Understanding Pressure Units in Vacuum Measurement (mbar, Torr, Pa)
Vacuum measurement underpins countless industrial and scientific processes, from semiconductor fabrication and thin-film deposition to vacuum heat treatment and mass spectrometry. Yet one of the most frequent sources of confusion for engineers and procurement teams is the bewildering array of pressure units: millibar (mbar), Torr, and Pascal (Pa). A misplaced decimal or incorrect conversion can lead to specification errors, miscalibrated controllers, or even process failures. Poseidon Scientific’s VG-SP205 Pirani Vacuum Transmitter and VG-SM225 Cold Cathode Vacuum Transmitter are engineered with flexible 0–10 V analog outputs and customizable RS232 digital protocols, allowing users to work in their preferred units without compromise. This article explains the differences, provides exact conversion tools, and offers practical guidance for selecting and configuring vacuum gauges in real-world systems.
1. SI vs Non-SI Units
The Pascal (Pa) is the SI-derived unit of pressure, defined as one newton per square meter (1 Pa = 1 N/m²). It is the only unit fully consistent with the International System of Units and is preferred in scientific literature and regions that mandate metric compliance. In vacuum technology, however, non-SI units remain dominant because of historical precedent and instrument legacy.
The Torr originated from Evangelista Torricelli’s mercury barometer and equals the pressure exerted by a 1 mm column of mercury at 0 °C. The millibar (mbar) is a convenient decimal fraction of the standard atmosphere (1 bar = 10⁵ Pa, so 1 mbar = 100 Pa). Both Torr and mbar are non-SI but are still widely used because they produce numerically convenient values in the high- and ultra-high-vacuum ranges. Poseidon transmitters report pressure internally in Pa but allow output scaling or digital transmission in any of the three units, ensuring compatibility regardless of regional or legacy requirements.
2. Conversion Formulas
Accurate conversion is essential for specification sheets, controller programming, and cross-referencing datasheets. The exact relationships are:
\[ 1 \, \text{Torr} = 133.322 \, \text{Pa} \]
\[ 1 \, \text{mbar} = 100 \, \text{Pa} \]
\[ 1 \, \text{Torr} = 1.33322 \, \text{mbar} \]
For quick field calculations, the following simplified formulas are sufficiently accurate for most engineering work (within 0.1 %):
\[ P_{\text{Pa}} = P_{\text{Torr}} \times 133.322 \]
\[ P_{\text{Pa}} = P_{\text{mbar}} \times 100 \]
\[ P_{\text{mbar}} = P_{\text{Torr}} \times 1.33322 \]
Poseidon’s VG-SP205 and VG-SM225 store factory calibration constants in EEPROM and can be ordered with digital protocols that output directly in the user’s chosen unit, eliminating manual conversion inside the PLC or SCADA system. This feature is especially valuable when integrating with legacy controllers that expect Torr or mbar scaling.
3. Common Industry Preferences
Different sectors have converged on preferred units through decades of standardization:
- Semiconductor and thin-film deposition: mbar is dominant in Europe and Asia; Torr remains common in North American tools.
- Vacuum heat treatment and metallurgy: Torr is still the de-facto standard for furnace controllers and leak-rate specifications.
- Scientific instrumentation (mass spec, SEM): Pa is increasingly specified for traceability to SI standards, though Torr persists in many OEM manuals.
- Research laboratories: Pa is preferred for publication and international collaboration.
The VG-SP205 Pirani (atmosphere to 10⁻³ Torr) and VG-SM225 Cold Cathode (10⁻³ to 10⁻⁷ Torr) cover the full range required by these industries. Their customizable RS232 protocol (available from as few as 5–10 units) lets global customers standardize on a single hardware platform while receiving data in the unit native to their region or controller.
4. Display Configuration in Controllers
Modern PLCs, HMIs, and SCADA systems accept 0–10 V analog inputs (2–8 V active range on Poseidon transmitters) and map them to engineering units via scaling blocks. For the VG-SP205, the factory default maps 2 V to atmosphere and 8 V to 10⁻³ Torr. Changing the displayed unit requires only an adjustment in the PLC tag database:
Example scaling for Torr to Pa:
Pressure_Pa = (Analog_Voltage - 2) × (760 × 133.322) / 6Digital integration is even simpler. Poseidon’s RS232 output can stream pressure directly in Pa, mbar, or Torr along with status codes (filament health for Pirani, high-voltage status for cold cathode). This eliminates scaling errors entirely and allows the controller to display the exact unit required by the operator without reprogramming analog channels.
5. Avoiding Unit Misinterpretation
Common pitfalls include:
- Confusing mbar with bar (off by a factor of 1000).
- Using approximate conversions (e.g., 1 Torr ≈ 1 mbar) in the high-vacuum regime, where the 33 % difference becomes significant.
- Assuming a gauge calibrated in Torr will read correctly on a Pa-scaled controller without rescaling.
Always verify the gauge datasheet for the exact output scaling and request Poseidon’s calibration certificate, which lists both primary (Pa) and secondary (Torr/mbar) values. During commissioning, perform a single-point cross-check at a known pressure (e.g., 1 Torr) using a certified reference gauge. Poseidon transmitters include built-in over-range and fault detection that drops the output below 2 V, providing an unambiguous “invalid reading” regardless of unit setting.
6. Example Calculation Scenarios
Scenario 1 – Pump-down verification: A chamber is pumped from atmosphere (760 Torr) to crossover (1 Torr). Convert the target to Pa for an SI-based controller:
\[ 1 \, \text{Torr} = 133.322 \, \text{Pa} \]
The controller must therefore trip the roughing valve at approximately 133 Pa.
Scenario 2 – Leak-rate specification: A tool requires a leak rate < 10⁻⁶ mbar·L/s. Convert to Torr·L/s for a U.S. customer:
\[ 10^{-6} \, \text{mbar·L/s} = 10^{-6} \times 0.75006 \, \text{Torr·L/s} \approx 7.5 \times 10^{-7} \, \text{Torr·L/s} \]
Scenario 3 – Dual-gauge crossover: The VG-SP205 reads 0.8 Torr while the VG-SM225 reads 106 Pa. Verify consistency:
\[ 0.8 \, \text{Torr} \times 133.322 = 106.66 \, \text{Pa} \]
The 0.6 % difference is within normal gauge tolerance and confirms healthy system behavior.
These examples illustrate why Poseidon’s flexible output options—analog voltage plus digital engineering units—save hours of engineering time and prevent costly miscalculations.
7. Global Customer Considerations
International projects frequently mix U.S. Torr-based tools with European mbar specifications and Asian Pa documentation. Poseidon’s approach simplifies compliance: the same physical transmitter can be ordered with a protocol that outputs the unit required by each end user. For export-heavy manufacturers, this single-part-number strategy reduces inventory complexity and qualification paperwork. Calibration certificates are issued with dual or triple unit columns to satisfy both ISO 17025 accredited labs and customer audit requirements. The result is faster global deployment and fewer discrepancies during factory acceptance testing.
8. Practical Selection Advice
When specifying vacuum gauges, first identify the dominant unit in your control system and documentation. Then evaluate:
- Range overlap: Use the VG-SP205 for rough vacuum (atmosphere to 10⁻³ Torr) and the VG-SM225 for high vacuum (10⁻³ to 10⁻⁷ Torr).
- Output flexibility: Choose the RS232-customizable version if your controllers or customers require native units without PLC scaling.
- Cost and serviceability: Poseidon’s self-developed designs keep unit prices at 3000–3500 RMB while offering field-cleanable cold-cathode heads and 3–5 year Pirani filament life.
- Integration ease: 0–10 V analog plus RJ45 connector ensures drop-in compatibility with most PLC analog modules.
For new projects, request Poseidon’s unit-conversion templates and sample PLC function blocks. For existing systems, a simple firmware update or protocol customization can align legacy gauges with modern SI requirements without hardware replacement.
Understanding and correctly applying mbar, Torr, and Pa is fundamental to reliable vacuum measurement. Poseidon Scientific’s VG-SP205 Pirani and VG-SM225 Cold Cathode transmitters remove the unit-conversion burden through factory-default scaling, digital engineering-unit output, and full traceability. By selecting gauges that speak your preferred language—literally—engineers and procurement teams achieve faster integration, fewer errors, and lower total cost of ownership across global operations.
References & Further Reading
Lafferty, J. M. (Ed.). (1998). Foundations of Vacuum Science and Technology. John Wiley & Sons.
Peacock, R. N., et al. (1991). “Comparison of hot cathode and cold cathode ionization gauges.” Journal of Vacuum Science & Technology A, 9(3), 1977.
Redhead, P. A. (1959). “The magnetron gauge: A cold-cathode vacuum gauge.” Canadian Journal of Physics, 37(11), 1260.
Need help selecting the right unit configuration for your vacuum system? Poseidon applications engineers provide free unit-conversion audits, customized protocol files, and sample controller code. Contact us today to ensure your next gauge order arrives ready to speak your language—whether Pa, mbar, or Torr.
