O2 Probes and Failure Prediction

Which kind of probe?

Yokogawa zirconia sensors. Sorry about omitting the obvious.

Sorry, I don't have any experience with Yokogawa. But, generally, cracking and contamination are the greatest causes of failures and there's nothing I know of that will predict that. I'm not much help there. Have you talked with the field service manager at Yokogawa?

Our O2 meters came from 'Technical Tools' room. They were in charge of calibrating everything that needed calibrating. The O2 meters were signed out for not more than a five day period, at which time they were to be returned for charging and recalibration with a fixed concentration of test gasses. The meters were controlled by the on site 'Tool Room', logged out to a specific location, under a supervisor or team leader, and back in at the end of each shift. A missing meter would send a pair of workers to the last location to make sure no one was trapped or unconscious in a 'confined space'. "Not following this procedure would be cause for reprimand or worse".

Increasing resistance can be an indication of the ageing of the probe. Depending on the properties of the subsances which come into contact with the probe surface it might also be an indication of surface contamination. This is specifically important if you use the probe in an atmoshere containing silicone contaminants.

Another hint: Frequent on-off switching of the probe will increse thermal stress thus potentially reducing the operating life of the probe.

I would suggest that you aks the manufacturer for advise, specifying your application as detailed as possible.

Regards nudnik

Yes, I understood that the increasing resistance was a sign of sensor failure due to to the metallic probe deteriorating.

I assume this is in gas service. The O2 sensors I am familiar with have been problems. They are expensive to replace or repair.

In the worst applications, we have changed the operating philosophy away from continuous measurement to periodic. What I mean in that the time we needed to check oxygen was not the most corrosive part of the cycle. We installed a the oxygen sensor in a vent pipe and vent / purge with N2 when it is non critical. The level were watching in that application was <5 PPM for quality reasons. When it is time to test O2, a valve opens and the O2 sensor is in the gas stream. When not checking %O2, that valve closes and nitrogen is purged (giving a zero baseline for a nice reference check).

Applications where we look at oxygen to prevent an explosive atmoshere are much worse because they are on line all the time. We count on a weekly calibration. If operations needs to confirm (rough guess) during the week the piping allows them to be pulled out of the pipe and air is measured, Nitrogen can also be injected to test the zero end. they have a 0-20% go or no go test from the field.

You are correct, this is measuring boiler flue gas for combustion control- So it is continuous duty. Had you noticed any decreased accuracy of the probe prior to failure?

In my opinion, yes there is erratic behavior before failure. It was a batch operation measuring oxygen in ammonia, methane, nitrogen, H2S and some other nasty gases. Other people looking at the same data thought the behavior was simply the different products and batch cycles. Failure or recalibration often occurs before the same product was produced on the next cycle.

I&E group did a weekly official calibration.

They also did unofficial "pull the probe and check air (19-20%) and blow Nitrogen and check zero. Crude but effective!