The FIELDKIT 200 has been deployed since 2015. Roughly 3,400 nodes have been in the ground, some for six years. The 400 is what we learned.
Water got in. Not through the seals, which held, but through the cable gland on units where a technician had not fully torqued it in the rain at the end of a long day. The 400 uses a quarter-turn connector that is either engaged or visibly not.
Batteries died early at high latitude. The 200's charge controller assumed a duty cycle that does not survive a Nordic winter with six weeks of no useful sun. The 400 has a larger battery, a smarter controller and a documented latitude-dependent deployment life.
Commissioning took too long. It was a laptop, a serial cable and a fifteen-minute procedure that field technicians reasonably resented. The 400 pairs by QR code from a phone in about four minutes.
And cartridges. The 200's sensors were fixed at manufacture, which meant a soil moisture node could not become a turbidity node. The 400 has four swappable bays. This is the change customers asked for most and the one that cost the most engineering.