Oxygen Probes

ST Oxygen Probe

The probe 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.

Model	Range (°C)	Sheath	Diameter (mm)	Thermocouple
ST	700 - 1000	steel	26	type K, N or R

Output

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.

Installation

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.

Ordering & Information

In the U.S.A., please contact
MacDhui Probe of America
7867 Enterprise Drive
Mentor, OH 44060
Phone: 440-942-5597
Fax: 440-953-8862
Cell: 440-666-0335 (Nancy Janasek, Vice President)
eMail: macdhuiprobe@aol.com

Anywhere in the rest of the world, please contact Australian Oxytrol Systems

Probes are normally ready for despatch by international air freight within one week of order.

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Oxygen Probes

Trouble-shooting Oxygen Probes

Introduction

If a probe is under suspicion, some simple tests in service can diagnose the problem. Most carbon potential controllers can indicate temperature and probe oxygen mV. Use the controller to check that these readings are plausible, to establish whether it is temperature or mV which should be investigated.

Basic checks - no signal

Check the thermocouple
Unplug the connection cable and use a mV meter to check the probe thermocouple output at the instrument end of the cable. Work back to the probe terminals at the Cannnon plug, then to the thermocouple wires inside the probe head, to establish where the fault lies. If the thermocouple is faulty inside the probe, dismantle the probe and replace the thermocouple. If the inner 4-bore insulator is broken, replace it.

Check the oxygen signal
Unplug the connection cable and use a mV meter to check the mV signal at the instrument end of the cable. Work back to the probe terminals at the Cannnon plug, then to the oxygen mV signal wires inside the probe head, to establish where the fault lies. Repair the connections or replace the sensor, as required.

More advanced checks - signal may not be accurate

If the probe is producing an oxygen signal, but you suspect that the signal might not be accurate, perform the following checks. All of these checks can be performed while the probe is in the furnace. These checks are not a calibration, but they all give an indication of the condition of the sensor.

Disconnect the reference air line at the probe head and check that reference air is flowing. Re-connect the reference air.

Check the response time
Measure the oxygen mV signal. Leave the mV meter on the terminals and short out the probe oxygen mV pins, then remove the short. The mV signal should recover immediately. Slow recovery indicates a contaminated sensor, which should be replaced.

Check the impedance
Place a 50 kohm resistor across the oxygen mV pins. The mV signal will drop. If the decrease is less than 20%, there is no problem. If the signal drops to less than 50% of the original mV reading, the sensor is probably contaminated and should be replaced.

Check for leakage
Shut off the reference air, or disconnect the reference air at the probe head. The mV signal should stay constant or drop slowly. If the mV signal drops faster than 25 mV in the first minute, the sensor is probably cracked or leaky and must be replaced.

If the sensor passes the above test, restore the reference air supply. The mV signal should recover to its previous value within a few seconds. A slow recovery indicates that the internal 4-bore insulator is broken. Increasing the reference air flow rate may enable the probe to be used temporarily, but it is better to replace the insulator.

If the probe passes all the above tests

• Fast recovery from a short-circuit
• Low impedance
• Small effect from loss of reference air
• Fast recovery from resumption of reference air

Then the probe is almost certainly in good condition.

Check purge effectiveness
Finally, check the oxygen signal during a purge (burn-out) cycle. During burn-out, the mV signal should drop to less than 250 mV for at least one minute. It is not possible to give a blanket recommendation on purge air flow rates. The crucial parameter is not the amount of reference air, but the result of the reference air. The correct flow rate is whatever is required to bring the mV down to 250 mV for one minute. The purge air flow rate must not be so high as to cause excessive temperature excursions.

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