Why do I need an Interface Block?

The Interface Block is what receives the signal from your flow switch or flow meter and then transmits wirelessly to the Sniper Software on your handheld unitl.

What is the difference in these these Interface Blocks?

The difference is the capacity to accept unique signals (ie, number of booms on your 4 wheeler) and compatibilty with your equipment (ie, flow switch vs. flow meter, etc...).