Engineering Unit Conversions for Seals | MPa, bar, psi

Hydraulic and sealing specifications cross US customary, metric and ISO units daily. A groove drawing in millimeters, a system pressure in bar, a customer RFQ in psi, and a material datasheet in MPa create conversion errors that propagate into wrong compression calculations and incorrect material selection.

This reference consolidates the conversions seal engineers use most often when specifying O-rings, preparing quote requests, and reviewing supplier datasheets.

Pressure Conversions

Seal datasheets and hydraulic system specs mix psi, bar, and MPa. Use these exact factors (not rounded shortcuts) for specification documents.

Quick mental checks

• 1 bar ≈ 14.5 psi (atmospheric reference)
• 1 MPa = 10 bar ≈ 145 psi
• 3,000 psi ≈ 207 bar ≈ 20.7 MPa (common mobile hydraulic ceiling)

When comparing O-ring extrusion resistance to system pressure, always convert to a single unit before checking gap tables. A 0.15 mm extrusion gap safe at 150 bar is not safe at 150 MPa.

Temperature Conversions

Material temperature limits are published in °C or °F depending on supplier region.

Common seal temperature anchors

Always specify continuous vs peak temperature when converting — a seal rated to +150°C continuous may tolerate +175°C for minutes only.

Force and Load (Bolt / Gland)

Face seal and flange designs sometimes express preload in force rather than torque.

Torque-to-clamp-force depends on thread pitch, friction coefficient, and washer stack — do not convert torque to O-ring compression percentage without a joint diagram.

Hardness Scales

O-rings are specified in Shore A (ASTM D2240). Some European datasheets use IRHD (ISO 48).

Shore A and IRHD are not identical scales — for critical specifications, require supplier measurement on the same scale (Shore A Type A durometer, 1 kg indenter).

PTFE is measured on Shore D, not Shore A. A PTFE "90 Shore A equivalent" in marketing literature usually means ~55–65 Shore D.

Length and Groove Dimensions

AS568 sizes are defined in inches; ISO 3601 in millimeters. When cross-referencing, convert both ID and CS — do not mix inch CS with metric ID on the same specification line.

Surface finish conversions:

See the surface finish guide for Ra recommendations by motion type.

Volume Swell (Fluid Compatibility Testing)

ASTM D471 immersion results are often reported as % volume change. Interpretation guide:

Negative volume change (shrinkage) can be equally damaging — the seal loses contact stress. Always record hardness change and tensile change alongside volume swell.

Related Resources

• O-Ring Engineering Hub
• Compression Calculator
• Hydraulic Fluid Compatibility
• Glossary

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Q1: How do I convert 3,000 psi hydraulic pressure to bar for an O-ring spec sheet?

Multiply by 0.0689476: 3,000 × 0.0689476 = 206.8 bar (or ~20.7 MPa). Many European datasheets list extrusion gap limits in bar — use the converted value before comparing to your system pressure.

Q2: What is 70 Shore A in IRHD?

Approximately 67 IRHD — close but not identical. For procurement specifications, state "70 ± 5 Shore A per ASTM D2240 Type A" rather than converting to IRHD unless your customer requires ISO 48.

Q3: How many bar is 1 MPa?

10 bar exactly. This is the most common conversion error in multinational RFQs — always verify whether the document uses MPa or bar before specifying backup rings or hardness upgrades.

Q4: How do I convert groove dimensions from inches to millimeters for ISO 3601?

Multiply each dimension by 25.4. Example: 0.103" CS → 2.62 mm. Round only after conversion; do not round intermediate values when calculating compression percentage.

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Preparing an RFQ with mixed units? Request a quote with your groove drawing — we accept inch, metric, and AS568 dash numbers and return specifications in your preferred unit system.