I need to replace a ZO21D probe or a ZR22 probe with a /AV or /ZA on the end of the Model Number.

Yokogawa has been making Zirconia Oxygen Analyzers for over 30 years. The older O2 probes and analyzers used a transistor temperature sensor. The newer units use a PT1000 RTD. We discontinued the ZO21D probe and both the AV8G and ZA8C analyzers over 15 years ago. To make it easier for our customers who still had the older ZA8C and AV8G analyzers we added an option to allow them to order the ZR22G probes with the old style sensors already installed. Some customers are still ordering ZR22 Probes with the /ZR or /AV option (see the photo to the right). In many cases they are actually using them on AV550s or ZR402Gs but they have the analyzers set to “ZO21D” mode so that they will work with the old cold junction sensors. They were purchasing probes with the old sensor because they still had some ZA8C Analyzers or AV8G Analyzers in operation along with some of the newer ZR402s and AV550s.

Now the old style Cold Junction sensors are no longer manufactured, and we have not been able to locate another supplier.

These customers need to replace any ZA8Cs or AV8Gs they still have with ZR402s or AV550s.

The new Cold Junction sensor is a PT1000 RTD. It is available as a spare part. The part number is K9470BG - Cold Junction (RTD) for ZR22.

We have a limited stock of the ZO21D style cold junction sensors. The part number is E7042AU-J1, and the list price is $250.00. We will continued to sell them but they will be gone in a short time. The few customers that still have ZA8Cs or AV8Gs can purchase the parts but they need to plan to change to the ZR402 or AV550.

Identifying the Cold Junction Sensors

The Cold Junction sensor may be a PT1000 RTD or a Transistor Cold Junction sensor.

There are two styles of RTD that have been used. Both are PT1000:

The newer units have a PT1000 sensor with a Red & Black wire


The older units have a PT1000 sensor with a Red & White wire.


The Transistor sensor is mounted on a small metal bracket with 2 wires connected to one terminal and 1 wire connected to the other one.


For the ZR402G or AV550 with a PT1000 sensor set the Detector Setting to ZR22G.

For the ZR402G or AV550 with a Transistor sensor set the Detector Setting to ZO21C.

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