Frenquently asked questions
How does an oxygen probe work? What is reference air?
The operating principle of an oxygen probe is the comparison of two partial pressures of oxygen in two separate gaseous media. The zirconium oxide (ZrO2) in the measuring cell has lattice defects – in other words, some of the sites that could be occupied by oxygen ions are missing. In order to compare these partial pressures, the oxygen probe must be supplied with a gas whose oxygen content is known (ambient air: %O2 = 20.9%), this is the so-called baseline air.
The property of this ceramic is that it allows the displacement of these oxygen ions at a temperature above 600 °C. The measuring element then becomes conductive. The voltage thus generated expresses a ratio between the relative difference in oxygen concentrations and the temperature of the process atmosphere.
Once the voltage value (which varies between 0 and 1200 mV) has been measured, the oxygen concentration can be easily calculated using the Nernst equation to obtain %O2 or by knowing the CO content (either by analysis or by theoretical calculation). The carbon potential (%C) of its heat treatment atmosphere can then be determined.
What is purge air (or cleaning air) and why are not all probes equipped with this feature?
Purge air is used on our %C probes (CarboProbe ZI Pro, ZS Pro, ST and ST-E) only to clean the ZrO2 measuring element with a fresh air flow.
In heat treatment, the measuring element can become clogged with carbon soot deposits (due to the carbon-rich atmosphere in which these probes are used). A supply of fresh air “burns off” the soot and frees the measuring element from this deposit. If this cleaning is not carried out at regular intervals, the probe measurement will be distorted.
Important: The burning of soot leads to a significant increase in temperature which can destroy the measuring element if left uncontrolled (see the probe’s operating instructions for more information).
Purge (or cleaning) air is not available for the CP, DS, LT and HT CarboProbes. These cannot be used in carburising atmospheres (with carbon). This means that there are no soot deposits that can be burnt off by the cleaning process. Cleaning air cannot clean deposits other than carbon deposits (e.g. incineration residues).
What is the difference between measuring %C (carbon potential) and %O2 (percentage oxygen)?
There is no difference in the measurement of the probe itself!
All probes generate the same millivolts (mV) whether they are in a carbon- or oxygen-rich atmosphere. It is the calculation downstream of the measurement that allows the calculation of either %C, in the heat treatment field, or %O2 in other fields.
It should be noted that the probes intended for the measurement of oxygen (%O2) have electrodes that are not resistant to carburising atmospheres (with carbon). For this reason, these probes cannot be used in the field of heat treatment.
The opposite is not the case. A probe for heat treatment (%C) can be used in an oxygen measurement process, but up to a maximum temperature of 1100°C, above which the metal electrode will melt.
Why are some probes sold with a thermocouple to measure temperature?
The equation for calculating %O2 or %C requires knowledge of the temperature at which the measurement is made. This temperature can be obtained by a thermocouple inside the probe or by a thermocouple elsewhere on the furnace.
Important: If your kiln is not homogeneous from a temperature point of view, it is important that the temperature measurement is made as close as possible to the oxygen probe to avoid errors in the calculation of %C or %O2.
An internal thermocouple in the probe is therefore ideal for accurate measurement.
Discover our oxygen probes %O2 and carbon %C
Why does Econox use two different zirconium oxide (ZrO2) systems in its oxygen probes?
Historically, Econox has always used a zirconium oxide ball for %C measurement in heat treatment atmospheres. Many years later, another system was introduced.
It appears that in some particular cases, the use of either ZrO2 cell may give a slightly longer life. The strength of Econox is therefore that it can offer you two systems if one of them does not give the expected results.
What are the standard lengths of Econox probes?
Heat treatment probes have metal outer tubes. The lengths are therefore fixed between 350mm and 1300mm, for obvious stock and manufacturing reasons.
For oxygen probes, although the lengths are normally fixed, we can change them by cutting the ceramic to the desired length. It is then possible to supply a probe to the exact length required by the customer.
Why do Econox probes only work from 600 °C?
This limitation is both physical and chemical, as the zirconium oxide (ZrO2) measuring cell only becomes reactive at 600 °C and above. This applies to all probes on the market that use a zirconium oxide cell.
What is the service life of an oxygen probe?
The ZrO2 measuring element of the oxygen probes has a limited service life. Its surface layer is altered by the impurities suspended in the process atmosphere. For these reasons, the user should change this measurement element as soon as the measurement no longer corresponds to what is expected.
Depending on use and the atmosphere in which it is used, the service life of a probe can vary from approximately 6 months to 24 months.
What are the advantages of working with Econox?
We are a small family company with quality and our customers at the heart of our business. We are not part of a large group, which allows us to be very responsive in solving your problems, in our delivery times, support or special requests that our customers may have.