string s = "abcd ewr wertertretery Bar Level(Ref. b1) 620.1 mV * 650.0 750.0";
            Regex reg = new Regex(@"(?<=Bar Level\(Ref. b1\).*?)-?\d+(.\d+)?");

            Console.WriteLine(reg.Match(s));

------解决方案--------------------
C# code

string str = "abcd ewr wertertretery Bar Level(Ref. b1) 620.1 mV * 650.0 750.0";

           Match m = Regex.Match(str, @"-?\d+\.?\d+\b");
           Console.WriteLine(m);