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Material Properties constants

82 values, each with its units, its uncertainty, and where it came from.

Electromagnetic 6

Relative permittivity of air measured

εr,air=1.00059 —\varepsilon_{r,\mathrm{air}} = 1.00059\ \text{—}

—Typical dielectric constant of dry air at 0 °C and one atmosphere — so close to vacuum that most capacitor and antenna work ignores the difference.

Relative permittivity of water measured

εr,H2O=80.1 —\varepsilon_{r,\mathrm{H_2O}} = 80.1\ \text{—}

—Typical static dielectric constant of liquid water at 20 °C — an outlier among common liquids and the reason water dissolves salts so well.

Relative permittivity of PTFE measured

εr,PTFE=2.1 —\varepsilon_{r,\mathrm{PTFE}} = 2.1\ \text{—}

—Typical dielectric constant of PTFE (Teflon), about 2.1 and almost flat from DC to tens of gigahertz — the benchmark low-loss RF insulator.

Relative permittivity of FR-4 measured

εr,FR-4=4.4 —\varepsilon_{r,\mathrm{FR\text{-}4}} = 4.4\ \text{—}

—Typical dielectric constant of FR-4 circuit-board laminate near 1 GHz — the number behind every microstrip impedance and trace-delay calculation.

Relative permeability of iron measured

μr,Fe=5000 —\mu_{r,\mathrm{Fe}} = 5000\ \text{—}

—Typical maximum relative permeability of commercial soft iron — a wildly variable figure spanning roughly 200 to 5000 with purity and field level.

Relative permeability of mu-metal measured

μr,μ-metal=80,000 —\mu_{r,\mu\text{-metal}} = 80,000\ \text{—}

—Typical relative permeability of annealed mu-metal in weak fields — the nickel-iron alloy used to shield instruments from stray magnetic fields.

Thermodynamic 7

Normal boiling point of water

Tb=373.1243 KT_{\mathrm{b}} = 373.1243\ \text{K}

KWater boils at 99.9743 °C (373.1243 K) under one standard atmosphere — very slightly below 100 °C, and not by accident.

Specific heat capacity of liquid water

cp,water=4181.6 J/(kg⋅K)c_{p,\mathrm{water}} = 4181.6\ \text{J/(kg}{\cdot}\text{K)}

J/(kg·K)4181.6 J/(kg·K) for liquid water at 25 °C and 0.1 MPa — about 1.00 BTU/(lb·°F), the highest of any common liquid.

Specific heat capacity of ice

cp,ice=2108 J/(kg⋅K)c_{p,\mathrm{ice}} = 2108\ \text{J/(kg}{\cdot}\text{K)}

J/(kg·K)About 2108 J/(kg·K) for ice at 0 °C, roughly half the value for liquid water — the reason freezers cool loads far faster than they freeze them.

Latent heat of fusion of water

Lf=333,550 J/kgL_{\mathrm{f}} = 333,550\ \text{J/kg}

J/kg333.55 kJ/kg (143.4 BTU/lb) to melt ice at 0 °C without changing its temperature — equivalent to 80 K of sensible heating of water.

Latent heat of vaporisation of water

Lv=2,256,400 J/kgL_{\mathrm{v}} = 2,256,400\ \text{J/kg}

J/kg2256.4 kJ/kg (970 BTU/lb) to boil water at 100 °C and 1 atm, nearly seven times the heat of fusion and the basis of all steam heating.

Maximum density of water (4 °C) measured

ρmax⁡=999.975 kg/m3\rho_{\max} = 999.975\ \text{kg/m}^{3}

kg/m³Water is densest at about 3.98 °C, 999.975 kg/m³ — the anomaly that makes ice float and keeps deep lakes from freezing solid.

Density of water at 20 °C measured

ρ20=998.207 kg/m3\rho_{20} = 998.207\ \text{kg/m}^{3}

kg/m³998.207 kg/m³ at 20 °C and 1 atm (62.316 lb/ft³), the reference density behind specific gravity and most laboratory calibrations.

Material Properties 69

Density of Seawater measured

ρsw=1025 kg/m3\rho_{\mathrm{sw}} = 1025\ \text{kg/m}^{3}

kg/m³Representative density of open-ocean seawater at 35 g/kg salinity and 15 °C, about 1025 kg/m³ or 64 lb/ft³ at the surface.

Density of Dry Air at 20 °C measured

ρair=1.204 kg/m3\rho_{\mathrm{air}} = 1.204\ \text{kg/m}^{3}

kg/m³Density of dry air at 20 °C and 101.325 kPa, 1.204 kg/m³ — the standard-air value behind the 1.08 sensible-heat factor.

Density of Structural Steel measured

ρsteel=7850 kg/m3\rho_{\mathrm{steel}} = 7850\ \text{kg/m}^{3}

kg/m³Typical density of carbon and low-alloy structural steel at 20 °C, 7850 kg/m³ or 490 lb/ft³, essentially independent of grade.

Density of Aluminium Alloy 6061 measured

ρAl=2700 kg/m3\rho_{\mathrm{Al}} = 2700\ \text{kg/m}^{3}

kg/m³Typical density of 6061 aluminium alloy at 20 °C, 2700 kg/m³ or 169 lb/ft³ — about 35 % of steel for the same volume.

Density of Reinforced Concrete measured

ρRC=2400 kg/m3\rho_{\mathrm{RC}} = 2400\ \text{kg/m}^{3}

kg/m³Typical density of normal-weight reinforced concrete, 2400 kg/m³ or 150 lb/ft³, including ordinary reinforcing steel content.

Density of Ice at 0 °C measured

ρice=917 kg/m3\rho_{\mathrm{ice}} = 917\ \text{kg/m}^{3}

kg/m³Density of ordinary hexagonal ice at 0 °C and 1 atm, about 917 kg/m³ — roughly 8 % lighter than the water it freezes from.

Density of Mercury at 20 °C measured

ρHg=13,534 kg/m3\rho_{\mathrm{Hg}} = 13,534\ \text{kg/m}^{3}

kg/m³Density of liquid mercury at 20 °C and 1 atm, 13 534 kg/m³ — 13.5 times water, and the basis of the mmHg pressure unit.

Density of Softwood Timber measured

ρwood=500 kg/m3\rho_{\mathrm{wood}} = 500\ \text{kg/m}^{3}

kg/m³Representative density of construction softwood such as spruce-pine-fir or Douglas fir at 12 % moisture, roughly 500 kg/m³.

Young's Modulus of Structural Steel measured

Esteel=2×1011 PaE_{\mathrm{steel}} = 2 \times 10^{11}\ \text{Pa}

PaElastic modulus of carbon and low-alloy structural steel at room temperature, 200 GPa or 29 000 ksi, effectively grade-independent.

Young's Modulus of Type 304 Stainless Steel measured

ESS304=1.93×1011 PaE_{\mathrm{SS304}} = 1.93 \times 10^{11}\ \text{Pa}

PaElastic modulus of annealed Type 304 austenitic stainless steel at 20 °C, about 193 GPa or 28 000 ksi — 3 % below carbon steel.

Young's Modulus of Aluminium Alloy 6061 measured

EAl=6.89×1010 PaE_{\mathrm{Al}} = 6.89 \times 10^{10}\ \text{Pa}

PaElastic modulus of 6061 aluminium at room temperature, 68.9 GPa or 10 000 ksi — roughly one third the stiffness of steel.

Young's Modulus of Normal-Weight Concrete measured

Ec=2.5×1010 PaE_{c} = 2.5 \times 10^{10}\ \text{Pa}

PaSecant elastic modulus of 28 MPa (4000 psi) normal-weight concrete, about 25 GPa — computed from strength, not measured directly.

Shear Modulus of Structural Steel measured

Gsteel=7.72×1010 PaG_{\mathrm{steel}} = 7.72 \times 10^{10}\ \text{Pa}

PaShear (rigidity) modulus of structural steel at room temperature, 77.2 GPa or 11 200 ksi — the value used in torsion and shear.

Shear Modulus of Aluminium Alloy measured

GAl=2.6×1010 PaG_{\mathrm{Al}} = 2.6 \times 10^{10}\ \text{Pa}

PaShear modulus of common wrought aluminium alloys at room temperature, about 26 GPa or 3800 ksi — one third of steel's value.

Poisson's Ratio of Steel measured

νsteel=0.3 —\nu_{\mathrm{steel}} = 0.3\ \text{—}

—Poisson's ratio of carbon and alloy steel in the elastic range, 0.30 — the lateral contraction per unit of axial extension.

Poisson's Ratio of Aluminium measured

νAl=0.33 —\nu_{\mathrm{Al}} = 0.33\ \text{—}

—Poisson's ratio of wrought aluminium alloys in the elastic range, about 0.33 — slightly higher than steel's 0.30.

Poisson's Ratio of Concrete measured

νc=0.2 —\nu_{c} = 0.2\ \text{—}

—Poisson's ratio of hardened normal-weight concrete under service compression, typically 0.15–0.25 with 0.20 used in design.

Poisson's Ratio of Rubber measured

νrubber=0.499 —\nu_{\mathrm{rubber}} = 0.499\ \text{—}

—Poisson's ratio of natural and synthetic rubber, about 0.499 — nearly incompressible, the practical limit for isotropic solids.

Yield Strength of ASTM A36 Steel

Fy,A36=248,000,000 PaF_{y,\mathrm{A36}} = 248,000,000\ \text{Pa}

PaSpecified minimum yield strength of ASTM A36 structural steel, 36 ksi or 248 MPa — a floor guaranteed by the mill, not a measurement.

Yield Strength of ASTM A992 Steel

Fy,A992=345,000,000 PaF_{y,\mathrm{A992}} = 345,000,000\ \text{Pa}

PaSpecified minimum yield strength of ASTM A992 wide-flange steel, 50 ksi or 345 MPa — the default grade for W-shapes since 1998.

Yield Strength of 6061-T6 Aluminium measured

Fy,6061-T6=276,000,000 PaF_{y,\mathrm{6061\text{-}T6}} = 276,000,000\ \text{Pa}

PaTypical 0.2 % offset yield strength of 6061-T6 aluminium, 276 MPa or 40 ksi, with an ultimate tensile strength near 310 MPa.

Compressive Strength of Normal-Weight Concrete measured

fc′=28,000,000 Paf'_{c} = 28,000,000\ \text{Pa}

PaTypical specified 28-day cylinder strength of ordinary structural concrete, about 28 MPa (4000 psi), with 20–40 MPa the usual range.

Tensile Strength of a Grade 5 Bolt

Sut,Gr.5=827,000,000 PaS_{ut,\mathrm{Gr.5}} = 827,000,000\ \text{Pa}

PaMinimum ultimate tensile strength of an SAE Grade 5 bolt up to 1 in diameter, 120 ksi or 827 MPa, with 85 ksi proof strength.

Tensile Strength of a Grade 8 Bolt

Sut,Gr.8=1,034,000,000 PaS_{ut,\mathrm{Gr.8}} = 1,034,000,000\ \text{Pa}

PaMinimum ultimate tensile strength of an SAE Grade 8 bolt, 150 ksi or 1034 MPa, with 130 ksi proof — the high-strength shop fastener.

Thermal Conductivity of Copper measured

kCu=401 W/(m⋅K)k_{\mathrm{Cu}} = 401\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of pure annealed copper at 25 °C, about 401 W/(m·K) — the benchmark for practical heat-transfer materials.

Thermal Conductivity of Aluminium measured

kAl=237 W/(m⋅K)k_{\mathrm{Al}} = 237\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of pure aluminium at 25 °C, about 237 W/(m·K); alloy 6061-T6 is markedly lower at roughly 167 W/(m·K).

Thermal Conductivity of Carbon Steel measured

ksteel=50 W/(m⋅K)k_{\mathrm{steel}} = 50\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of plain carbon steel near room temperature, roughly 50 W/(m·K) — about an eighth of copper's value.

Thermal Conductivity of Type 304 Stainless Steel measured

kSS304=16.2 W/(m⋅K)k_{\mathrm{SS304}} = 16.2\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of Type 304 austenitic stainless steel at 20 °C, about 16 W/(m·K) — roughly a third of carbon steel's.

Thermal Conductivity of Water measured

kw=0.598 W/(m⋅K)k_{w} = 0.598\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of liquid water at 20 °C and 1 atm, about 0.60 W/(m·K) — high for a liquid, still 700× worse than copper.

Thermal Conductivity of Air measured

kair=0.0257 W/(m⋅K)k_{\mathrm{air}} = 0.0257\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of dry air at 20 °C and 1 atm, about 0.026 W/(m·K) — the benchmark every insulation is measured against.

Thermal Conductivity of Fibreglass Batt Insulation measured

kfg=0.04 W/(m⋅K)k_{\mathrm{fg}} = 0.04\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of standard fibreglass batt at 24 °C, about 0.040 W/(m·K) — roughly R-3.6 per inch in US units.

Thermal Conductivity of Rigid Foam Board measured

kfoam=0.024 W/(m⋅K)k_{\mathrm{foam}} = 0.024\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Aged thermal conductivity of rigid polyisocyanurate or XPS board, roughly 0.024–0.029 W/(m·K), about R-5 to R-6 per inch.

Thermal Conductivity of Concrete measured

kc=1.7 W/(m⋅K)k_{c} = 1.7\ \text{W/(m}{\cdot}\text{K)}

W/(m·K)Thermal conductivity of normal-weight structural concrete, roughly 1.4–2.0 W/(m·K) depending on aggregate and moisture content.

Specific Heat of Carbon Steel measured

csteel=486 J/(kg⋅K)c_{\mathrm{steel}} = 486\ \text{J/(kg}{\cdot}\text{K)}

J/(kg·K)Specific heat of plain carbon steel near room temperature, about 486 J/(kg·K) or 0.116 BTU/(lb·°F), rising with temperature.

Specific Heat of 30 % Propylene Glycol measured

cPG30=3850 J/(kg⋅K)c_{\mathrm{PG30}} = 3850\ \text{J/(kg}{\cdot}\text{K)}

J/(kg·K)Specific heat of a 30 % by volume propylene glycol/water mix near 40 °C, about 3850 J/(kg·K) — some 8 % below plain water.

Thermal Expansion Coefficient of Carbon Steel measured

αsteel=0.0000117 1/K\alpha_{\mathrm{steel}} = 0.0000117\ \text{1/K}

1/KLinear thermal expansion coefficient of carbon steel near room temperature, 11.7 µm/(m·K) or 6.5 µin/(in·°F).

Thermal Expansion Coefficient of Type 304 Stainless measured

αSS304=0.0000173 1/K\alpha_{\mathrm{SS304}} = 0.0000173\ \text{1/K}

1/KLinear expansion coefficient of Type 304 austenitic stainless steel, 17.3 µm/(m·K) — about 50 % more than carbon steel.

Thermal Expansion Coefficient of Aluminium measured

αAl=0.0000234 1/K\alpha_{\mathrm{Al}} = 0.0000234\ \text{1/K}

1/KLinear expansion coefficient of aluminium near room temperature, 23.4 µm/(m·K) or 13 µin/(in·°F) — twice that of steel.

Thermal Expansion Coefficient of Copper measured

αCu=0.0000168 1/K\alpha_{\mathrm{Cu}} = 0.0000168\ \text{1/K}

1/KLinear expansion coefficient of copper near room temperature, 16.8 µm/(m·K) or 9.3 µin/(in·°F) — 44 % more than steel.

Thermal Expansion Coefficient of Concrete measured

αc=0.00001 1/K\alpha_{c} = 0.00001\ \text{1/K}

1/KLinear expansion coefficient of normal-weight concrete, roughly 8–12 µm/(m·K) — close enough to steel to make reinforcing work.

Thermal Expansion Coefficient of PVC measured

αPVC=0.000054 1/K\alpha_{\mathrm{PVC}} = 0.000054\ \text{1/K}

1/KLinear expansion coefficient of rigid PVC pipe, about 54 µm/(m·K) or 3.0 µin/(in·°F) — nearly five times that of steel.

Dynamic Viscosity of Water at 20 °C measured

μw=0.001002 Pa⋅s\mu_{w} = 0.001002\ \text{Pa}{\cdot}\text{s}

Pa·sDynamic viscosity of pure water at 20 °C and 1 atm, 1.002 mPa·s — the value that made the centipoise a de facto standard.

Dynamic Viscosity of Air at 20 °C measured

μair=0.0000181 Pa⋅s\mu_{\mathrm{air}} = 0.0000181\ \text{Pa}{\cdot}\text{s}

Pa·sDynamic viscosity of dry air at 20 °C and 1 atm, 18.1 µPa·s — about 1/55 of water's, though its kinematic viscosity is 15× larger.

Dynamic Viscosity of SAE 30 Oil measured

μSAE30=0.088 Pa⋅s\mu_{\mathrm{SAE30}} = 0.088\ \text{Pa}{\cdot}\text{s}

Pa·sDynamic viscosity of a typical SAE 30 mineral engine oil at 40 °C, roughly 0.088 Pa·s — about 90 times that of water at 20 °C.

Electrical Resistivity of Copper at 20 °C measured

ρCu=1.678×10−8 Ω⋅m\rho_{\mathrm{Cu}} = 1.678 \times 10^{-8}\ \text{Ω}{\cdot}\text{m}

Ω·mResistivity of pure annealed copper at 20 °C, 1.678 × 10⁻⁸ Ω·m; the commercial IACS reference is 1.7241 × 10⁻⁸ Ω·m.

Electrical Resistivity of Aluminium at 20 °C measured

ρAl=2.65×10−8 Ω⋅m\rho_{\mathrm{Al}} = 2.65 \times 10^{-8}\ \text{Ω}{\cdot}\text{m}

Ω·mResistivity of pure aluminium at 20 °C, 2.65 × 10⁻⁸ Ω·m; EC-grade 1350 conductor is about 2.83 × 10⁻⁸ Ω·m (61 % IACS).

Electrical Resistivity of Carbon Steel measured

ρsteel=1.6×10−7 Ω⋅m\rho_{\mathrm{steel}} = 1.6 \times 10^{-7}\ \text{Ω}{\cdot}\text{m}

Ω·mResistivity of plain carbon steel at 20 °C, roughly 1.4–1.8 × 10⁻⁷ Ω·m — about ten times copper's, and composition-sensitive.

Temperature Coefficient of Resistance, Copper measured

αCu=0.00393 1/K\alpha_{\mathrm{Cu}} = 0.00393\ \text{1/K}

1/KTemperature coefficient of resistance for annealed copper referenced to 20 °C, 0.00393 per kelvin — 0.393 % more resistance per degree.

Speed of Sound in Dry Air at 20 °C measured

cair=343.2 m/sc_{\mathrm{air}} = 343.2\ \text{m/s}

m/sSpeed of sound in dry air at 20 °C and 1 atm, 343 m/s or 1125 ft/s — set by temperature, essentially not by pressure.

Speed of Sound in Water at 20 °C measured

cw=1482 m/sc_{w} = 1482\ \text{m/s}

m/sSpeed of sound in fresh water at 20 °C and 1 atm, about 1482 m/s — 4.3 times faster than in air, and rising with temperature.

Speed of Sound in Steel measured

csteel=5900 m/sc_{\mathrm{steel}} = 5900\ \text{m/s}

m/sLongitudinal (bulk) wave speed in carbon steel, about 5900 m/s; the thin-bar wave speed √(E/ρ) is lower, near 5100 m/s.

Friction Coefficient, Dry Steel on Steel measured

μsteel/steel=0.6 —\mu_{\mathrm{steel/steel}} = 0.6\ \text{—}

—Representative static friction coefficient for clean dry steel on steel, about 0.6 with a legitimate range of 0.4 to 0.8.

Friction Coefficient, Lubricated Steel on Steel measured

μsteel/steel,lub=0.1 —\mu_{\mathrm{steel/steel,lub}} = 0.1\ \text{—}

—Representative friction coefficient for oil-lubricated steel on steel in boundary lubrication, about 0.10 (range 0.05–0.15).

Friction Coefficient, Rubber Tyre on Dry Asphalt measured

μdry=0.8 —\mu_{\mathrm{dry}} = 0.8\ \text{—}

—Representative peak friction coefficient for a passenger tyre on dry asphalt, about 0.8 — competition tyres exceed 1.0.

Friction Coefficient, Rubber Tyre on Wet Asphalt measured

μwet=0.5 —\mu_{\mathrm{wet}} = 0.5\ \text{—}

—Representative friction coefficient for a passenger tyre on wet asphalt, about 0.5, falling to 0.3 or below on worn tyres.

Emissivity of Polished Aluminium measured

εAl=0.05 —\varepsilon_{\mathrm{Al}} = 0.05\ \text{—}

—Total hemispherical emissivity of bright polished aluminium near room temperature, about 0.05 — an excellent radiant barrier.

Emissivity of Oxidised Steel measured

εsteel=0.8 —\varepsilon_{\mathrm{steel}} = 0.8\ \text{—}

—Total emissivity of oxidised or mill-scaled carbon steel, about 0.8 — against roughly 0.1 for the same steel polished bright.

Emissivity of Flat Black Paint measured

εblack=0.96 —\varepsilon_{\mathrm{black}} = 0.96\ \text{—}

—Total hemispherical emissivity of flat black paint near room temperature, about 0.96 — the practical stand-in for a black body.

Absolute Roughness of Commercial Steel Pipe measured

ϵsteel=0.000046 m\epsilon_{\mathrm{steel}} = 0.000046\ \text{m}

mAbsolute roughness ε of new commercial steel or wrought-iron pipe, 0.046 mm (0.00015 ft) — the Moody-chart default.

Absolute Roughness of Drawn Tubing and Plastic Pipe measured

ϵtube=0.0000015 m\epsilon_{\mathrm{tube}} = 0.0000015\ \text{m}

mAbsolute roughness ε of drawn copper tube, glass and smooth plastic pipe, about 0.0015 mm — 30 times smoother than steel.

Absolute Roughness of Cast Iron Pipe measured

ϵCI=0.00026 m\epsilon_{\mathrm{CI}} = 0.00026\ \text{m}

mAbsolute roughness ε of new uncoated cast iron pipe, about 0.26 mm — five times rougher than steel, and far worse when tuberculated.

Hazen-Williams C for Plastic Pipe measured

CPVC=150 —C_{\mathrm{PVC}} = 150\ \text{—}

—Hazen-Williams roughness coefficient for smooth plastic pipe such as PVC, PE and CPVC, conventionally taken as 150 for design.

Hazen-Williams C for Cast Iron Pipe measured

CCI=100 —C_{\mathrm{CI}} = 100\ \text{—}

—Hazen-Williams roughness coefficient for aged unlined cast iron pipe, typically 100 — new pipe is near 130, badly tuberculated near 60.

Manning's n for Concrete Channels measured

nconcrete=0.013 —n_{\mathrm{concrete}} = 0.013\ \text{—}

—Manning roughness coefficient for finished concrete channels and pipe, typically 0.013 with a defensible range of 0.011 to 0.016.

Unit Weight of Loose Sand measured

γsand=15,500 N/m3\gamma_{\mathrm{sand}} = 15,500\ \text{N/m}^{3}

N/m³Typical moist bulk unit weight of loose sand, about 15.5 kN/m³ or 99 lbf/ft³; dense sand runs nearer 19.5 kN/m³.

Unit Weight of Soft Clay measured

γclay=16,000 N/m3\gamma_{\mathrm{clay}} = 16,000\ \text{N/m}^{3}

N/m³Typical saturated unit weight of soft normally consolidated clay, about 16 kN/m³ or 102 lbf/ft³, with 14–18 kN/m³ the usual range.

Specific Gravity of Soil Solids measured

Gs=2.65 —G_{s} = 2.65\ \text{—}

—Specific gravity of the mineral solids in most soils, about 2.65 for quartz sands and 2.70–2.75 for clays — remarkably consistent.

Bulk Modulus of Water measured

Kw=2,180,000,000 PaK_{w} = 2,180,000,000\ \text{Pa}

PaIsothermal bulk modulus of liquid water at 20 °C and 1 atm, about 2.18 GPa — a 0.005 % volume change per bar of pressure.

Vapour Pressure of Water at 20 °C measured

pv=2339 Pap_{v} = 2339\ \text{Pa}

PaSaturation vapour pressure of water at 20 °C, 2339 Pa (0.339 psia) — the absolute pressure at which 20 °C water boils.