§1 — Pressure units in high-pressure science
The SI unit of pressure is the pascal (Pa). High-pressure science works in
gigapascals (1 GPa = 10⁹ Pa); the
kilobar survives from older literature (1 GPa = 10 kbar) and the
megabar (1 Mbar = 100 GPa) marks the static-compression frontier. Atmosphere, bar, psi and torr appear mainly in gas-loading and vacuum work.
| Unit |
= in GPa |
Note |
| 1 GPa |
1 |
SI working unit in HP |
| 1 MPa |
1×10⁻³ |
1 GPa = 1000 MPa |
| 1 kbar |
0.1 |
legacy literature; 1 GPa = 10 kbar |
| 1 Mbar |
100 |
static-compression frontier |
| 1 bar |
1×10⁻⁴ |
≈ atmospheric pressure |
| 1 atm |
1.01325×10⁻⁴ |
= 101 325 Pa (exact) |
| 1 psi |
6.8948×10⁻⁶ |
pound-force per inch² |
| 1 torr |
1.3332×10⁻⁷ |
= 1 mmHg; vacuum / gas loading |
Exact: 1 GPa = 10 kbar = 1000 MPa = 10 000 bar = 9 869.23 atm = 145 037.7 psi.
§2 — Temperature scales
Kelvin (K) is the SI absolute scale;
°C = K − 273.15 and
°F = °C × 9/5 + 32. Absolute zero is 0 K = −273.15 °C. The ruby pressure gauge is referenced at 298 K.
| Reference point |
K |
°C |
Note |
| Absolute zero |
0 |
−273.15 |
thermodynamic floor |
| Liquid helium |
4.2 |
−268.95 |
cryogenic DAC |
| Liquid nitrogen |
77 |
−196.15 |
gas loading, cryostats |
| Dry ice (CO₂) |
194.7 |
−78.5 |
— |
| Ice point |
273.15 |
0 |
— |
| Room temperature |
298 |
25 |
ruby gauge reference |
| Water boiling (1 atm) |
373.15 |
100 |
— |
| LH-DAC typical |
1500–3000 |
1227–2727 |
laser-heated DAC |
§3 — Pressure ↔ depth & planetary interiors
Pressure inside a planet rises with depth; the Earth's interior spans the full DAC range, which is why diamond cells reproduce deep-Earth conditions. Earth values follow PREM [
Dziewonski & Anderson 1981]; the solar-core value is ~26.5 million GPa [standard solar model].
| Environment |
Pressure |
Category |
| Bicycle tyre |
0.0009 GPa |
Everyday |
| Pressure cooker |
0.0002 GPa |
Everyday |
| Mariana Trench (11 km) |
0.11 GPa |
Geology |
| MeOH:EtOH 4:1 limit (DAC) |
9.8 GPa |
DAC / HP |
| Upper mantle (100–410 km) |
3–13 GPa |
Geology |
| HPHT diamond synthesis |
5.5–7 GPa |
DAC / HP |
| Lower mantle (660–2900 km) |
24–135 GPa |
Geology |
| Outer core (liquid Fe) |
136–329 GPa |
Geology |
| Inner core (solid Fe) |
329–364 GPa |
Geology |
| BETSA MDAC record |
400+ GPa |
BETSA MDAC |
| Jupiter centre (metallic H) |
~4000 GPa |
Planetology |
| Sun centre (H→He fusion) |
~2.6×10⁷ GPa |
Stars |
| Neutron-star core |
~10²⁶ GPa |
Stars |
§4 — Energy, work & related units
Pressure × volume is an energy:
1 GPa·Å³ = 0.16022 eV — handy in EOS work (see the Energy / λ page). Useful links: 1 eV = 1.602177×10⁻¹⁹ J = 96.485 kJ/mol; thermal energy k_BT = 25.7 meV at 298 K [
NIST CODATA].
§5 — SI prefixes & constants
| Symbol |
Name |
Factor |
| G |
giga |
10⁹ |
| M |
mega |
10⁶ |
| k |
kilo |
10³ |
| m |
milli |
10⁻³ |
| µ |
micro |
10⁻⁶ |
| n |
nano |
10⁻⁹ |
| p |
pico |
10⁻¹² |
k_B = 1.380649×10⁻²³ J/K = 8.617333×10⁻⁵ eV/K · N_A = 6.02214076×10²³ mol⁻¹ · R = 8.314462 J/mol·K (k_B and N_A are exact since the 2019 SI revision).
§6 — References and official sources
·
BIPM — The International System of Units (SI brochure) — definitions of the pascal and the kelvin
·
NIST CODATA 2018 — fundamental constants (k_B, N_A, exact values)
·
Dziewonski & Anderson 1981 — PREM, Earth pressure–depth profile
·
Solar core — central pressure ≈ 26.5 million GPa
·
BETSA — membrane DAC, 400+ GPa