Use the following picture to determine the version of your O2 sensor. The sensor on the left (old) was manufactured before April 2007 but could have been sold depending on company stock.
The sensor on the right (current) has been manufactured since April 2007 and Vernier is currently shipping.
The old version of the sensor is not repairable. The current version is repairable. See TIL 1605: Can my O2 Gas Sensor (O2-BTA) be repaired?.
If you need to measure dissolved oxygen, consider the Vernier Optical DO Probe, order code ODO-BTA, $299.