezing point of water at 0° and initially calibrated the boiling point of his alcohol mixture at 80°, creating an 80-degree span between water's phase transitions. This 0-80 scale proved mathematically convenient for 18th-century scientists using base-8 and duodecimal calculations. Réaumur's thermometers gained immediate acceptance in France and spread to Prussia, the Holy Roman Empire, and Imperial Russia by the mid-18th century. The scale became standard in German-speaking lands, Russian meteorology (until 1917), and French scientific institutions until the late 1800s. France officially replaced Réaumur with Celsius in 1794 during the French Revolution's metric reforms, though Réaumur remained popular in rural areas until the mid-19th century. Germany abandoned it following unification (1871), and Russia converted to Celsius after the 1917 Revolution. Today, the Réaumur scale survives primarily in traditional Italian dairy regulation (Parmigiano-Reggiano DOP requires Réaumur temperature specifications) and appears in historical scientific documents, 18th-19th century literature, and antique thermometer collections.
Quick Answer
Réaumur scale: 0°Ré (water freezes) to 80°Ré (water boils)
One degree Réaumur = 1.25 degrees Celsius. The scale uses an 80-degree span between water's freezing and boiling points, compared to Celsius's 100 degrees.
Quick Comparison Table
| Temperature Reference | Réaumur (°Ré) | Celsius (°C) | Fahrenheit (°F) |
|---|---|---|---|
| Absolute Zero | -218.52°Ré | -273.15°C | -459.67°F |
| Dry Ice Sublimation | -62.2°Ré | -78.5°C | -109.3°F |
| Water Freezes | 0°Ré | 0°C | 32°F |
| Room Temperature | 16°Ré | 20°C | 68°F |
| Human Body | 29.6°Ré | 37°C | 98.6°F |
| Water Boils | 80°Ré | 100°C | 212°F |
| Parmigiano Milk | 26.4-27.2°Ré | 33-34°C | 91.4-93.2°F |
| Candy Soft-Ball | 91.2°Ré | 114°C | 237°F |
Note: Room temperature at 16°Ré demonstrates why Europeans found this scale intuitive for everyday use.
Definition
The Réaumur scale (symbol: °Ré, °Re, or °R) is a temperature scale that divides the interval between the freezing point and boiling point of water into 80 equal divisions under standard atmospheric pressure (1 atmosphere).
Scale Calibration
Fixed Points:
- Freezing point of water: 0 degrees Réaumur (0°Ré)
- Boiling point of water: 80 degrees Réaumur (80°Ré)
- Degree size: Each Réaumur degree = 1.25 Celsius degrees (or 5/4 °C)
Mathematical Relationships:
- Réaumur to Celsius: °C = °Ré × 5/4 (or °Ré × 1.25)
- Celsius to Réaumur: °Ré = °C × 4/5 (or °C × 0.8)
- Réaumur to Fahrenheit: °F = (°Ré × 9/4) + 32 (or °Ré × 2.25 + 32)
- Fahrenheit to Réaumur: °Ré = (°F - 32) × 4/9
Why 80 Degrees?
Réaumur's choice of 80 degrees between water's freezing and boiling points was not arbitrary:
- Base-8 arithmetic: 80 = 10 × 8, facilitating calculations in the duodecimal and base-8 systems common in 18th-century commerce
- Divisibility: 80 has many factors (1, 2, 4, 5, 8, 10, 16, 20, 40, 80), making fraction conversions easier
- Alcohol expansion: Réaumur's diluted ethanol expanded approximately 8% per 10 degrees, making 80 degrees a natural calibration
- Practical range: Most European weather falls between -20°Ré and +30°Ré, yielding manageable numbers
Thermometric Fluid
Unlike Fahrenheit's mercury thermometers, Réaumur used diluted ethanol (alcohol-water mixture) because:
- Consistent expansion: 80 parts expansion per 1000 parts volume per 10°Ré
- Visibility: Clear alcohol + dye was easier to read than mercury
- Lower freezing point: Alcohol mixture remains liquid far below water's freezing point
- Safety: Less toxic than mercury for household thermometers
History
The Réaumur scale's 300-year history mirrors the development of scientific measurement, European political changes, and the eventual triumph of the metric system.
René-Antoine Ferchault de Réaumur (1683-1757)
Born in La Rochelle, France, Réaumur was a polymath whose work spanned entomology, metallurgy, and experimental physics. Appointed to the French Academy of Sciences at age 25 (1708), he gained fame for inventing processes to produce opaque "Réaumur porcelain" glass and techniques for steel production.
1730: Creation of the Scale
Réaumur's thermometer research began as an investigation into improving scientific instruments. His 1730 paper to the French Academy, "Règles pour construire des thermomètres dont les degrés soient comparables" ("Rules for Constructing Thermometers Whose Degrees Are Comparable"), proposed:
- Universal standard: All thermometers should use identical calibration points
- Reproducibility: Freezing and boiling water provided reliable fixed points
- Alcohol-based: Diluted ethanol expansion was more linear than wine spirit
- 80-degree scale: Practical for calculation and measurement precision
Réaumur's thermometers quickly became standard in French scientific institutions, replacing inconsistent instruments calibrated to "blood heat" or "deepest winter cold."
European Adoption (1730-1800)
The Réaumur scale spread across continental Europe within decades:
France (1730-1794):
- French Academy of Sciences adopted Réaumur as standard (1732)
- Weather observations recorded in Réaumur at royal observatories
- Public thermometers in Paris showed Réaumur readings
- Pharmacies and hospitals used Réaumur for medication storage
German States (1740s-1871):
- Prussia adopted Réaumur for meteorological observations (1740s)
- German scientific journals published temperatures in Réaumur
- Instrument makers in Nuremberg, Dresden, and Berlin standardized on Réaumur
- Persisted in German-speaking regions until unification standardization
Imperial Russia (1740s-1917):
- Russian Academy of Sciences adopted Réaumur (1740s)
- St. Petersburg and Moscow weather stations used Réaumur exclusively
- Remained official scale until Bolshevik Revolution (1917)
- Russian literature and documents reference Réaumur (Tolstoy, Dostoevsky novels)
Holy Roman Empire/Austria-Hungary:
- Vienna Observatory used Réaumur (1750s-1870s)
- Austrian meteorological network standardized on Réaumur
- Persisted in rural Austria and Hungary into the early 20th century
Competition with Other Scales (1742-1850)
The mid-18th century saw multiple temperature scales competing:
Fahrenheit (1714): Dominated Britain, Netherlands, and English-speaking world Celsius/Centigrade (1742): Proposed by Anders Celsius, initially inverted (100° freeze, 0° boil) Réaumur (1730): Dominant in France, Germany, Russia, Italy
Scientific preference gradually shifted toward Celsius due to:
- Decimal logic: 100 degrees matched metric system's base-10 philosophy
- International standardization: Celsius gained support from international scientific congresses
- Simplicity: 0-100 was conceptually cleaner than 0-80
Decline and Official Abandonment (1794-1900)
France (1794):
- French Revolution's metric system officially adopted Celsius (centigrade)
- Réaumur declared obsolete by Revolutionary government
- Rural France continued using Réaumur into the 1850s
- Instrument makers produced dual-scale thermometers (Réaumur/Celsius) through 1870s
Germany (1871-1880s):
- German unification prompted measurement standardization
- Meteorological services switched to Celsius (1876)
- Industrial and commercial sectors gradually converted (1880s-1900s)
- Last German Réaumur thermometers manufactured circa 1920
Russia (1917-1920):
- Bolshevik Revolution brought metric system adoption
- Soviet government mandated Celsius for all official purposes (1918-1920)
- Complete conversion by 1925
20th Century Survival (1900-Present)
Despite official abandonment, Réaumur persisted in niche applications:
Italian Dairy (1900s-present):
- Parmigiano-Reggiano DOP (Protected Designation of Origin) regulations specify Réaumur
- Traditional cheesemakers measure milk temperature in Réaumur for authenticity
- Consortium rules reference specific Réaumur temperatures for curd formation
Historical Literature:
- 18th-19th century scientific papers require Réaumur conversion
- Historical meteorological data recorded in Réaumur
- Antique thermometer collecting preserves knowledge
Symbolism and Tradition:
- European heritage associations preserve Réaumur knowledge
- Museum exhibits explaining pre-metric measurement systems
- Educational demonstrations of historical scientific practice
Real-World Examples
Daily Life Temperatures
- Comfortable room: 16°Ré (20°C / 68°F)
- Warm summer day: 24°Ré (30°C / 86°F)
- Cold winter day: -8°Ré (-10°C / 14°F)
- Fever temperature: 32°Ré (40°C / 104°F)
- Hot bath: 36-40°Ré (45-50°C / 113-122°F)
Cheesemaking (Traditional Italian Dairy)
- Parmigiano-Reggiano milk heating: 26.4-27.2°Ré (33-34°C)
- Whey removal temperature: 44-44.8°Ré (55-56°C)
- Curd cooking: 54.4°Ré (68°C)
- Aging room: 12.8-14.4°Ré (16-18°C)
Historical Scientific Examples
- Réaumur's laboratory: 10-14°Ré typical working temperature
- 18th-century Parisian winter: -12 to -16°Ré common (Mercure de France, 1780s)
- Russian steppe summer: 28-32°Ré (35-40°C) recorded by explorers
- German brewery cellars: 6-8°Ré (7.5-10°C) for fermentation
Cooking and Confectionery
- Candy soft-ball stage: 91.2°Ré (114°C / 237°F)
- Caramel temperature: 136°Ré (170°C / 338°F)
- Dutch sugar syrup: 102.4°Ré (128°C) in traditional recipes
- Oil for frying: 144-152°Ré (180-190°C)
Extreme Temperatures (Conversion Reference)
- Liquid nitrogen: -156.6°Ré (-195.8°C)
- Dry ice: -62.2°Ré (-78.5°C)
- Pizza oven: 200-240°Ré (250-300°C)
- Steel melting point: 1,172°Ré (1,465°C)
Common Uses
Historical Scientific Literature
Researchers studying 18th-19th century European science must convert Réaumur temperatures:
Meteorological Records:
- French, German, Russian weather observations (1730-1900)
- Climate historians reconstructing historical weather patterns
- Agricultural records linking crop yields to temperature data
Industrial Documentation:
- Metallurgy research from German states
- French textile dyeing process documentation
- Russian glass manufacturing temperature logs
Italian Artisan Cheesemaking
The Parmigiano-Reggiano Consortium (Consorzio del Formaggio Parmigiano-Reggiano) maintains traditional Réaumur specifications:
DOP Regulations Referencing Réaumur:
- Milk heating: Must reach 26.4-27.2°Ré (33-34°C) before rennet addition
- Curd cutting: Performed at specific Réaumur temperatures
- Whey separation: Temperature-critical step measured in Réaumur
Traditional cheesemakers use antique or reproduction Réaumur thermometers to maintain authenticity and comply with centuries-old recipes. Modern producers convert Celsius measurements but reference Réaumur in documentation.
Antique Thermometer Collecting
Réaumur thermometers are prized collectibles:
Value Factors:
- Age: 18th-century Réaumur thermometers: $500-5,000+
- Maker: Instruments by famous makers (Fahrenheit, Dollond): $2,000-20,000
- Condition: Working alcohol column increases value significantly
- Provenance: Scientific institution provenance adds premium
Collectors seek:
- Wall-mounted wooden-case thermometers (1750-1850)
- Brass-framed scientific instruments
- Dual-scale Réaumur/Celsius transition models (1850-1900)
- Russian Imperial thermometers with Cyrillic markings
Historical Reenactment and Museums
Living history sites and science museums demonstrate Réaumur thermometers:
- Colonial Williamsburg-style European village recreations
- Science museum historical instrument exhibits
- University physics department antique collections
- Historical society educational programs
Literary and Historical Research
Translators and historians must understand Réaumur references in:
Literature:
- Tolstoy's War and Peace: Russian temperatures in Réaumur
- Goethe's scientific writings: Réaumur measurements
- 19th-century French novels: Weather and fever descriptions
Historical Documents:
- Napoleon's Russian campaign weather logs (1812): -30°Ré cold
- French Revolution period documents
- Industrial Revolution factory records
Traditional European Confectionery
Some traditional European candy makers reference Réaumur in heritage recipes:
- Dutch sugar boiling techniques
- French confectionery historical recipes
- German marzipan production documentation (pre-1900)
Modern practitioners convert to Celsius but may cite Réaumur for historical authenticity.
Conversion Guide
Basic Conversion Formulas
Réaumur to Celsius:
- Formula: °C = °Ré × 5/4 (or °Ré × 1.25)
- Example: 16°Ré × 1.25 = 20°C
Celsius to Réaumur:
- Formula: °Ré = °C × 4/5 (or °C × 0.8)
- Example: 25°C × 0.8 = 20°Ré
Réaumur to Fahrenheit:
- Formula: °F = (°Ré × 9/4) + 32 (or °Ré × 2.25 + 32)
- Example: 20°Ré × 2.25 + 32 = 77°F
Fahrenheit to Réaumur:
- Formula: °Ré = (°F - 32) × 4/9
- Example: (68°F - 32) × 4/9 = 16°Ré
Quick Conversion Table
| Réaumur (°Ré) | Celsius (°C) | Fahrenheit (°F) | Description |
|---|---|---|---|
| -40°Ré | -50°C | -58°F | Extreme cold |
| -20°Ré | -25°C | -13°F | Severe winter |
| 0°Ré | 0°C | 32°F | Water freezes |
| 8°Ré | 10°C | 50°F | Cool day |
| 16°Ré | 20°C | 68°F | Room temperature |
| 24°Ré | 30°C | 86°F | Hot summer day |
| 29.6°Ré | 37°C | 98.6°F | Human body |
| 40°Ré | 50°C | 122°F | Very hot |
| 64°Ré | 80°C | 176°F | Near boiling |
| 80°Ré | 100°C | 212°F | Water boils |
Historical Conversion Examples
Russian Winter (Tolstoy's War and Peace):
- Historical: -30°Ré during Napoleon's retreat
- Modern: -30°Ré × 1.25 = -37.5°C = -35.5°F
Parisian Summer (1780s Mercure de France):
- Historical: 25°Ré heat wave
- Modern: 25°Ré × 1.25 = 31.25°C ≈ 88°F
German Brewery (1850s):
- Historical: Lager fermentation at 7°Ré
- Modern: 7°Ré × 1.25 = 8.75°C ≈ 48°F
Cheesemaking Conversion
Parmigiano-Reggiano DOP Specification:
- Traditional: 27°Ré milk temperature
- Modern conversion: 27°Ré × 1.25 = 33.75°C ≈ 92.75°F
Curd cooking:
- Traditional: 54.4°Ré
- Modern: 54.4°Ré × 1.25 = 68°C = 154.4°F
Common Conversion Mistakes
1. Confusing Réaumur Symbol with Rankine
Wrong: Treating °R as Réaumur in American engineering contexts
- °R in US engineering = Rankine (absolute Fahrenheit scale)
- °Ré or °Re = Réaumur (European historical scale)
- Rankine: °R = °F + 459.67 (absolute zero at 0°R)
- Réaumur: °Ré = (°C × 4/5) (water freezes at 0°Ré)
Right: Check context and notation:
- Modern US engineering: °R = Rankine
- European historical documents: °R, °Ré, °Re = Réaumur
- Verify date and location before assuming scale
2. Using Addition Instead of Multiplication
Wrong: Converting Réaumur like Fahrenheit (adding 32)
- Réaumur to Celsius is pure multiplication: °C = °Ré × 1.25
- NO addition/subtraction for Réaumur ↔ Celsius (both use 0° for freezing)
Right:
- Réaumur ↔ Celsius: Multiply only (×1.25 or ×0.8)
- Réaumur ↔ Fahrenheit: Requires both multiply and add (°F = °Ré × 2.25 + 32)
3. Reversing the Multiplication Factor
Wrong: °C = °Ré × 0.8 (backwards)
- This gives Celsius to Réaumur, not Réaumur to Celsius
- 20°Ré × 0.8 = 16 (incorrect; should be 25°C)
Right:
- Réaumur → Celsius: Multiply by 1.25 (or 5/4)
- Celsius → Réaumur: Multiply by 0.8 (or 4/5)
- Remember: Celsius has MORE degrees (100) than Réaumur (80), so Ré × 1.25 = C
4. Forgetting Réaumur is Larger Than Celsius Degrees
Wrong: Thinking 10°Ré is colder than 10°C
- Each Réaumur degree is LARGER (1.25×) than Celsius
- 10°Ré = 12.5°C (warmer, not colder)
Right: Réaumur degrees are 25% larger:
- 1°Ré change = 1.25°C change
- Fewer total degrees (0-80) cover the same range as Celsius (0-100)
5. Applying Celsius Intuition to Réaumur
Wrong: Assuming 20°Ré is "room temperature"
- 20°Ré = 25°C = 77°F (warm, not comfortable room temp)
- Room temperature (20°C) = 16°Ré
Right: Recalibrate intuition:
- 16°Ré = comfortable room (20°C)
- 24°Ré = hot day (30°C)
- 0°Ré = freezing (same as Celsius)
Réaumur Conversion Formulas
To Celsius:
To Fahrenheit:
To Kelvin:
To Rankine:
To Rømer:
To Newton:
To Delisle:
Frequently Asked Questions
Water freezes at 0°Ré and boils at 80°Ré under standard atmospheric pressure (1 atm at sea level). This 80-degree span is the defining feature of the Réaumur scale, compared to Celsius's 100-degree span.
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