5.1.1 Probe and Probe Heating. The probe is usually a 1-to 2-m length of 5 mm bore fused silica or borosilicate tubing with an 18/9 standard socket joint on the downstream (filter) end. The inlet end of the probe is left flat and open. The glass probe is wrapped with heating tape and inserted into a metal sheath (stainless is recommended) along with a thermocouple. The thermocouple and heating tape are connected to a temperature controller and heater power supply. The temperature controller is set to provide a probe temperature above the acid dew point.