Gas　controlled　heat　pipes　(GCHPs)　provide　a　uniform,　stable　and　reproducible　temperature　zone　to　calibrate　thermometers　and　thermocouples,　and　to　realize　defining　fixed　points　using　a　calorimetric　method.　Therefore,　to　perform　such　investigations,　a　GCHP　furnace　using　sodium　as　its　working　fluid　was　constructed　at　the　National　Institute　of　Metrology　(NIM),　China.　Also,　investigations　into　the　thermal　characteristics　of　the　NIM　gas　controlled　sodium　heat　pipe　were　carried　out.　The　temperature　stability　over　5　hours　was　better　than　+/-　0.25　mK　while　controlling　the　pressure　at　111250　Pa.　The　temperature　uniformity　within　14　cm　from　the　bottom　of　the　thermometer　well　was　within　0.3　mK.　While　keeping　the　pressure　stable　at　the　same　value,　17　temperature　determinations　were　performed　over　14　days,　obtaining　a　temperature　reproducibility　of　1.27　mK.　Additionally,　the　NIM　gas　controlled　sodium　heat　pipe　was　compared　with　the　sodium　heat　pipe　produced　by　INRiM.　The　temperature　in　the　INRiM　sodium　heat　pipe　operating　at　111250　Pa　was　determined,　obtaining　a　difference　of　21　mK　with　respect　to　the　NIM　GCHP.　This　difference　was　attributed　to　sodium　impurities,　pressure　controller　capabilities　and　reproducibility,　and　instabilities　of　high　temperature　standard　platinum　resistance　thermometers　(HTSPRTs).　Further　investigations　will　be　carried　out　on　extending　the　pressure/temperature　range　and　connecting　both　GCHPs　to　the　same　pressure　line.