TDS meters are extremely helpful when trying to balance your nute solution, especially if you are using multiple products. It really isnt that useful tho after you change the water because like zandor said the plant will take up certain nutes and discard others, throwing off the ppm. Anyway if you are low on cash right now then def invest in a ph pen. They are around 30 bucks and can be found on ebay. Believe me if you want to do it right and not end up with crap then you have to check the ph twice a day until your plants can handle more nutes in which case the ph swings will be much more mild and easier to stabalize. If you cant afford to buy a ppm meter then always use half the reccomended amount for the nutes. Remember it is easier to add nutes than it is to change out the res. because of nute burn. Just my 2C and good luck:thumbsup: