Australian Oxytrol Systems

ST Probe

ST Probe - for control of carburising furnaces

  ST probe - overview

The probie of choice for control of carburising furnaces

The ST probe incorporates a long life steel sheath especially formulated for carburising atmospheres. The electrode connections, contact springs and proprietary seal are designed for accuracy and reliability under the most severe conditions of atmosphere and thermal cycling.

Probes are normally available in lengths up to 1200 mm, or 2000 mm on special order.

The standard configuration includes a K-type thermocouple of 1.0 mm wire gauge, saving the need for a separate thermocouple.

ModelRange (°C)SheathDiameter (mm)Thermocouple
ST 700 - 1000 steel 26 type K, N or R


The signal from the probe is the same as for all standard zirconia oxygen probes. The ST probe is compatible with all common carbon potential controllers. At most, a simple change of connection plug or socket might be required.

ST Probe, showing electrical connector and reference air connector. The standard sheath size is 3/4" schedule 40, but 1/2" schedule 40 is also available.


The probe can be installed anywhere in the kiln or furnace, provided that the temperature is less than 1000 deg C. The probe will measure the oxygen concentration at the probe tip. The port for installation should be sealed sufficiently to prevent atmospheric air from causing a local variation in the oxygen concentration.

When fitting or removing a probe with a hot furnace, move the probe slowly to avoid thermal shock of the ceramic components. Slide the probe in at a rate of approximately 50 mm per minute. The rate is most easily regulated with probes incorporating a thermocouple. The thermocouple read-out can be used to ensure that the heating rate is less than 100 deg C per minute.

The probes must be provided with a reference air supply of 1 to 10 ml per minute.

Soot Purging

It is very important to burn out any soot build-up in the probe at regular intervals. It is advisable to do this at the beginning of each run. The air flow required for purging should be adjusted to suit the individual furnace. The airflow should be fast enough to result in a probe output of less than 150 mV for at least 3 minutes.

Probe Testing

There are two simple tests that can be performed regularly while the probe is in service. If the probe fails either of these two tests, it should be replaced.

1.Check Probe Impedance

The impedance of a probe will gradually increase as the probe is used. The impedance of the brand new probe should be measured at the normal operating temperature then at regular intervals, at the same temperature. If the impedance abruptly changes or increases to 10 times more than the initial value, there may be a problem with the probe.

2. Check reference air response

While the probe is in use, quickly shut off the reference air supply. The probe ouput signal should gradually fall by a few mV in one minute. If the change is more than 25 mV, the sensor has probably cracked and will give inaccurate readings.

  ST probe - view of air and signal connectionst

  ST probe - detail of sensor