I have some packs of the 4.0 and the 7.0 that I use when recalibrating my pH meter before using it. I keep the fluid in a container and get multiple uses out of it, so the cost isn't very much.

I use this and the 7.0 solution to calibrate my pH meter and get accurate results. No sense having a pH meter without taking care of it.

This and the 7.00 solution are needed each time you brew to get accurate pH readings. After using it I save it in a small jar and can get a couple of uses out of it.

pH meters need to be calibrated to be accurate and this is the fluid I use. I save the contents in a plastic vial so i can get a few uses from it.

Even though its "one use" I keep the contents in a glass jar and get several uses out of it. I use a new package after every 5 uses.