The ProCheck-S Solid Validation Standards are used as a quick and convenient “check” to determine if a Primary Calibration is needed prior to the next regularly scheduled primary calibration. Designed for use with HF scientific turbidimeters, they can be used as often as needed and will last for many years. Although actual values will be determined with testing, the typical value for the low range ProCheck-S will be less than 1 NTU for drinking water and other applications requiring a very low NTU range and the mid range ProCheck-S will typically be around 7 NTU for higher range applications.