Milwaukee M10031B 1413µS/cm conductivity calibration solution in a sachet, 25 sachets á 20ml

It allows you to set up your measuring device correctly and ensures reliable readings.

That depends on how often you use it. If used frequently, regular calibration is recommended.

In many cases, yes, provided the same measurement range is used.

Yes, as long as it remains clean and is stored correctly.

It is best to store them in a cool, dry place and keep them tightly sealed.

If the readings seem unusual or change significantly.