Normally, when it comes to install thermocouple for measuring temperature on the surface of boiler or heat-exchanger, commonly used method was welding them with high temperature by attaching a metal plate or kinfe-edge type pad which has its thickness over 3 mm. This procedure can only be applied if the tube has the enough strength to endure high temperature welding process. However, this welding precess can't be performed if the tubes are filled with water or oil inside because it may cause the damage to the tube and breakage of thermal-capacity, response time will be delayed and be difficult to measure exact temperature changes. To overcome these issues, R990 series are suitable for performing resistant welding by using spot-welding machine with under 3.2 mm O.D sheath and sheet metal pad, therefore, user can tightly install the pad along the curved surface of the tube, even if the user is not an expert welding operator. Furthermore, due to its compact size and low thermal-capacity, R990 series can offer fast response time without delay even if the measuring temperature fluctuates. Most of all, since R990 series does not cause any thermal-effect, it can be installed on the tubes which carry water or oil inside without expecting any damages to the tube or welded area.