Aye, i was convinced when i was shopping to buy all the calibration fluids and to do it on a montly if not monthly basis. As it is i dip the meter in the fluids here and there and they all read correctly, tis all goodBecause for our purpose you don't need much precision, any of them are fine. Check ebay for one around $30. I prefer the Hanna
BTW, even if a meter doesn't need calibration it does need to be checked regularly. You don't need calibration fluids for that, again because of the precision, so just use distilled or ro water to calibrate to 7.0