The section from the bulb to the bourdon tube of the thermometers is filled withnitrogen gas that is under pressure. Any temperature change around the bulb willcause the bourdon tube to move thereby creating a change in the indicator needle.Gas actuated dial thermometers measure the temperature of an object regardless of the exterior temperature of environment. These temperatures are compensated for ambient temperature. Specifications: 1) Diameters: 2.5" (63mm), 3.5" (90mm), 4" (100mm), 6" (150mm)2) Case: a) 2.5", 3.5", 6": stainless steel b) 4": plastic, stainless steel3) Bulb: brass or 316 stainless steel4) Capillary: double braided copper or stainless steel producing area:China payment:T/T, L/C supplying capacity:2,000pcs/month