Measuring about 0.0001 psi(abs) (≈ 5.2 × 10^−3 torr) continuously: which gauge is typically used for this low-vacuum region?

Introduction / Context:
At a few millitorr (≈ 5 × 10^−3 torr), many industrial systems require continuous indication rather than occasional reference measurements. Thermal conductivity gauges (Pirani/thermocouple) are commonly used in this vacuum region.

Given Data / Assumptions:

• Target: ~0.0001 psi(abs) ≈ 0.0052 torr.
• Continuous, on-line indication is desired.
• Gas composition is reasonably constant (thermal gauges are gas-dependent).

Concept / Approach:
Pirani gauges operate by relating filament heat loss to gas pressure, covering roughly 10^−3 to 10 torr (instrument-dependent). Thermocouple gauges are similar. While a McLeod gauge can measure in this region, it is an intermittent, manual device and not suited for continuous monitoring. Thus, the practical, textbook answer for continuous measurement near 5 mTorr is the Pirani gauge.

Step-by-Step Solution:
Convert units: 0.0001 psi → ~5 mTorr.Match ranges: Pirani covers ~10^−3–10 torr.Select Pirani as the continuous-reading instrument.

Verification / Alternative check:
Vacuum practice charts place Pirani/thermocouple gauges in the “low to medium vacuum” window that includes a few millitorr; ionization gauges are used for high/ultra-high vacuum below ~10^−3 torr.

Why Other Options Are Wrong:
McLeod: Accurate but intermittent and unsuitable for continuous control.Mercury manometer: Far too insensitive at millitorr.“None”: Not true—thermal gauges are standard here.

Common Pitfalls:
Ignoring gas-dependence and contamination; calibration should match the actual gas mixture to minimize reading bias.

Final Answer:
Pirani (thermal conductivity) gauge