The dissolved oxygen probes have been redesigned so that they are all the same diameter. As a result of this redesign the new probes have a narrower diameter than the old probes. The new probes are easily identified with a B after the probe part number on the probe or meter packaging.
To compliment the new probe design the dissolved oxygen membranes have also been redesigned. The new membranes will not fit on old probes and old membranes will not fit on new probes.