Can I calibrate my 1720E using an offset from my laboratory turbidimeter?

Document ID

Document ID TE6665

Version

Version 1.0

Status

Status Published

Published Date

Published Date 13/03/2017
Question
Can I calibrate my 1720E using an offset from my laboratory turbidimeter?
Summary
Information describing why process turbidimeters should not be calibrated using slope offsets based on laboratory turbidimeters.
Answer
The Hach 1720E process turbidimeter and 2100N/AN laboratory turbidimeters are different optical design implementations of the EPA Turbidity Method 180.1. Despite this, Hach believes that the different use models, design features, and performance variations support the view of the USEPA that one of these instrument's calibrations should not be used to calibrate the other. Only instruments with identical optical designs can be used to verify instruments, and then only when each of the units have been previously calibrated following Hach's instructions, and in accordance with the approved EPA Method and EPA guidance documents. The notes below clarify this in detail.
  • One of the interferences of turbidity instruments is stray light. Stray light is light that contributes to the turbiditiy measurement outside of the intended optical design of the instrument. Stray light offsets can become significant, particularly at low turbidities. The 2100N/AN turbidimeters have higher stray light than the 1720E. Using a 2100N/AN turbidimeter to adjust the calibration of a 1720E is thus introducing a stray light error due to the 2100N/AN at whatever turbidity standard is being used to calibrate the instruments.
  • Whereas the 2100N/AN instruments have a small volume glass sample vial, the 1720E instruments have a 1L measurement cell with no glass vial. Despite the high quality of manufacture, each individual 2100N/AN glass vial will contribute different small variations to the sample turbidity measured by the 2100N/AN that are not experienced by the 1720E. Thus, an error from the glass vial will be introduced into the measurement of the 1720E.
  • Turbidity is very much an in-situ measurement. Low level turbidity in particular is very difficult to create and maintain. Taking samples of low turbidity water for measurement on a laboratory turbidimeter presents several problems. The use  model of taking a sample and transporting it to a laboratory turbidimeter has the potential to alter the turbidity value. Also, with a laboratory turbidimeter, the skill level of the operator affects the accuracy of measurement.
For these reasons, Hach and the USEPA recommends that a laboratory turbidimeter not be used to calibrate a process turbidimeter. 1

1
EPA 815-R-99-010 - Guidance Manual for Compliance with the Interim Enhanced Surface Water Treatment Rule: Turbidity Provisions. Section 3.3.2 Calibration states “EPA does not recommend calibrating on-line instruments by comparison with a bench-top turbidimeter. It has been determined that this procedure is likely to introduce unacceptable levels of error into the calibration.”