I have the same tds meter it was £3 on ebay. I also have had 3 yellow PH pens £6. Replaced the PH pen with a £60 unit, and although i rarely use it im resonably confident of my PH ranges.
That tds pen is only good for throwing at a wall or putting in the bin, sorry.
edit... A ph pen or tds meter is useless without the various fluids to store/test/clean the unit.