You can borrow mine if you need to know quickly. I wish I had known about that inline model before I got the hand held one I use. It works fine, I just have to disconnect and take samples at different points to check the RO and DI.
My DI cartridge is just that, a cartridge. When I bought it, I thought it used loose material, but it doesn't. I use about 20 gallons a week and am seeing 6 to 8 months before the TDS start coming up and my water is Edmond/NW Okc (178th and Penn). I pay $35.00 including shipping for a replacement insert cartridge.
I mention all this in case someone has a significantly cheaper alternative...
Tony