The AP Probe is suitable for high temperature carburizing heat treating, carbonitriding and endothermic gas generators. The AP Probe is NOT suitable for nitriding applications.

This AP Probe consists of % oxygen sensor, thermocouple sensor, their protection tube and the cold end termination head.

Zirconia is a solid state electrolyte which conducts oxygen ions at temperatures above 1400°F. The ion conduction results in a voltage between the two electrodes. The magnitude of the voltage is dependent on the temperature of the Zirconia and on the ratio of the oxygen partial pressure on the reference side of the electrolyte to the oxygen partial pressure on the process side of the electrolyte.

Two electrodes make contact with the platinum coated zirconium element at the tip of the sensor, one outer electrode, one inner electrode.

The special alloy protection tube is one conductor for the oxygen probe signal, which eliminates the need of a small signal wire normally exposed to the the furnace environment. The protection tube has excellent resistance to corrosion and oxidation at high temperature, and has good mechanical strength (since Hydrogen causes embrittlement).

Two connections to the Zirconia cell conduct the voltage to the four pin connector.

Probe Location on Furnace
If the enriching gas supply has a separate inlet, locate the probe in the following manner:

• close to atmosphere intake, where atmosphere is well mixed and at furnace temperature
• as far away from the fumes of quench oil as possible
• as far away from door opening as possible
• work load should be clean – free from oils
• probe should not be located in a “hot spot”
• avoid hot spots in the furnace
• the probe can be mounted at any angle

Generally, locate the oxygen probe in an area where the furnace gases are known to be thoroughly mixed. Locate in a wall area reasonably free from other pipes and valves to simplify installation and replacement.

A cable containing wiring for the thermocouple compensating cable and leadwire for the probe and reference air supply tube should be free from interference from other electrical or heat sources.

Thermocouple Wiring
NEVER wire thermocouples using ordinary copper wire. Wire only with thermocouple extension wire.

Observe the correct thermocouple and thermocouple extension wire polarity. The negative wire of both thermocouple and thermocouple extension wire is coded red.

Recommended TC Extension Wire: 20 gauge, solid, duplex, glass braid insulated.