Deciding the correct length of cable for a suspended IP68 transducer is an important part of specifying, if the cable is too short it is difficult and expensive to join two cables together and make the seal liquid tight.
There are two separate parts of the installation that need to be considered to determine the overal length which is the suspended length and any extra length that is needed to reach the termination point from the point of suspension.
The top is fairly easy to define and is the point where the cable would be most securely suspended, whilst providing easy access for removal and fitting. The Bottom is less defined since it may not be advantageous to have the sensor reach the very bottom, but instead suspended at a higher point to prevent build up of sediment or to increase sensitivity to the changes in liquid level.
If the transducer cable is clamped at the hanging point without electrical termination, it will then have to be run to instrumentation elsewhere which might be a long distance away.
The transducer cable is relatively high cost because it often includes a strain relief core made from Kevlar fibre to prevent the cable stretching due to its own weight which would affect the calibration reference point especially with very deep installations. There is also one or possibly two plastic vent tubes for allowing atmospheric pressure to reach the reference side of the transducer sensing diaphragm. To keep the cost down the length of the transducer cable should be kept to a minimum by adding a termination junction box so that a lower cost signal cable can be run to where the signal conditioning instrumentation is located.
Using a junction box has the added benefit of providing a dry enclosure for the vented reference tube which must be allowed to breath to atmosphere but sufficiently protected from condensation.