problem in serial port reading

actually i am trying to establish communication between serial port and a power sensor which is of ophir make. now after learning from example i have established communication. i write SP(send power) then sensor transmit the value of power and i read it. now problem is that
1) i want to monitor power continuously because once i read the value again i have to transmit SP command for the new value. how should i transmit SP Command to serial port so that it always give me power value.
2) now i want to plot these value on the chart with data logging. how can i do this.