So I have got away with it for years by just changing reservoirs once a week with one top up a week of fresh water and keeping an eye on PH. For once I have decided to want to know my nutrient strength. After doing a bit of research I have invested in a top of the range chinese ppm pen for £4.50, done a few tests and even if it were 10 to 20 ppm out which it isn't, I would still be in a safe zone. I went for the high end of nutrient measuring pens cos of funds and because I saw a video with a top guy saying about current trends with regards to measuring your nutrient strength, as in must have kit etc and what can we sell um next? Using Spock and Holmes logic It seems to me that it just comes down to numbers, with CF and EC you have low numbers and with ppm you have high numbers.....Fascinating..... , Which means when using a ppm scale you have a lot more rope to play with and stand a less chance if any at all of making a mistake. When I used to grow other crops I also favoured Fahrenheit opposed to celsius for the same reason, which I still do,..... call me retro, some folk do. Elementary my dear Watson, more rope, more dope. Gunna stick with ppm, write down the numbers the pen displays /mls per litre as a guide and maybe buy a better one down the line, or another one of these when it feks up. It's weed Jim but not as we know it, not as we no it, not as we know it, It's gange Jim but not as we know it, not as we know it, Captain, There's truncheons in the hydro shop, hydro shop, hydro shop There's truncheons,in the hydro shop, but do we really need one? Answers on a postcard please and send to 221B Baker street or USS Enterprise NCC1-1701