Product review (2 stars): Stale calibration powder ?
The seller replied promptly to question re: filter paper; however when I attempted to calibrate to 6.86 it took a LOT of stirring of RODI H2O with a chopstick to dissolve the hard crumbles of powder. There is no expiration date on packet of powder. 7Pros did not respond to my request as to where I would get more packets of powder.