LSU Life Study cases

This section may provide useful for people contemplating using oxygen sensors in situations other than vehicle exhaust systems. We present photos and other information we have collected over years of receiving user feedback. If you have a sensor that fails unexpectedly then feel free to contact us to ask if we'd like to see it for inclusion in this section.

Please note that images presented (unless otherwise noted) here are all copyright © Tech Edge Pty. Ltd. 2011.

non-Automotive Environment with a lot of Ash (Case 1)

At left is a standard 7200 sensor (0-258-007-200) and the outer protection tube has been removed. It shows (click here or on the image) the space between the outer and inner protection tubes has been almost completely filled with ash from the flue it was installed in.

At right is another view of the same sensor showing ash spilling from the small holes at the end of the sensor.

Here's a view of the same sensor with the inner protection tube removed. There are some interesting things to note.

  • Our customer had found, unsurprisingly, that the sensor had "failed" in service. However, we found that once the sensor actually got some gas to it to sense, it was working fine (we tested in free-air and eventually got a reasonable free-air calibration value).
  • You will note there are what look likes cracks in the ceramic in front of the small bluish circle. The circle indicates where the diffusion chamber is located. The entrance to this chamber (the diffusion gap)is the small hole in the centre of the circle. It looks like there is a small blob of ash covering maybe 30% of the diffusion gap.
  • The cracks mentioned above are not directly above the diffusion chamber and could quite reasonably have been there since manufacture and have no adverse effect on operation.

Environment with High Moisture (?) Content (Case 2)

Here's a sensor we received from a flue application that appears to have lots of condensation on its external surface. We can only speculate that it was in a high humidity/moisture environment.

We took off the two protection tubes in one operation and noted that whatever had blocked the space between the inner and out protection tube was not loose ash (like we found in case 1). We speculate that a high moisture (or perhaps even tar) content somehow conspired to cement the ash into a hard and gas-tight material.

In fact, as this close up shows the ash or tar (or mixture) that found its way past the inner protection tube has also coated the sensor's outer surface. Remember that the sensor runs at a dull red heat - the ash/tar appears to have "bonded" to the surface.

Although we said the sensor worked fine once we took off the protection tubes exposing it to free-air. In fact, once we connected the sensor to a controller, the sensor's calibration point changed continuously (and quite rapidly) over a period of several hours the calibration value (as determined by WButils's calibration function) changed to the point at which we thought the sensor was performing correctly. This may be due to impurities or contaminants affecting the diffusion chamber's catalytic surfaces now having a chance to escape, or it may be due to oxygen now being able to combine with impurities that found their way into the sensor's diffusion chamber.

This sensor, presented to us as "dead", was bought back to life by simply removing the physical obstruction, caused by soot/ash/tar, lodging between the inner and outer protection tubes.

Automotive Condensation (?) LSU-4.2 Failure (Case 3)

This sensor was returned to us experiencing a symptom where the display showed just lean of stoich (Lambda=1.01). This is a classic problem that presents when the external Vs or Ip circuits are damaged, and is usually associated with damaged controller-to-sensor cables.

We see at left that some of the sensor's fine laminated structure has been exposed. The inset shows a small section of the structure that was found loose within the inner protection tube's body.

This damage appears to be a simple case of the LSU-4.2 (7200) suffering mild localised thermal shock. Not enough to destroy the whole sensor (we have seen them with the whole body broken), but perhaps a small droplet of condensation has caused localised mechanical failure.

Another potential cause (not confirmed, but possible in theory) is (temporary) failure of the controller's heater circuitry/firmware causing overheating.

The top image at right is of the hole into the diffusion chamber showing combustion products that have stuck to the sensor's surface, but the hole itself is quite clear.

Aeronautical Condensation (?) LSU-4.9 Failure (Case 4)

Here's another example of what looks like damage due to operating environment. In this case it is from an aeroplane engine application.

It's worth noting that this sensor is an LSU-4.9 (type 0-258-017-123, or Tech Edge part # [017123]). See the different laminations when compared with the case 3 LSU-4.2 sensor (Tech Edge [007200]). This is shown at right and far right. You can see the newer LSU-4.9 sensor (top of both images) has a smaller structure and also there's less ceramic (about 60% the thickness - which may explain why it heats up in a shorter time).

If you ever wondered about the ceramic getting hot, the image above far right, taken with a white background & office lighting, shows the sensor gets red hot (the scratches are from a metal needle we used to probe the surface for an experiment!).