Hi, I'm new on here and new to window cleaning. Hope someone can help with this. I bought a van mounted, Ionics Systems V4 600 litre last December. Haven't used is much as I've been setting up a new business but had to change 3 filters last month, carbon, sediment and DI. Today I have a PPM reading of 1 having only filled it half a dozen times since the filter changes! I'm really worried that it's the RO that needs changing now. The booklet provided is quite vague about how often the filters need changing. Does anyone know how long they last?