LNG (Liquefied Natural Gas) has widely been used as an efficient and clean energy source. However, the storage and transportation of LNG is a big problem that the vacuum degree is difficult to measure in LNG pipeline transportation, especially without the vacuum test port. A measurement method for the vacuum degree of the LNG pipeline based on the second law of thermodynamics (the principle of temperature field) was proposed, and the vacuum degree of the pipeline was measured in this paper. The vacuum degree was obtained as a function of the ambient temperature T0 and the outer wall temperature T1 of the outer tube. The feasibility of this method was verified through test bench and field experiments, and the functional expressions of the outer wall temperature of the outer pipe and pipeline vacuum degree at 22, 24, and 26 °C were combined and verified, which provided a powerful way for measuring LNG pipeline vacuum.
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