control system art essentially the output of a controller may be scaled to represent any desired factor or variable an example of this is where a desired temperature and an actual measured temperature are compared by a controller the controller output could be a signal representative of a change in the flow rate of some gas stream necessary to make the desired and actual temperatures equal on the other hand the same output signal could be scaled to represent a percentage change if the controller output can range from 0 to 10 volts which is common then the output signal could be scaled so that an output signal having a voltage level of 5 volts corresponds to 50 percent some specific flow rate or some specific temperature in a preferred embodiment for a closed temperature control loop a temperature transducer 125 in combination with a temperature measuring device such as a thermocouple which is operably located in conduit 11 provides an output signal 127 which is representative of the actual temperature of the vapor stream flowing in conduit 11 signal 127 is provided from the flow transducer 125 as the process variable input to the pid proportional integral derivative temperature controller 129 the pid temperature controller 129 is also provided with a set point signal 131 which is representative of the desired temperature of the vapor flowing in conduit 11 signal 131 may.