Why is a calibration with a Primary standard recommended by Hach at least every 90 days for the 1720E analyzer?
1720 and EPA Calibration Method
The 1720E is based on US EPA methods (180.1) and this is the requirement in the method. A Primary Standard is a liquid calibration with StablCal or Formazine. The liquid standard can also be used as a wet verification or we offer the Ice-Pic for a dry verification. The dry verification is considered a Secondary standard and should not be used for a Primary calibration if you are reporting to a regulatory agency. When you calibrate with the Primary Standard, the 1720E will offer a chance for a dry verification. If done then, this will set a "baseline" reading in the Ice-Pic which can be used as often as you wish to see if the calibration is holding. You can set the pass/fail criteria for the dry verification between 5-10% depending on your own quality procedures. If you do a dry verification and it fails, that would indicate you need to do a Primary calibration and then a new dry verification to reset the "baseline" reading, Refer to your instrument manual for further details. We also state that you should always check with your local regulator as they may have more calibration requirements.