TDS reading 3 why???


Reefing newb
I have a 5 stage BRS ro di system 75 GPD. I piggy backed two 90 GPD membrane from Spectrum and my TDS is reading 3 form product water.

All other filter cartridges are only a month old along with my DI filter.

Why is my TDS not reading 0

It reed 0 before i added the two RO membranes

heed help please
it is a inline tds meter i have looked on line but looks like everything i found are hand held meters do you still calibrate a inline tds meter.