therefore , the main heat exchanger carries the gas streams to and from the constant-flow pump that operates at room temperature . then nitrogen flow rate is set to either 5 or 10 l/min by the variable flow controller . after returning to the cold end of the heat exchanger ,the gas temperature is regulated by a heater and sensor before entering the nozzle of the cryostream . the gas then flows along the isothermal nozzzle and out over the sample . the temperature indicated on the controller is a mapped temperature for the crystal position(see section 6.1)