Convert String From Serial Data to Numerical Value

Using Arduino Programming Questions
21

I'm not understanding why I'm unable to convert the Liters/Minute to a numberical value.

7.320,4.734,284.057
4.7340 Liters/Minute
0 in HEX

   if (sensor_stringcomplete){                                                 //if a string from the Atlas Scientific product has been received in its entirety
      Serial.println(sensorstring);                                             //use the hardware serial port to send that data to the PC

      
  for (int i=6; i<11; i++)  {
    Serial.print(sensorstring[i]);     //print each string character to verify
    float flowLong = (sensorstring[i] - '0');
                }
    Serial.print(flowLong);Serial.println(" Liters/Minute"); 
    if (flowLong < 4.5) {
    Serial.println("FLOW WARNING!!!");
    }
       Serial.print(flowLong,HEX);Serial.println("  in HEX");
       sensorstring = "";                                                      //clear the string buffer:
       sensor_stringcomplete = false;                                          //reset the flag used to tell if we have received a completed string from the Atlas Scientific product
       }