Seems that the probe/sensor is included and is attached with a 1 meter cable. It seems it is ready to go out of the box. The membrane needs to be replaced after you can't calibrate correctly anymore. No word on lifespan of membrane but it comes with 5 spares also. The only accessory that you would need, depending, is the s/w to chart/record your results over time. Or just write it down and keep a spreadsheet.
Here is where I got the information:
Extach Manual
