Guidance Manual for Compliance with the Interim Enhanced Surface Water Treatment Rule: Turbidity Provisions

After calibration, performance of the turbidimeter should be verified with a secondary standard. If the instrument has internal electronic diagnostics designed to assist in determining proper calibration, the operator should use these tools to verify proper calibration and operation.

Calibration Standards

A calibration standard must be used to conduct a calibration. Standards are materials with a known value which, when placed in the instrument, should be used to adjust the instrument to read the known value.

There are a variety of standards on the market today which are used to calibrate turbidimeters. They are most often characterized as Primary,Secondary, or Alternative standards. Standard Methods describes Primary Standards as a standard which is prepared by the user from traceable raw materials, using precise methodologies and under controlled environmental conditions. (Standard Methods, 1995) Standard Methods defines Secondary Standards as those standards a manufacturer (or an independent testing organization) has certified to give instrument calibration results equivalent (within certain limits) to results obtained when an instrument is calibrated with a primary standard.

Standard Methods and EPA differ in their definitions of each of these standards. EPA recognizes the following three Standards for approved use in the calibration of turbidimeters.

  1. Formazin (user prepared and commercially produced)
  2. AMCO-AEPA-1® Microspheres
  3. Stablcal® ( Stabilized Formazin)

Users need to realize that some instruments have been designed and calibrated on specific primary standard(s) listed above. For optimal results, users should contact the manufacturer of the instrument to determine the recommended primary standard to be used for calibration.

Additionally, EPA recognizes Secondary Standards for use in monitoring day-to-day accuracy of turbidimeters by checking the calibration. This check is used to determine if calibration with a Primary Standard is necessary. Secondary Standards are used to check whether an instrument produces measurements within acceptable limits around a nominal value (typically 10%). Example of SECONDARY STANDARDS include:

  • Gelex®
  • Glass/Ceramic Cubes
  • Manufactures Provided Instruments Specific Secondary Standards

EPA Statement

The attached is in response to your request for clarification on the use of AMCO AEPA-1 turbidity standards for drinking water compliance monitoring. This response was prepared by the James O'Dell, editor of EPA Method 180.1, and reviewed by Office of Ground Water and Drinking Water personnel. Please feel free to contact me if you have any questions.

Jim O'Dell

AMCO Turbidity Standards
AMCO AEPA -1 standards are approved and accepted turbidity calibration standards for USEPA Method 180.1 and can be used for all drinking water compliance monitoring. These standards should meet the calibration requirements of all instruments manufactured to be compliant with Method 180.1. The use of the any of the approved standards should not adversely effect the performance any Method 180.1 compliant turbidimeter. It is expected that the choice of which EPA approved standard to use for calibration can and should be decided by the end user.