If you are getting a 0 reading post DI your DI unit is doing its job. Do you only have 1 probe? I have 2 with mine and there is a huge difference between pre and post Di readings. Pre may be 135 post is 0 ish.
Hope that helps some.
Rob
I don't have DI just RO Sorry for the confusion!!

The probes are pre / post RO.
Sorry
