The oxygen probe

Short summary

• 4 years warranty*
• high precission Platinum thermelelement
• No reference air, no , no cleaning, no turning, no repairs needed
• Outputlevel is independent of reference air (if used)
• Ceramical protection tube (ø 17 mm)
• Available lengths: 500, 600, 700, 800, 900, 1000, 1100 und 1200 mm
• Hardening furnaces, Slot furnaces and vacuum furnaces (max 5bar)
• Variable mounting position : horizontal, vertikal or diagonal

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• Specification
• Drawing

Detailed description

The principle of all oxygen probes is the same. Oxygen migrates from the ambient air through the zirconia into the furnace gases and produces a volage of 0 to 1,2 Volt. This voltage is the same at all build oxygen probes in the first ours, as it follws the Nernst-law which is a physical law such as gravity or the theory oif relativity. Materials and dimensions do not influence this law and the voltage respectively.

However reality shows a differnt pichture. So the questions are

• Why do oxygen probes different voltages after some weeks?
• Why do some life for some hours, some months and few for years?

Unless these questions are understood, it becomes clear why thermo-control oxygen probes has such a long lifetime, a 4 year warranty, and suits perfectly into your furnace setup.

Common issues of oxygen probes and the solutions applied in the thermo-control xygen probes.

Impermeability

The end of the lifetime is reachs as soon the zirconia tube is not sealed. The oxygen of the reference air gets diluted by the incomming furncae gases. A too low probe voltage is the result, however the electronic still calculates with 20,9% oxygen. The consequence is an over-carbonization of the charge.

In a thermo-control made oxygen probe, a 5-6mm thin zirconia tibe is build in, which makes it probably to the thinnest one worldwide. The benefits of this design are an unreached resistance against thermal-shock and vibration. It can be produced out of one piece to a record length of 1200mm without any junctions. This is whythese can be used also in a vacuum furnace up to 5bar.

However, the diameter of 5-6mm is not freely choosen. It was made possible by the long experience of thermo-control to build long-life thermoelements with thinn thermo-wires (patented EP3045625). The thin positive and a thickned negative thermo-wire, the probe's voltage conductor, the reference air tube, the electrical insulation occupy not more than 2,5mm room.

Gaselectrode

The gas-electrode (Image 1 below) is often dsinged very robust in order to resist corrosion. However this design is ver expensive due to a high amount of noble metals. After 6-9 months they burn-in into the zirconia and increase the risk of generating hairline cracks in the creamic due to their thermal expansion. The result is a leaking oxygen probe and an over-carbonizing.

To reduce costs some oxygen probes are build with an electrode which is alloed with an oxidation metal. This cost reduction turns into a unsecure performance of the oxygen probe as a thin oxide layer is build with every opening of the furnace after operation. This oxide layer changes the electrical resistance in an uncotrollable way, producing incorrect signals.

A short term solution is the use of vibration (sometime a small knocking in the probes head is enough). The effect is that the hard ceramic body grinds off the oxide layer - the volatge of the probes jumps up. However on the next day the oxide layer is back again.

The worse case are loose gas-electrodes, which deliver a too low voltage after few weeks already, since the oxide (an oxygen contributor) sits between the electrode and the zirconia. In this case the voltage can jump by more than 100 mV at knocking.

The solution to the above mentioned issues uses thermo-control is called "protected helical electrode" (granted patent P 32 0 1796) as shown in Image 2. The above picture shows an thermo-control build electrode after 4 years of service. The electrode is still in an "as-new" condition with very few signs of oxidation. The electode was protected properly.

Also no signs of soot or rust flakes which is due to the patented design of the protection tube (P 33 23 241). The asymetrically set holes in the protection tube have individual objectives.

Two "side-holes" suck the gas out of the tube, where the third one lets new gas in without any solid particles. The nest-alike electrode-fastener remained 4 years without any soot despite of the fact that the protection tube was strongly covered with soot (Picture 5).

This oxygen probe was never flushed, turned or cleaned in an other way, nor were parts exchanged. It will work for several years for sure.

The position in the furncae

Surprisingly little is known about this problem. The early oxygen probes with ceramic protection tubes in late 70's were mounted near to the thermoelememts above the charge to get a simialr temperature. As a temporary solution, an oxygen probe was monted beneath the grit.

Later in time, stainless-steel protection tubes were introduced. they have the advantage of being resistant against thermal-shocks or other mechanical forces. Also they are electrically conducting, therefore they could substitue the expensive Pt wire.

The backside of the stainless-steel tube is that they can only be mounted in a hanging position, otherwise the bending of the tube will breake the zirconia tube. At this position the tube is relatively cold which makes is very attractive for soot, covering pastes and other materials which enhances the corrosion of the tube. The result is a falling oxygen probe voltage and an increasing sooting of the furnace.

The above mentioned oxygen probe beneath the grit was operating at the highest temperatur over a decade. The temporary solution turned out to be a direct hit.

The thermo-control build oxygen probe in the pictures above was mounted specifically in that position under the grit. Due to the use of an more expensive ceramic protection tube and the ideal alignment of the holes a flushing of the probe was not necessary, giving the probe a very long lifetime.

Another advantage of mounting position under the charge is the minimal thermal insulation in the head. In comparison to the hanging position the head is working at 100K lower temperature. the porbe remains sealed and hardly needy reference air.

thermo-control oxygen probes were the first ones working in vacuum furnaces up to 5bar.

As a summary three major design features are build in a thermo-control probe:

• A high precision thermoelement provides an measurement with an tolerance of ± 1 Kelvin.
  
  
• A protection of the gas-electrode against corrosion provides an average lifetime of 10 years.
  
  
• An ideal filtering of the gases ensures that the soot-line can be approached. Even if everything else is black from soot, the electrode remains clean.
  
• The vacuum-sealed construction requires almost no additional reference air (appr. 1mm³ / hour) and is independent of the pressure of the reference air or the pressure in the furnace.