This is from Gaps Water's website and I tried a pair.
Ref: PRF-750-RO
Upgrade the performance of your on-demand PRF-RO or Merlin System by fitting the new SPECTRUM PRF-RO membrane.
Delivery improved water production and quality with a 70% increase in flow, 94% rejection rate (dependant on pressure) and 50% improved recovery.
The PRF-750-RO out performs the standard TLC-350 on many levels.
Well I did this as my membranes needed replacing and although I have only used them for a month at 55psi approx the input is 250ppm approx and the output is 5ppm.
The old ones (4 sets from different suppliers - Gardiners when they did them, Soap national later) gave an output of 8 or 9 to start with before settling down to about 12 to 14 for a year or so and then crept up to about 18 before I replaced them.
Early days but looking good so far.
Edited to add: I haven't calibrated my TDS meter but it is the one I've used for the past few years.