What is spatial pressure mapping?
In an ideal high-pressure experiment, pressure is identical at every point of the sample chamber. In practice, once the pressure-transmitting medium (PTM) solidifies, it can no longer flow and redistribute stresses: the sample chamber becomes non-hydrostatic, with pressure higher at the centre and lower at the edges (or vice versa depending on the anvil geometry). This spatial pressure gradient — typically 0.5 to several GPa across the chamber — is a direct signature of deviatoric (non-hydrostatic) stress, which biases all pressure-dependent measurements and can produce artefacts in diffraction patterns that mimic structural phase transitions.

Why use multiple ruby beads?
A single ruby bead gives only the local pressure at its position. To map the full pressure field, BETSA places up to 4 ruby microspheres (typically 3–5 µm diameter, BETSA type) at selected radial positions across the chamber — centre, mid-radius, and near-edge. Each bead independently measures its local pressure via its R1 fluorescence wavelength λ. BETSA Wave Fit then reconstructs the continuous pressure field by Shepard inverse-distance-squared interpolation: every pixel of the chamber image is assigned the weighted average of the four bead pressures, with weights proportional to 1/d². The resulting colour map gives an immediate visual assessment of the pressure distribution.

Reading the colour map
The map appears automatically as soon as 2 beads have a valid λ entered: ● Red = high pressure  ·  ● Green = low pressure
A uniform green-to-red gradient across a small pressure range (ΔP < 0.5 GPa) indicates near-hydrostatic conditions. A steep gradient, or abrupt colour changes between beads, signals strong non-hydrostaticity. The gradient statistics panel quantifies this with ΔP = P_max − P_min and a colour-coded hydrostatic quality indicator.

Hydrostatic limits of common PTMs (Klotz et al. 2009)
· Helium: solidifies at 12.1 GPa — remains quasi-hydrostatic to >100 GPa (ΔP < 0.5 GPa)
· Neon: solidifies at 4.8 GPa — excellent to ~50 GPa
· Argon / N₂: solidifies at 1.5–2.4 GPa — gradients detectable from solidification pressure
· MeOH:EtOH 4:1: hydrostatic to ~10 GPa, then rapidly non-hydrostatic
· No PTM: severe gradients (ΔP > 2 GPa) even at low pressure

Practical procedure
1. Select bead B1 (pre-placed at centre). Click the chamber to reposition if needed.
2. Enter its measured λ R1 in the field below — pressure is computed automatically.
3. Select B2, click to place at mid-radius, enter λ R2.
4. Repeat for B3 and B4 at different azimuthal or radial positions.
5. The colour map and gradient statistics update in real time.

References:
Klotz et al. (2009) J. Phys. D: Appl. Phys. 42:075413 — Hydrostatic limits of 11 PTMs; σ across multiple beads as solidification indicator
Dewaele & Loubeyre (2007) High Press. Res. 27:419 — Non-hydrostatic stress and EOS bias in DAC with helium medium
Piermarini, Block & Barnett (1973) J. Appl. Phys. 44:5377 — Ruby fluorescence as pressure gauge; spatial mapping concept