Cryogenic LNG Tank Monitoring During Commissioning

Scenario

Project Type: Commissioning support for LNG storage tank instrumentation Location Context: Coastal LNG terminal near Sabine Pass, LA — humid subtropical climate; tank under nitrogen purge with internal vapor space at −162°C Constraints: No physical access to thermocouple junctions during cooldown; must verify sensor integrity before liquid nitrogen introduction; safety-critical — false high reading could trigger unnecessary venting.

Given Data

• Measured EMF = −5.87 mV (negative polarity confirms sub-zero measurement)
• Reference junction temperature (t_ref) = 18.6°C (verified via traceable digital thermometer at marshalling cabinet)

Calculation

Using the same Type-K inverse model (validated down to −200°C):

1. Convert t_ref = 18.6°C → V_cjc ≈ 0.754 mV
2. Compensated EMF = −5.87 mV + 0.754 mV = −5.116 mV
3. Inverse polynomial solution for −5.116 mV yields −161.82°C
4. Rounded to tool’s precision: −161.82°C → −161.82°C (displayed as −161.82, reported as −161.82°C)

Result and Decision

The result matched the expected equilibrium temperature (−162°C ±0.5°C) derived from independent pressure–temperature saturation tables. This confirmed thermocouple continuity, correct polarity wiring, and absence of thermal shorts. The commissioning team cleared the tank for controlled LNG introduction and added redundant fiber-optic temperature sensing at three radial levels for long-term monitoring.

Lesson

Negative EMF values are valid and essential for cryogenic work — verify polarity before installation; reversed leads produce erroneous positive readings that mask dangerous low-temperature excursions.